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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>Programming FAQ — Python 2.7.18 documentation</title> <link rel="stylesheet" href="../_static/classic.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/language_data.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.18 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="Design and History FAQ" href="design.html" /> <link rel="prev" title="General Python FAQ" href="general.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="file:///usr/share/doc/python2.7/html/faq/programming.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div id="outdated-warning" style="padding: .5em; text-align: center; background-color: #FFBABA; color: #6A0E0E;"> This document is for an old version of Python that is <a href="https://devguide.python.org/devcycle/#end-of-life-branches">no longer supported</a>. You should install the python3 and python3-doc packages and read the <a href="file:///usr/share/doc/python3-doc/html/faq/programming.html"> Python documentation for the Python3 version packaged in this release</a>. </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="design.html" title="Design and History FAQ" accesskey="N">next</a> |</li> <li class="right" > <a href="general.html" title="General Python FAQ" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.18 documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">Python Frequently Asked Questions</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="programming-faq"> <h1><a class="toc-backref" href="#id1">Programming FAQ</a><a class="headerlink" href="#programming-faq" title="Permalink to this headline">¶</a></h1> <div class="contents topic" id="contents"> <p class="topic-title">Contents</p> <ul class="simple"> <li><a class="reference internal" href="#programming-faq" id="id1">Programming FAQ</a><ul> <li><a class="reference internal" href="#general-questions" id="id2">General Questions</a><ul> <li><a class="reference internal" href="#is-there-a-source-code-level-debugger-with-breakpoints-single-stepping-etc" id="id3">Is there a source code level debugger with breakpoints, single-stepping, etc.?</a></li> <li><a class="reference internal" href="#is-there-a-tool-to-help-find-bugs-or-perform-static-analysis" id="id4">Is there a tool to help find bugs or perform static analysis?</a></li> <li><a class="reference internal" href="#how-can-i-create-a-stand-alone-binary-from-a-python-script" id="id5">How can I create a stand-alone binary from a Python script?</a></li> <li><a class="reference internal" href="#are-there-coding-standards-or-a-style-guide-for-python-programs" id="id6">Are there coding standards or a style guide for Python programs?</a></li> <li><a class="reference internal" href="#my-program-is-too-slow-how-do-i-speed-it-up" id="id7">My program is too slow. How do I speed it up?</a></li> </ul> </li> <li><a class="reference internal" href="#core-language" id="id8">Core Language</a><ul> <li><a class="reference internal" href="#why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value" id="id9">Why am I getting an UnboundLocalError when the variable has a value?</a></li> <li><a class="reference internal" href="#what-are-the-rules-for-local-and-global-variables-in-python" id="id10">What are the rules for local and global variables in Python?</a></li> <li><a class="reference internal" href="#why-do-lambdas-defined-in-a-loop-with-different-values-all-return-the-same-result" id="id11">Why do lambdas defined in a loop with different values all return the same result?</a></li> <li><a class="reference internal" href="#how-do-i-share-global-variables-across-modules" id="id12">How do I share global variables across modules?</a></li> <li><a class="reference internal" href="#what-are-the-best-practices-for-using-import-in-a-module" id="id13">What are the “best practices” for using import in a module?</a></li> <li><a class="reference internal" href="#why-are-default-values-shared-between-objects" id="id14">Why are default values shared between objects?</a></li> <li><a class="reference internal" href="#how-can-i-pass-optional-or-keyword-parameters-from-one-function-to-another" id="id15">How can I pass optional or keyword parameters from one function to another?</a></li> <li><a class="reference internal" href="#what-is-the-difference-between-arguments-and-parameters" id="id16">What is the difference between arguments and parameters?</a></li> <li><a class="reference internal" href="#why-did-changing-list-y-also-change-list-x" id="id17">Why did changing list ‘y’ also change list ‘x’?</a></li> <li><a class="reference internal" href="#how-do-i-write-a-function-with-output-parameters-call-by-reference" id="id18">How do I write a function with output parameters (call by reference)?</a></li> <li><a class="reference internal" href="#how-do-you-make-a-higher-order-function-in-python" id="id19">How do you make a higher order function in Python?</a></li> <li><a class="reference internal" href="#how-do-i-copy-an-object-in-python" id="id20">How do I copy an object in Python?</a></li> <li><a class="reference internal" href="#how-can-i-find-the-methods-or-attributes-of-an-object" id="id21">How can I find the methods or attributes of an object?</a></li> <li><a class="reference internal" href="#how-can-my-code-discover-the-name-of-an-object" id="id22">How can my code discover the name of an object?</a></li> <li><a class="reference internal" href="#what-s-up-with-the-comma-operator-s-precedence" id="id23">What’s up with the comma operator’s precedence?</a></li> <li><a class="reference internal" href="#is-there-an-equivalent-of-c-s-ternary-operator" id="id24">Is there an equivalent of C’s “?:” ternary operator?</a></li> <li><a class="reference internal" href="#is-it-possible-to-write-obfuscated-one-liners-in-python" id="id25">Is it possible to write obfuscated one-liners in Python?</a></li> </ul> </li> <li><a class="reference internal" href="#numbers-and-strings" id="id26">Numbers and strings</a><ul> <li><a class="reference internal" href="#how-do-i-specify-hexadecimal-and-octal-integers" id="id27">How do I specify hexadecimal and octal integers?</a></li> <li><a class="reference internal" href="#why-does-22-10-return-3" id="id28">Why does -22 // 10 return -3?</a></li> <li><a class="reference internal" href="#how-do-i-convert-a-string-to-a-number" id="id29">How do I convert a string to a number?</a></li> <li><a class="reference internal" href="#how-do-i-convert-a-number-to-a-string" id="id30">How do I convert a number to a string?</a></li> <li><a class="reference internal" href="#how-do-i-modify-a-string-in-place" id="id31">How do I modify a string in place?</a></li> <li><a class="reference internal" href="#how-do-i-use-strings-to-call-functions-methods" id="id32">How do I use strings to call functions/methods?</a></li> <li><a class="reference internal" href="#is-there-an-equivalent-to-perl-s-chomp-for-removing-trailing-newlines-from-strings" id="id33">Is there an equivalent to Perl’s chomp() for removing trailing newlines from strings?</a></li> <li><a class="reference internal" href="#is-there-a-scanf-or-sscanf-equivalent" id="id34">Is there a scanf() or sscanf() equivalent?</a></li> <li><a class="reference internal" href="#what-does-unicodeerror-ascii-decoding-encoding-error-ordinal-not-in-range-128-mean" id="id35">What does ‘UnicodeError: ASCII [decoding,encoding] error: ordinal not in range(128)’ mean?</a></li> </ul> </li> <li><a class="reference internal" href="#sequences-tuples-lists" id="id36">Sequences (Tuples/Lists)</a><ul> <li><a class="reference internal" href="#how-do-i-convert-between-tuples-and-lists" id="id37">How do I convert between tuples and lists?</a></li> <li><a class="reference internal" href="#what-s-a-negative-index" id="id38">What’s a negative index?</a></li> <li><a class="reference internal" href="#how-do-i-iterate-over-a-sequence-in-reverse-order" id="id39">How do I iterate over a sequence in reverse order?</a></li> <li><a class="reference internal" href="#how-do-you-remove-duplicates-from-a-list" id="id40">How do you remove duplicates from a list?</a></li> <li><a class="reference internal" href="#how-do-you-make-an-array-in-python" id="id41">How do you make an array in Python?</a></li> <li><a class="reference internal" href="#how-do-i-create-a-multidimensional-list" id="id42">How do I create a multidimensional list?</a></li> <li><a class="reference internal" href="#how-do-i-apply-a-method-to-a-sequence-of-objects" id="id43">How do I apply a method to a sequence of objects?</a></li> <li><a class="reference internal" href="#why-does-a-tuple-i-item-raise-an-exception-when-the-addition-works" id="id44">Why does a_tuple[i] += [‘item’] raise an exception when the addition works?</a></li> </ul> </li> <li><a class="reference internal" href="#dictionaries" id="id45">Dictionaries</a><ul> <li><a class="reference internal" href="#how-can-i-get-a-dictionary-to-display-its-keys-in-a-consistent-order" id="id46">How can I get a dictionary to display its keys in a consistent order?</a></li> <li><a class="reference internal" href="#i-want-to-do-a-complicated-sort-can-you-do-a-schwartzian-transform-in-python" id="id47">I want to do a complicated sort: can you do a Schwartzian Transform in Python?</a></li> <li><a class="reference internal" href="#how-can-i-sort-one-list-by-values-from-another-list" id="id48">How can I sort one list by values from another list?</a></li> </ul> </li> <li><a class="reference internal" href="#objects" id="id49">Objects</a><ul> <li><a class="reference internal" href="#what-is-a-class" id="id50">What is a class?</a></li> <li><a class="reference internal" href="#what-is-a-method" id="id51">What is a method?</a></li> <li><a class="reference internal" href="#what-is-self" id="id52">What is self?</a></li> <li><a class="reference internal" href="#how-do-i-check-if-an-object-is-an-instance-of-a-given-class-or-of-a-subclass-of-it" id="id53">How do I check if an object is an instance of a given class or of a subclass of it?</a></li> <li><a class="reference internal" href="#what-is-delegation" id="id54">What is delegation?</a></li> <li><a class="reference internal" href="#how-do-i-call-a-method-defined-in-a-base-class-from-a-derived-class-that-overrides-it" id="id55">How do I call a method defined in a base class from a derived class that overrides it?</a></li> <li><a class="reference internal" href="#how-can-i-organize-my-code-to-make-it-easier-to-change-the-base-class" id="id56">How can I organize my code to make it easier to change the base class?</a></li> <li><a class="reference internal" href="#how-do-i-create-static-class-data-and-static-class-methods" id="id57">How do I create static class data and static class methods?</a></li> <li><a class="reference internal" href="#how-can-i-overload-constructors-or-methods-in-python" id="id58">How can I overload constructors (or methods) in Python?</a></li> <li><a class="reference internal" href="#i-try-to-use-spam-and-i-get-an-error-about-someclassname-spam" id="id59">I try to use __spam and I get an error about _SomeClassName__spam.</a></li> <li><a class="reference internal" href="#my-class-defines-del-but-it-is-not-called-when-i-delete-the-object" id="id60">My class defines __del__ but it is not called when I delete the object.</a></li> <li><a class="reference internal" href="#how-do-i-get-a-list-of-all-instances-of-a-given-class" id="id61">How do I get a list of all instances of a given class?</a></li> <li><a class="reference internal" href="#why-does-the-result-of-id-appear-to-be-not-unique" id="id62">Why does the result of <code class="docutils literal notranslate"><span class="pre">id()</span></code> appear to be not unique?</a></li> </ul> </li> <li><a class="reference internal" href="#modules" id="id63">Modules</a><ul> <li><a class="reference internal" href="#how-do-i-create-a-pyc-file" id="id64">How do I create a .pyc file?</a></li> <li><a class="reference internal" href="#how-do-i-find-the-current-module-name" id="id65">How do I find the current module name?</a></li> <li><a class="reference internal" href="#how-can-i-have-modules-that-mutually-import-each-other" id="id66">How can I have modules that mutually import each other?</a></li> <li><a class="reference internal" href="#import-x-y-z-returns-module-x-how-do-i-get-z" id="id67">__import__(‘x.y.z’) returns <module ‘x’>; how do I get z?</a></li> <li><a class="reference internal" href="#when-i-edit-an-imported-module-and-reimport-it-the-changes-don-t-show-up-why-does-this-happen" id="id68">When I edit an imported module and reimport it, the changes don’t show up. Why does this happen?</a></li> </ul> </li> </ul> </li> </ul> </div> <div class="section" id="general-questions"> <h2><a class="toc-backref" href="#id2">General Questions</a><a class="headerlink" href="#general-questions" title="Permalink to this headline">¶</a></h2> <div class="section" id="is-there-a-source-code-level-debugger-with-breakpoints-single-stepping-etc"> <h3><a class="toc-backref" href="#id3">Is there a source code level debugger with breakpoints, single-stepping, etc.?</a><a class="headerlink" href="#is-there-a-source-code-level-debugger-with-breakpoints-single-stepping-etc" title="Permalink to this headline">¶</a></h3> <p>Yes.</p> <p>The pdb module is a simple but adequate console-mode debugger for Python. It is part of the standard Python library, and is <a class="reference internal" href="../library/pdb.html#module-pdb" title="pdb: The Python debugger for interactive interpreters."><code class="xref py py-mod docutils literal notranslate"><span class="pre">documented</span> <span class="pre">in</span> <span class="pre">the</span> <span class="pre">Library</span> <span class="pre">Reference</span> <span class="pre">Manual</span></code></a>. You can also write your own debugger by using the code for pdb as an example.</p> <p>The IDLE interactive development environment, which is part of the standard Python distribution (normally available as Tools/scripts/idle), includes a graphical debugger.</p> <p>PythonWin is a Python IDE that includes a GUI debugger based on pdb. The Pythonwin debugger colors breakpoints and has quite a few cool features such as debugging non-Pythonwin programs. Pythonwin is available as part of the <a class="reference external" href="https://sourceforge.net/projects/pywin32/">Python for Windows Extensions</a> project and as a part of the ActivePython distribution (see <a class="reference external" href="https://www.activestate.com/activepython">https://www.activestate.com/activepython</a>).</p> <p><a class="reference external" href="http://boa-constructor.sourceforge.net/">Boa Constructor</a> is an IDE and GUI builder that uses wxWidgets. It offers visual frame creation and manipulation, an object inspector, many views on the source like object browsers, inheritance hierarchies, doc string generated html documentation, an advanced debugger, integrated help, and Zope support.</p> <p><a class="reference external" href="http://eric-ide.python-projects.org/">Eric</a> is an IDE built on PyQt and the Scintilla editing component.</p> <p>Pydb is a version of the standard Python debugger pdb, modified for use with DDD (Data Display Debugger), a popular graphical debugger front end. Pydb can be found at <a class="reference external" href="http://bashdb.sourceforge.net/pydb/">http://bashdb.sourceforge.net/pydb/</a> and DDD can be found at <a class="reference external" href="https://www.gnu.org/software/ddd">https://www.gnu.org/software/ddd</a>.</p> <p>There are a number of commercial Python IDEs that include graphical debuggers. They include:</p> <ul class="simple"> <li>Wing IDE (<a class="reference external" href="https://wingware.com/">https://wingware.com/</a>)</li> <li>Komodo IDE (<a class="reference external" href="https://komodoide.com/">https://komodoide.com/</a>)</li> <li>PyCharm (<a class="reference external" href="https://www.jetbrains.com/pycharm/">https://www.jetbrains.com/pycharm/</a>)</li> </ul> </div> <div class="section" id="is-there-a-tool-to-help-find-bugs-or-perform-static-analysis"> <h3><a class="toc-backref" href="#id4">Is there a tool to help find bugs or perform static analysis?</a><a class="headerlink" href="#is-there-a-tool-to-help-find-bugs-or-perform-static-analysis" title="Permalink to this headline">¶</a></h3> <p>Yes.</p> <p>PyChecker is a static analysis tool that finds bugs in Python source code and warns about code complexity and style. You can get PyChecker from <a class="reference external" href="http://pychecker.sourceforge.net/">http://pychecker.sourceforge.net/</a>.</p> <p><a class="reference external" href="https://www.pylint.org/">Pylint</a> is another tool that checks if a module satisfies a coding standard, and also makes it possible to write plug-ins to add a custom feature. In addition to the bug checking that PyChecker performs, Pylint offers some additional features such as checking line length, whether variable names are well-formed according to your coding standard, whether declared interfaces are fully implemented, and more. <a class="reference external" href="https://docs.pylint.org/">https://docs.pylint.org/</a> provides a full list of Pylint’s features.</p> </div> <div class="section" id="how-can-i-create-a-stand-alone-binary-from-a-python-script"> <h3><a class="toc-backref" href="#id5">How can I create a stand-alone binary from a Python script?</a><a class="headerlink" href="#how-can-i-create-a-stand-alone-binary-from-a-python-script" title="Permalink to this headline">¶</a></h3> <p>You don’t need the ability to compile Python to C code if all you want is a stand-alone program that users can download and run without having to install the Python distribution first. There are a number of tools that determine the set of modules required by a program and bind these modules together with a Python binary to produce a single executable.</p> <p>One is to use the freeze tool, which is included in the Python source tree as <code class="docutils literal notranslate"><span class="pre">Tools/freeze</span></code>. It converts Python byte code to C arrays; a C compiler you can embed all your modules into a new program, which is then linked with the standard Python modules.</p> <p>It works by scanning your source recursively for import statements (in both forms) and looking for the modules in the standard Python path as well as in the source directory (for built-in modules). It then turns the bytecode for modules written in Python into C code (array initializers that can be turned into code objects using the marshal module) and creates a custom-made config file that only contains those built-in modules which are actually used in the program. It then compiles the generated C code and links it with the rest of the Python interpreter to form a self-contained binary which acts exactly like your script.</p> <p>Obviously, freeze requires a C compiler. There are several other utilities which don’t. One is Thomas Heller’s py2exe (Windows only) at</p> <blockquote> <div><a class="reference external" href="http://www.py2exe.org/">http://www.py2exe.org/</a></div></blockquote> <p>Another tool is Anthony Tuininga’s <a class="reference external" href="http://cx-freeze.sourceforge.net/">cx_Freeze</a>.</p> </div> <div class="section" id="are-there-coding-standards-or-a-style-guide-for-python-programs"> <h3><a class="toc-backref" href="#id6">Are there coding standards or a style guide for Python programs?</a><a class="headerlink" href="#are-there-coding-standards-or-a-style-guide-for-python-programs" title="Permalink to this headline">¶</a></h3> <p>Yes. The coding style required for standard library modules is documented as <span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0008"><strong>PEP 8</strong></a>.</p> </div> <div class="section" id="my-program-is-too-slow-how-do-i-speed-it-up"> <h3><a class="toc-backref" href="#id7">My program is too slow. How do I speed it up?</a><a class="headerlink" href="#my-program-is-too-slow-how-do-i-speed-it-up" title="Permalink to this headline">¶</a></h3> <p>That’s a tough one, in general. There are many tricks to speed up Python code; consider rewriting parts in C as a last resort.</p> <p>In some cases it’s possible to automatically translate Python to C or x86 assembly language, meaning that you don’t have to modify your code to gain increased speed.</p> <p><a class="reference external" href="http://www.cosc.canterbury.ac.nz/~greg/python/Pyrex/">Pyrex</a> can compile a slightly modified version of Python code into a C extension, and can be used on many different platforms.</p> <p><a class="reference external" href="http://psyco.sourceforge.net">Psyco</a> is a just-in-time compiler that translates Python code into x86 assembly language. If you can use it, Psyco can provide dramatic speedups for critical functions.</p> <p>The rest of this answer will discuss various tricks for squeezing a bit more speed out of Python code. <em>Never</em> apply any optimization tricks unless you know you need them, after profiling has indicated that a particular function is the heavily executed hot spot in the code. Optimizations almost always make the code less clear, and you shouldn’t pay the costs of reduced clarity (increased development time, greater likelihood of bugs) unless the resulting performance benefit is worth it.</p> <p>There is a page on the wiki devoted to <a class="reference external" href="https://wiki.python.org/moin/PythonSpeed/PerformanceTips">performance tips</a>.</p> <p>Guido van Rossum has written up an anecdote related to optimization at <a class="reference external" href="https://www.python.org/doc/essays/list2str">https://www.python.org/doc/essays/list2str</a>.</p> <p>One thing to notice is that function and (especially) method calls are rather expensive; if you have designed a purely OO interface with lots of tiny functions that don’t do much more than get or set an instance variable or call another method, you might consider using a more direct way such as directly accessing instance variables. Also see the standard module <a class="reference internal" href="../library/profile.html#module-profile" title="profile: Python source profiler."><code class="xref py py-mod docutils literal notranslate"><span class="pre">profile</span></code></a> which makes it possible to find out where your program is spending most of its time (if you have some patience – the profiling itself can slow your program down by an order of magnitude).</p> <p>Remember that many standard optimization heuristics you may know from other programming experience may well apply to Python. For example it may be faster to send output to output devices using larger writes rather than smaller ones in order to reduce the overhead of kernel system calls. Thus CGI scripts that write all output in “one shot” may be faster than those that write lots of small pieces of output.</p> <p>Also, be sure to use Python’s core features where appropriate. For example, slicing allows programs to chop up lists and other sequence objects in a single tick of the interpreter’s mainloop using highly optimized C implementations. Thus to get the same effect as:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">L2</span> <span class="o">=</span> <span class="p">[]</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span> <span class="n">L2</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">L1</span><span class="p">[</span><span class="n">i</span><span class="p">])</span> </pre></div> </div> <p>it is much shorter and far faster to use</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">L2</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="n">L1</span><span class="p">[:</span><span class="mi">3</span><span class="p">])</span> <span class="c1"># "list" is redundant if L1 is a list.</span> </pre></div> </div> <p>Note that the functionally-oriented built-in functions such as <a class="reference internal" href="../library/functions.html#map" title="map"><code class="xref py py-func docutils literal notranslate"><span class="pre">map()</span></code></a>, <a class="reference internal" href="../library/functions.html#zip" title="zip"><code class="xref py py-func docutils literal notranslate"><span class="pre">zip()</span></code></a>, and friends can be a convenient accelerator for loops that perform a single task. For example to pair the elements of two lists together:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">zip</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">],</span> <span class="p">[</span><span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">])</span> <span class="go">[(1, 4), (2, 5), (3, 6)]</span> </pre></div> </div> <p>or to compute a number of sines:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">map</span><span class="p">(</span><span class="n">math</span><span class="o">.</span><span class="n">sin</span><span class="p">,</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">))</span> <span class="go">[0.841470984808, 0.909297426826, 0.14112000806, -0.756802495308]</span> </pre></div> </div> <p>The operation completes very quickly in such cases.</p> <p>Other examples include the <code class="docutils literal notranslate"><span class="pre">join()</span></code> and <code class="docutils literal notranslate"><span class="pre">split()</span></code> <a class="reference internal" href="../library/stdtypes.html#string-methods"><span class="std std-ref">methods of string objects</span></a>. For example if s1..s7 are large (10K+) strings then <code class="docutils literal notranslate"><span class="pre">"".join([s1,s2,s3,s4,s5,s6,s7])</span></code> may be far faster than the more obvious <code class="docutils literal notranslate"><span class="pre">s1+s2+s3+s4+s5+s6+s7</span></code>, since the “summation” will compute many subexpressions, whereas <code class="docutils literal notranslate"><span class="pre">join()</span></code> does all the copying in one pass. For manipulating strings, use the <code class="docutils literal notranslate"><span class="pre">replace()</span></code> and the <code class="docutils literal notranslate"><span class="pre">format()</span></code> <a class="reference internal" href="../library/stdtypes.html#string-methods"><span class="std std-ref">methods on string objects</span></a>. Use regular expressions only when you’re not dealing with constant string patterns. You may still use <a class="reference internal" href="../library/stdtypes.html#string-formatting"><span class="std std-ref">the old % operations</span></a> <code class="docutils literal notranslate"><span class="pre">string</span> <span class="pre">%</span> <span class="pre">tuple</span></code> and <code class="docutils literal notranslate"><span class="pre">string</span> <span class="pre">%</span> <span class="pre">dictionary</span></code>.</p> <p>Be sure to use the <code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code> built-in method to do sorting, and see the <a class="reference external" href="https://wiki.python.org/moin/HowTo/Sorting">sorting mini-HOWTO</a> for examples of moderately advanced usage. <code class="xref py py-meth docutils literal notranslate"><span class="pre">list.sort()</span></code> beats other techniques for sorting in all but the most extreme circumstances.</p> <p>Another common trick is to “push loops into functions or methods.” For example suppose you have a program that runs slowly and you use the profiler to determine that a Python function <code class="docutils literal notranslate"><span class="pre">ff()</span></code> is being called lots of times. If you notice that <code class="docutils literal notranslate"><span class="pre">ff()</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">ff</span><span class="p">(</span><span class="n">x</span><span class="p">):</span> <span class="o">...</span> <span class="c1"># do something with x computing result...</span> <span class="k">return</span> <span class="n">result</span> </pre></div> </div> <p>tends to be called in loops like:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">list</span> <span class="o">=</span> <span class="nb">map</span><span class="p">(</span><span class="n">ff</span><span class="p">,</span> <span class="n">oldlist</span><span class="p">)</span> </pre></div> </div> <p>or:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">sequence</span><span class="p">:</span> <span class="n">value</span> <span class="o">=</span> <span class="n">ff</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">...</span> <span class="c1"># do something with value...</span> </pre></div> </div> <p>then you can often eliminate function call overhead by rewriting <code class="docutils literal notranslate"><span class="pre">ff()</span></code> to:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">ffseq</span><span class="p">(</span><span class="n">seq</span><span class="p">):</span> <span class="n">resultseq</span> <span class="o">=</span> <span class="p">[]</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">seq</span><span class="p">:</span> <span class="o">...</span> <span class="c1"># do something with x computing result...</span> <span class="n">resultseq</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">result</span><span class="p">)</span> <span class="k">return</span> <span class="n">resultseq</span> </pre></div> </div> <p>and rewrite the two examples to <code class="docutils literal notranslate"><span class="pre">list</span> <span class="pre">=</span> <span class="pre">ffseq(oldlist)</span></code> and to:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">value</span> <span class="ow">in</span> <span class="n">ffseq</span><span class="p">(</span><span class="n">sequence</span><span class="p">):</span> <span class="o">...</span> <span class="c1"># do something with value...</span> </pre></div> </div> <p>Single calls to <code class="docutils literal notranslate"><span class="pre">ff(x)</span></code> translate to <code class="docutils literal notranslate"><span class="pre">ffseq([x])[0]</span></code> with little penalty. Of course this technique is not always appropriate and there are other variants which you can figure out.</p> <p>You can gain some performance by explicitly storing the results of a function or method lookup into a local variable. A loop like:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">key</span> <span class="ow">in</span> <span class="n">token</span><span class="p">:</span> <span class="nb">dict</span><span class="p">[</span><span class="n">key</span><span class="p">]</span> <span class="o">=</span> <span class="nb">dict</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span> <span class="o">+</span> <span class="mi">1</span> </pre></div> </div> <p>resolves <code class="docutils literal notranslate"><span class="pre">dict.get</span></code> every iteration. If the method isn’t going to change, a slightly faster implementation is:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">dict_get</span> <span class="o">=</span> <span class="nb">dict</span><span class="o">.</span><span class="n">get</span> <span class="c1"># look up the method once</span> <span class="k">for</span> <span class="n">key</span> <span class="ow">in</span> <span class="n">token</span><span class="p">:</span> <span class="nb">dict</span><span class="p">[</span><span class="n">key</span><span class="p">]</span> <span class="o">=</span> <span class="n">dict_get</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span> <span class="o">+</span> <span class="mi">1</span> </pre></div> </div> <p>Default arguments can be used to determine values once, at compile time instead of at run time. This can only be done for functions or objects which will not be changed during program execution, such as replacing</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">degree_sin</span><span class="p">(</span><span class="n">deg</span><span class="p">):</span> <span class="k">return</span> <span class="n">math</span><span class="o">.</span><span class="n">sin</span><span class="p">(</span><span class="n">deg</span> <span class="o">*</span> <span class="n">math</span><span class="o">.</span><span class="n">pi</span> <span class="o">/</span> <span class="mf">180.0</span><span class="p">)</span> </pre></div> </div> <p>with</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">degree_sin</span><span class="p">(</span><span class="n">deg</span><span class="p">,</span> <span class="n">factor</span><span class="o">=</span><span class="n">math</span><span class="o">.</span><span class="n">pi</span><span class="o">/</span><span class="mf">180.0</span><span class="p">,</span> <span class="n">sin</span><span class="o">=</span><span class="n">math</span><span class="o">.</span><span class="n">sin</span><span class="p">):</span> <span class="k">return</span> <span class="n">sin</span><span class="p">(</span><span class="n">deg</span> <span class="o">*</span> <span class="n">factor</span><span class="p">)</span> </pre></div> </div> <p>Because this trick uses default arguments for terms which should not be changed, it should only be used when you are not concerned with presenting a possibly confusing API to your users.</p> </div> </div> <div class="section" id="core-language"> <h2><a class="toc-backref" href="#id8">Core Language</a><a class="headerlink" href="#core-language" title="Permalink to this headline">¶</a></h2> <div class="section" id="why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value"> <h3><a class="toc-backref" href="#id9">Why am I getting an UnboundLocalError when the variable has a value?</a><a class="headerlink" href="#why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value" title="Permalink to this headline">¶</a></h3> <p>It can be a surprise to get the UnboundLocalError in previously working code when it is modified by adding an assignment statement somewhere in the body of a function.</p> <p>This code:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="mi">10</span> <span class="gp">>>> </span><span class="k">def</span> <span class="nf">bar</span><span class="p">():</span> <span class="gp">... </span> <span class="nb">print</span> <span class="n">x</span> <span class="gp">>>> </span><span class="n">bar</span><span class="p">()</span> <span class="go">10</span> </pre></div> </div> <p>works, but this code:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="mi">10</span> <span class="gp">>>> </span><span class="k">def</span> <span class="nf">foo</span><span class="p">():</span> <span class="gp">... </span> <span class="nb">print</span> <span class="n">x</span> <span class="gp">... </span> <span class="n">x</span> <span class="o">+=</span> <span class="mi">1</span> </pre></div> </div> <p>results in an UnboundLocalError:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">foo</span><span class="p">()</span> <span class="gt">Traceback (most recent call last):</span> <span class="c">...</span> <span class="gr">UnboundLocalError</span>: <span class="n">local variable 'x' referenced before assignment</span> </pre></div> </div> <p>This is because when you make an assignment to a variable in a scope, that variable becomes local to that scope and shadows any similarly named variable in the outer scope. Since the last statement in foo assigns a new value to <code class="docutils literal notranslate"><span class="pre">x</span></code>, the compiler recognizes it as a local variable. Consequently when the earlier <code class="docutils literal notranslate"><span class="pre">print</span> <span class="pre">x</span></code> attempts to print the uninitialized local variable and an error results.</p> <p>In the example above you can access the outer scope variable by declaring it global:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="mi">10</span> <span class="gp">>>> </span><span class="k">def</span> <span class="nf">foobar</span><span class="p">():</span> <span class="gp">... </span> <span class="k">global</span> <span class="n">x</span> <span class="gp">... </span> <span class="nb">print</span> <span class="n">x</span> <span class="gp">... </span> <span class="n">x</span> <span class="o">+=</span> <span class="mi">1</span> <span class="gp">>>> </span><span class="n">foobar</span><span class="p">()</span> <span class="go">10</span> </pre></div> </div> <p>This explicit declaration is required in order to remind you that (unlike the superficially analogous situation with class and instance variables) you are actually modifying the value of the variable in the outer scope:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span> <span class="n">x</span> <span class="go">11</span> </pre></div> </div> </div> <div class="section" id="what-are-the-rules-for-local-and-global-variables-in-python"> <h3><a class="toc-backref" href="#id10">What are the rules for local and global variables in Python?</a><a class="headerlink" href="#what-are-the-rules-for-local-and-global-variables-in-python" title="Permalink to this headline">¶</a></h3> <p>In Python, variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.</p> <p>Though a bit surprising at first, a moment’s consideration explains this. On one hand, requiring <a class="reference internal" href="../reference/simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> for assigned variables provides a bar against unintended side-effects. On the other hand, if <code class="docutils literal notranslate"><span class="pre">global</span></code> was required for all global references, you’d be using <code class="docutils literal notranslate"><span class="pre">global</span></code> all the time. You’d have to declare as global every reference to a built-in function or to a component of an imported module. This clutter would defeat the usefulness of the <code class="docutils literal notranslate"><span class="pre">global</span></code> declaration for identifying side-effects.</p> </div> <div class="section" id="why-do-lambdas-defined-in-a-loop-with-different-values-all-return-the-same-result"> <h3><a class="toc-backref" href="#id11">Why do lambdas defined in a loop with different values all return the same result?</a><a class="headerlink" href="#why-do-lambdas-defined-in-a-loop-with-different-values-all-return-the-same-result" title="Permalink to this headline">¶</a></h3> <p>Assume you use a for loop to define a few different lambdas (or even plain functions), e.g.:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">>>> </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span> <span class="gp">... </span> <span class="n">squares</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="k">lambda</span><span class="p">:</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span> </pre></div> </div> <p>This gives you a list that contains 5 lambdas that calculate <code class="docutils literal notranslate"><span class="pre">x**2</span></code>. You might expect that, when called, they would return, respectively, <code class="docutils literal notranslate"><span class="pre">0</span></code>, <code class="docutils literal notranslate"><span class="pre">1</span></code>, <code class="docutils literal notranslate"><span class="pre">4</span></code>, <code class="docutils literal notranslate"><span class="pre">9</span></code>, and <code class="docutils literal notranslate"><span class="pre">16</span></code>. However, when you actually try you will see that they all return <code class="docutils literal notranslate"><span class="pre">16</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span><span class="p">[</span><span class="mi">2</span><span class="p">]()</span> <span class="go">16</span> <span class="gp">>>> </span><span class="n">squares</span><span class="p">[</span><span class="mi">4</span><span class="p">]()</span> <span class="go">16</span> </pre></div> </div> <p>This happens because <code class="docutils literal notranslate"><span class="pre">x</span></code> is not local to the lambdas, but is defined in the outer scope, and it is accessed when the lambda is called — not when it is defined. At the end of the loop, the value of <code class="docutils literal notranslate"><span class="pre">x</span></code> is <code class="docutils literal notranslate"><span class="pre">4</span></code>, so all the functions now return <code class="docutils literal notranslate"><span class="pre">4**2</span></code>, i.e. <code class="docutils literal notranslate"><span class="pre">16</span></code>. You can also verify this by changing the value of <code class="docutils literal notranslate"><span class="pre">x</span></code> and see how the results of the lambdas change:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="mi">8</span> <span class="gp">>>> </span><span class="n">squares</span><span class="p">[</span><span class="mi">2</span><span class="p">]()</span> <span class="go">64</span> </pre></div> </div> <p>In order to avoid this, you need to save the values in variables local to the lambdas, so that they don’t rely on the value of the global <code class="docutils literal notranslate"><span class="pre">x</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">>>> </span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span> <span class="gp">... </span> <span class="n">squares</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="k">lambda</span> <span class="n">n</span><span class="o">=</span><span class="n">x</span><span class="p">:</span> <span class="n">n</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span> </pre></div> </div> <p>Here, <code class="docutils literal notranslate"><span class="pre">n=x</span></code> creates a new variable <code class="docutils literal notranslate"><span class="pre">n</span></code> local to the lambda and computed when the lambda is defined so that it has the same value that <code class="docutils literal notranslate"><span class="pre">x</span></code> had at that point in the loop. This means that the value of <code class="docutils literal notranslate"><span class="pre">n</span></code> will be <code class="docutils literal notranslate"><span class="pre">0</span></code> in the first lambda, <code class="docutils literal notranslate"><span class="pre">1</span></code> in the second, <code class="docutils literal notranslate"><span class="pre">2</span></code> in the third, and so on. Therefore each lambda will now return the correct result:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span><span class="p">[</span><span class="mi">2</span><span class="p">]()</span> <span class="go">4</span> <span class="gp">>>> </span><span class="n">squares</span><span class="p">[</span><span class="mi">4</span><span class="p">]()</span> <span class="go">16</span> </pre></div> </div> <p>Note that this behaviour is not peculiar to lambdas, but applies to regular functions too.</p> </div> <div class="section" id="how-do-i-share-global-variables-across-modules"> <h3><a class="toc-backref" href="#id12">How do I share global variables across modules?</a><a class="headerlink" href="#how-do-i-share-global-variables-across-modules" title="Permalink to this headline">¶</a></h3> <p>The canonical way to share information across modules within a single program is to create a special module (often called config or cfg). Just import the config module in all modules of your application; the module then becomes available as a global name. Because there is only one instance of each module, any changes made to the module object get reflected everywhere. For example:</p> <p>config.py:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="o">=</span> <span class="mi">0</span> <span class="c1"># Default value of the 'x' configuration setting</span> </pre></div> </div> <p>mod.py:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">config</span> <span class="n">config</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="mi">1</span> </pre></div> </div> <p>main.py:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">config</span> <span class="kn">import</span> <span class="nn">mod</span> <span class="nb">print</span> <span class="n">config</span><span class="o">.</span><span class="n">x</span> </pre></div> </div> <p>Note that using a module is also the basis for implementing the Singleton design pattern, for the same reason.</p> </div> <div class="section" id="what-are-the-best-practices-for-using-import-in-a-module"> <h3><a class="toc-backref" href="#id13">What are the “best practices” for using import in a module?</a><a class="headerlink" href="#what-are-the-best-practices-for-using-import-in-a-module" title="Permalink to this headline">¶</a></h3> <p>In general, don’t use <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">modulename</span> <span class="pre">import</span> <span class="pre">*</span></code>. Doing so clutters the importer’s namespace, and makes it much harder for linters to detect undefined names.</p> <p>Import modules at the top of a file. Doing so makes it clear what other modules your code requires and avoids questions of whether the module name is in scope. Using one import per line makes it easy to add and delete module imports, but using multiple imports per line uses less screen space.</p> <p>It’s good practice if you import modules in the following order:</p> <ol class="arabic simple"> <li>standard library modules – e.g. <code class="docutils literal notranslate"><span class="pre">sys</span></code>, <code class="docutils literal notranslate"><span class="pre">os</span></code>, <code class="docutils literal notranslate"><span class="pre">getopt</span></code>, <code class="docutils literal notranslate"><span class="pre">re</span></code></li> <li>third-party library modules (anything installed in Python’s site-packages directory) – e.g. mx.DateTime, ZODB, PIL.Image, etc.</li> <li>locally-developed modules</li> </ol> <p>Only use explicit relative package imports. If you’re writing code that’s in the <code class="docutils literal notranslate"><span class="pre">package.sub.m1</span></code> module and want to import <code class="docutils literal notranslate"><span class="pre">package.sub.m2</span></code>, do not just write <code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">m2</span></code>, even though it’s legal. Write <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">package.sub</span> <span class="pre">import</span> <span class="pre">m2</span></code> or <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">.</span> <span class="pre">import</span> <span class="pre">m2</span></code> instead.</p> <p>It is sometimes necessary to move imports to a function or class to avoid problems with circular imports. Gordon McMillan says:</p> <blockquote> <div>Circular imports are fine where both modules use the “import <module>” form of import. They fail when the 2nd module wants to grab a name out of the first (“from module import name”) and the import is at the top level. That’s because names in the 1st are not yet available, because the first module is busy importing the 2nd.</div></blockquote> <p>In this case, if the second module is only used in one function, then the import can easily be moved into that function. By the time the import is called, the first module will have finished initializing, and the second module can do its import.</p> <p>It may also be necessary to move imports out of the top level of code if some of the modules are platform-specific. In that case, it may not even be possible to import all of the modules at the top of the file. In this case, importing the correct modules in the corresponding platform-specific code is a good option.</p> <p>Only move imports into a local scope, such as inside a function definition, if it’s necessary to solve a problem such as avoiding a circular import or are trying to reduce the initialization time of a module. This technique is especially helpful if many of the imports are unnecessary depending on how the program executes. You may also want to move imports into a function if the modules are only ever used in that function. Note that loading a module the first time may be expensive because of the one time initialization of the module, but loading a module multiple times is virtually free, costing only a couple of dictionary lookups. Even if the module name has gone out of scope, the module is probably available in <a class="reference internal" href="../library/sys.html#sys.modules" title="sys.modules"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.modules</span></code></a>.</p> </div> <div class="section" id="why-are-default-values-shared-between-objects"> <h3><a class="toc-backref" href="#id14">Why are default values shared between objects?</a><a class="headerlink" href="#why-are-default-values-shared-between-objects" title="Permalink to this headline">¶</a></h3> <p>This type of bug commonly bites neophyte programmers. Consider this function:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">mydict</span><span class="o">=</span><span class="p">{}):</span> <span class="c1"># Danger: shared reference to one dict for all calls</span> <span class="o">...</span> <span class="n">compute</span> <span class="n">something</span> <span class="o">...</span> <span class="n">mydict</span><span class="p">[</span><span class="n">key</span><span class="p">]</span> <span class="o">=</span> <span class="n">value</span> <span class="k">return</span> <span class="n">mydict</span> </pre></div> </div> <p>The first time you call this function, <code class="docutils literal notranslate"><span class="pre">mydict</span></code> contains a single item. The second time, <code class="docutils literal notranslate"><span class="pre">mydict</span></code> contains two items because when <code class="docutils literal notranslate"><span class="pre">foo()</span></code> begins executing, <code class="docutils literal notranslate"><span class="pre">mydict</span></code> starts out with an item already in it.</p> <p>It is often expected that a function call creates new objects for default values. This is not what happens. Default values are created exactly once, when the function is defined. If that object is changed, like the dictionary in this example, subsequent calls to the function will refer to this changed object.</p> <p>By definition, immutable objects such as numbers, strings, tuples, and <code class="docutils literal notranslate"><span class="pre">None</span></code>, are safe from change. Changes to mutable objects such as dictionaries, lists, and class instances can lead to confusion.</p> <p>Because of this feature, it is good programming practice to not use mutable objects as default values. Instead, use <code class="docutils literal notranslate"><span class="pre">None</span></code> as the default value and inside the function, check if the parameter is <code class="docutils literal notranslate"><span class="pre">None</span></code> and create a new list/dictionary/whatever if it is. For example, don’t write:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">mydict</span><span class="o">=</span><span class="p">{}):</span> <span class="o">...</span> </pre></div> </div> <p>but:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">mydict</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span> <span class="k">if</span> <span class="n">mydict</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span> <span class="n">mydict</span> <span class="o">=</span> <span class="p">{}</span> <span class="c1"># create a new dict for local namespace</span> </pre></div> </div> <p>This feature can be useful. When you have a function that’s time-consuming to compute, a common technique is to cache the parameters and the resulting value of each call to the function, and return the cached value if the same value is requested again. This is called “memoizing”, and can be implemented like this:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># Callers will never provide a third parameter for this function.</span> <span class="k">def</span> <span class="nf">expensive</span><span class="p">(</span><span class="n">arg1</span><span class="p">,</span> <span class="n">arg2</span><span class="p">,</span> <span class="n">_cache</span><span class="o">=</span><span class="p">{}):</span> <span class="k">if</span> <span class="p">(</span><span class="n">arg1</span><span class="p">,</span> <span class="n">arg2</span><span class="p">)</span> <span class="ow">in</span> <span class="n">_cache</span><span class="p">:</span> <span class="k">return</span> <span class="n">_cache</span><span class="p">[(</span><span class="n">arg1</span><span class="p">,</span> <span class="n">arg2</span><span class="p">)]</span> <span class="c1"># Calculate the value</span> <span class="n">result</span> <span class="o">=</span> <span class="o">...</span> <span class="n">expensive</span> <span class="n">computation</span> <span class="o">...</span> <span class="n">_cache</span><span class="p">[(</span><span class="n">arg1</span><span class="p">,</span> <span class="n">arg2</span><span class="p">)]</span> <span class="o">=</span> <span class="n">result</span> <span class="c1"># Store result in the cache</span> <span class="k">return</span> <span class="n">result</span> </pre></div> </div> <p>You could use a global variable containing a dictionary instead of the default value; it’s a matter of taste.</p> </div> <div class="section" id="how-can-i-pass-optional-or-keyword-parameters-from-one-function-to-another"> <h3><a class="toc-backref" href="#id15">How can I pass optional or keyword parameters from one function to another?</a><a class="headerlink" href="#how-can-i-pass-optional-or-keyword-parameters-from-one-function-to-another" title="Permalink to this headline">¶</a></h3> <p>Collect the arguments using the <code class="docutils literal notranslate"><span class="pre">*</span></code> and <code class="docutils literal notranslate"><span class="pre">**</span></code> specifiers in the function’s parameter list; this gives you the positional arguments as a tuple and the keyword arguments as a dictionary. You can then pass these arguments when calling another function by using <code class="docutils literal notranslate"><span class="pre">*</span></code> and <code class="docutils literal notranslate"><span class="pre">**</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span> <span class="o">...</span> <span class="n">kwargs</span><span class="p">[</span><span class="s1">'width'</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'14.3c'</span> <span class="o">...</span> <span class="n">g</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span> </pre></div> </div> <p>In the unlikely case that you care about Python versions older than 2.0, use <a class="reference internal" href="../library/functions.html#apply" title="apply"><code class="xref py py-func docutils literal notranslate"><span class="pre">apply()</span></code></a>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span> <span class="o">...</span> <span class="n">kwargs</span><span class="p">[</span><span class="s1">'width'</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'14.3c'</span> <span class="o">...</span> <span class="n">apply</span><span class="p">(</span><span class="n">g</span><span class="p">,</span> <span class="p">(</span><span class="n">x</span><span class="p">,)</span><span class="o">+</span><span class="n">args</span><span class="p">,</span> <span class="n">kwargs</span><span class="p">)</span> </pre></div> </div> </div> <div class="section" id="what-is-the-difference-between-arguments-and-parameters"> <span id="faq-argument-vs-parameter"></span><span id="index-1"></span><h3><a class="toc-backref" href="#id16">What is the difference between arguments and parameters?</a><a class="headerlink" href="#what-is-the-difference-between-arguments-and-parameters" title="Permalink to this headline">¶</a></h3> <p><a class="reference internal" href="../glossary.html#term-parameter"><span class="xref std std-term">Parameters</span></a> are defined by the names that appear in a function definition, whereas <a class="reference internal" href="../glossary.html#term-argument"><span class="xref std std-term">arguments</span></a> are the values actually passed to a function when calling it. Parameters define what types of arguments a function can accept. For example, given the function definition:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="n">foo</span><span class="p">,</span> <span class="n">bar</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span> <span class="k">pass</span> </pre></div> </div> <p><em>foo</em>, <em>bar</em> and <em>kwargs</em> are parameters of <code class="docutils literal notranslate"><span class="pre">func</span></code>. However, when calling <code class="docutils literal notranslate"><span class="pre">func</span></code>, for example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">func</span><span class="p">(</span><span class="mi">42</span><span class="p">,</span> <span class="n">bar</span><span class="o">=</span><span class="mi">314</span><span class="p">,</span> <span class="n">extra</span><span class="o">=</span><span class="n">somevar</span><span class="p">)</span> </pre></div> </div> <p>the values <code class="docutils literal notranslate"><span class="pre">42</span></code>, <code class="docutils literal notranslate"><span class="pre">314</span></code>, and <code class="docutils literal notranslate"><span class="pre">somevar</span></code> are arguments.</p> </div> <div class="section" id="why-did-changing-list-y-also-change-list-x"> <h3><a class="toc-backref" href="#id17">Why did changing list ‘y’ also change list ‘x’?</a><a class="headerlink" href="#why-did-changing-list-y-also-change-list-x" title="Permalink to this headline">¶</a></h3> <p>If you wrote code like:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">>>> </span><span class="n">y</span> <span class="o">=</span> <span class="n">x</span> <span class="gp">>>> </span><span class="n">y</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">y</span> <span class="go">[10]</span> <span class="gp">>>> </span><span class="n">x</span> <span class="go">[10]</span> </pre></div> </div> <p>you might be wondering why appending an element to <code class="docutils literal notranslate"><span class="pre">y</span></code> changed <code class="docutils literal notranslate"><span class="pre">x</span></code> too.</p> <p>There are two factors that produce this result:</p> <ol class="arabic simple"> <li>Variables are simply names that refer to objects. Doing <code class="docutils literal notranslate"><span class="pre">y</span> <span class="pre">=</span> <span class="pre">x</span></code> doesn’t create a copy of the list – it creates a new variable <code class="docutils literal notranslate"><span class="pre">y</span></code> that refers to the same object <code class="docutils literal notranslate"><span class="pre">x</span></code> refers to. This means that there is only one object (the list), and both <code class="docutils literal notranslate"><span class="pre">x</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span></code> refer to it.</li> <li>Lists are <a class="reference internal" href="../glossary.html#term-mutable"><span class="xref std std-term">mutable</span></a>, which means that you can change their content.</li> </ol> <p>After the call to <code class="xref py py-meth docutils literal notranslate"><span class="pre">append()</span></code>, the content of the mutable object has changed from <code class="docutils literal notranslate"><span class="pre">[]</span></code> to <code class="docutils literal notranslate"><span class="pre">[10]</span></code>. Since both the variables refer to the same object, using either name accesses the modified value <code class="docutils literal notranslate"><span class="pre">[10]</span></code>.</p> <p>If we instead assign an immutable object to <code class="docutils literal notranslate"><span class="pre">x</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="mi">5</span> <span class="c1"># ints are immutable</span> <span class="gp">>>> </span><span class="n">y</span> <span class="o">=</span> <span class="n">x</span> <span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">1</span> <span class="c1"># 5 can't be mutated, we are creating a new object here</span> <span class="gp">>>> </span><span class="n">x</span> <span class="go">6</span> <span class="gp">>>> </span><span class="n">y</span> <span class="go">5</span> </pre></div> </div> <p>we can see that in this case <code class="docutils literal notranslate"><span class="pre">x</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span></code> are not equal anymore. This is because integers are <a class="reference internal" href="../glossary.html#term-immutable"><span class="xref std std-term">immutable</span></a>, and when we do <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">=</span> <span class="pre">x</span> <span class="pre">+</span> <span class="pre">1</span></code> we are not mutating the int <code class="docutils literal notranslate"><span class="pre">5</span></code> by incrementing its value; instead, we are creating a new object (the int <code class="docutils literal notranslate"><span class="pre">6</span></code>) and assigning it to <code class="docutils literal notranslate"><span class="pre">x</span></code> (that is, changing which object <code class="docutils literal notranslate"><span class="pre">x</span></code> refers to). After this assignment we have two objects (the ints <code class="docutils literal notranslate"><span class="pre">6</span></code> and <code class="docutils literal notranslate"><span class="pre">5</span></code>) and two variables that refer to them (<code class="docutils literal notranslate"><span class="pre">x</span></code> now refers to <code class="docutils literal notranslate"><span class="pre">6</span></code> but <code class="docutils literal notranslate"><span class="pre">y</span></code> still refers to <code class="docutils literal notranslate"><span class="pre">5</span></code>).</p> <p>Some operations (for example <code class="docutils literal notranslate"><span class="pre">y.append(10)</span></code> and <code class="docutils literal notranslate"><span class="pre">y.sort()</span></code>) mutate the object, whereas superficially similar operations (for example <code class="docutils literal notranslate"><span class="pre">y</span> <span class="pre">=</span> <span class="pre">y</span> <span class="pre">+</span> <span class="pre">[10]</span></code> and <code class="docutils literal notranslate"><span class="pre">sorted(y)</span></code>) create a new object. In general in Python (and in all cases in the standard library) a method that mutates an object will return <code class="docutils literal notranslate"><span class="pre">None</span></code> to help avoid getting the two types of operations confused. So if you mistakenly write <code class="docutils literal notranslate"><span class="pre">y.sort()</span></code> thinking it will give you a sorted copy of <code class="docutils literal notranslate"><span class="pre">y</span></code>, you’ll instead end up with <code class="docutils literal notranslate"><span class="pre">None</span></code>, which will likely cause your program to generate an easily diagnosed error.</p> <p>However, there is one class of operations where the same operation sometimes has different behaviors with different types: the augmented assignment operators. For example, <code class="docutils literal notranslate"><span class="pre">+=</span></code> mutates lists but not tuples or ints (<code class="docutils literal notranslate"><span class="pre">a_list</span> <span class="pre">+=</span> <span class="pre">[1,</span> <span class="pre">2,</span> <span class="pre">3]</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">a_list.extend([1,</span> <span class="pre">2,</span> <span class="pre">3])</span></code> and mutates <code class="docutils literal notranslate"><span class="pre">a_list</span></code>, whereas <code class="docutils literal notranslate"><span class="pre">some_tuple</span> <span class="pre">+=</span> <span class="pre">(1,</span> <span class="pre">2,</span> <span class="pre">3)</span></code> and <code class="docutils literal notranslate"><span class="pre">some_int</span> <span class="pre">+=</span> <span class="pre">1</span></code> create new objects).</p> <p>In other words:</p> <ul class="simple"> <li>If we have a mutable object (<code class="xref py py-class docutils literal notranslate"><span class="pre">list</span></code>, <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a>, <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate"><span class="pre">set</span></code></a>, etc.), we can use some specific operations to mutate it and all the variables that refer to it will see the change.</li> <li>If we have an immutable object (<a class="reference internal" href="../library/functions.html#str" title="str"><code class="xref py py-class docutils literal notranslate"><span class="pre">str</span></code></a>, <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>, <a class="reference internal" href="../library/functions.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate"><span class="pre">tuple</span></code></a>, etc.), all the variables that refer to it will always see the same value, but operations that transform that value into a new value always return a new object.</li> </ul> <p>If you want to know if two variables refer to the same object or not, you can use the <a class="reference internal" href="../reference/expressions.html#is"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">is</span></code></a> operator, or the built-in function <a class="reference internal" href="../library/functions.html#id" title="id"><code class="xref py py-func docutils literal notranslate"><span class="pre">id()</span></code></a>.</p> </div> <div class="section" id="how-do-i-write-a-function-with-output-parameters-call-by-reference"> <h3><a class="toc-backref" href="#id18">How do I write a function with output parameters (call by reference)?</a><a class="headerlink" href="#how-do-i-write-a-function-with-output-parameters-call-by-reference" title="Permalink to this headline">¶</a></h3> <p>Remember that arguments are passed by assignment in Python. Since assignment just creates references to objects, there’s no alias between an argument name in the caller and callee, and so no call-by-reference per se. You can achieve the desired effect in a number of ways.</p> <ol class="arabic"> <li><p class="first">By returning a tuple of the results:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">func2</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">):</span> <span class="n">a</span> <span class="o">=</span> <span class="s1">'new-value'</span> <span class="c1"># a and b are local names</span> <span class="n">b</span> <span class="o">=</span> <span class="n">b</span> <span class="o">+</span> <span class="mi">1</span> <span class="c1"># assigned to new objects</span> <span class="k">return</span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="c1"># return new values</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="o">=</span> <span class="s1">'old-value'</span><span class="p">,</span> <span class="mi">99</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="o">=</span> <span class="n">func2</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span> <span class="nb">print</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="c1"># output: new-value 100</span> </pre></div> </div> <p>This is almost always the clearest solution.</p> </li> <li><p class="first">By using global variables. This isn’t thread-safe, and is not recommended.</p> </li> <li><p class="first">By passing a mutable (changeable in-place) object:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">func1</span><span class="p">(</span><span class="n">a</span><span class="p">):</span> <span class="n">a</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'new-value'</span> <span class="c1"># 'a' references a mutable list</span> <span class="n">a</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="n">a</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">+</span> <span class="mi">1</span> <span class="c1"># changes a shared object</span> <span class="n">args</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'old-value'</span><span class="p">,</span> <span class="mi">99</span><span class="p">]</span> <span class="n">func1</span><span class="p">(</span><span class="n">args</span><span class="p">)</span> <span class="nb">print</span> <span class="n">args</span><span class="p">[</span><span class="mi">0</span><span class="p">],</span> <span class="n">args</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="c1"># output: new-value 100</span> </pre></div> </div> </li> <li><p class="first">By passing in a dictionary that gets mutated:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">func3</span><span class="p">(</span><span class="n">args</span><span class="p">):</span> <span class="n">args</span><span class="p">[</span><span class="s1">'a'</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'new-value'</span> <span class="c1"># args is a mutable dictionary</span> <span class="n">args</span><span class="p">[</span><span class="s1">'b'</span><span class="p">]</span> <span class="o">=</span> <span class="n">args</span><span class="p">[</span><span class="s1">'b'</span><span class="p">]</span> <span class="o">+</span> <span class="mi">1</span> <span class="c1"># change it in-place</span> <span class="n">args</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'a'</span><span class="p">:</span> <span class="s1">'old-value'</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">:</span> <span class="mi">99</span><span class="p">}</span> <span class="n">func3</span><span class="p">(</span><span class="n">args</span><span class="p">)</span> <span class="nb">print</span> <span class="n">args</span><span class="p">[</span><span class="s1">'a'</span><span class="p">],</span> <span class="n">args</span><span class="p">[</span><span class="s1">'b'</span><span class="p">]</span> </pre></div> </div> </li> <li><p class="first">Or bundle up values in a class instance:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">callByRef</span><span class="p">:</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">**</span><span class="n">args</span><span class="p">):</span> <span class="k">for</span> <span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="n">value</span><span class="p">)</span> <span class="ow">in</span> <span class="n">args</span><span class="o">.</span><span class="n">items</span><span class="p">():</span> <span class="nb">setattr</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">key</span><span class="p">,</span> <span class="n">value</span><span class="p">)</span> <span class="k">def</span> <span class="nf">func4</span><span class="p">(</span><span class="n">args</span><span class="p">):</span> <span class="n">args</span><span class="o">.</span><span class="n">a</span> <span class="o">=</span> <span class="s1">'new-value'</span> <span class="c1"># args is a mutable callByRef</span> <span class="n">args</span><span class="o">.</span><span class="n">b</span> <span class="o">=</span> <span class="n">args</span><span class="o">.</span><span class="n">b</span> <span class="o">+</span> <span class="mi">1</span> <span class="c1"># change object in-place</span> <span class="n">args</span> <span class="o">=</span> <span class="n">callByRef</span><span class="p">(</span><span class="n">a</span><span class="o">=</span><span class="s1">'old-value'</span><span class="p">,</span> <span class="n">b</span><span class="o">=</span><span class="mi">99</span><span class="p">)</span> <span class="n">func4</span><span class="p">(</span><span class="n">args</span><span class="p">)</span> <span class="nb">print</span> <span class="n">args</span><span class="o">.</span><span class="n">a</span><span class="p">,</span> <span class="n">args</span><span class="o">.</span><span class="n">b</span> </pre></div> </div> <p>There’s almost never a good reason to get this complicated.</p> </li> </ol> <p>Your best choice is to return a tuple containing the multiple results.</p> </div> <div class="section" id="how-do-you-make-a-higher-order-function-in-python"> <h3><a class="toc-backref" href="#id19">How do you make a higher order function in Python?</a><a class="headerlink" href="#how-do-you-make-a-higher-order-function-in-python" title="Permalink to this headline">¶</a></h3> <p>You have two choices: you can use nested scopes or you can use callable objects. For example, suppose you wanted to define <code class="docutils literal notranslate"><span class="pre">linear(a,b)</span></code> which returns a function <code class="docutils literal notranslate"><span class="pre">f(x)</span></code> that computes the value <code class="docutils literal notranslate"><span class="pre">a*x+b</span></code>. Using nested scopes:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">linear</span><span class="p">(</span><span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">):</span> <span class="k">def</span> <span class="nf">result</span><span class="p">(</span><span class="n">x</span><span class="p">):</span> <span class="k">return</span> <span class="n">a</span> <span class="o">*</span> <span class="n">x</span> <span class="o">+</span> <span class="n">b</span> <span class="k">return</span> <span class="n">result</span> </pre></div> </div> <p>Or using a callable object:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">linear</span><span class="p">:</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">a</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">b</span> <span class="o">=</span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="k">def</span> <span class="nf">__call__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">a</span> <span class="o">*</span> <span class="n">x</span> <span class="o">+</span> <span class="bp">self</span><span class="o">.</span><span class="n">b</span> </pre></div> </div> <p>In both cases,</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">taxes</span> <span class="o">=</span> <span class="n">linear</span><span class="p">(</span><span class="mf">0.3</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span> </pre></div> </div> <p>gives a callable object where <code class="docutils literal notranslate"><span class="pre">taxes(10e6)</span> <span class="pre">==</span> <span class="pre">0.3</span> <span class="pre">*</span> <span class="pre">10e6</span> <span class="pre">+</span> <span class="pre">2</span></code>.</p> <p>The callable object approach has the disadvantage that it is a bit slower and results in slightly longer code. However, note that a collection of callables can share their signature via inheritance:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">exponential</span><span class="p">(</span><span class="n">linear</span><span class="p">):</span> <span class="c1"># __init__ inherited</span> <span class="k">def</span> <span class="nf">__call__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">a</span> <span class="o">*</span> <span class="p">(</span><span class="n">x</span> <span class="o">**</span> <span class="bp">self</span><span class="o">.</span><span class="n">b</span><span class="p">)</span> </pre></div> </div> <p>Object can encapsulate state for several methods:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">counter</span><span class="p">:</span> <span class="n">value</span> <span class="o">=</span> <span class="mi">0</span> <span class="k">def</span> <span class="nf">set</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="n">x</span> <span class="k">def</span> <span class="nf">up</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">+</span> <span class="mi">1</span> <span class="k">def</span> <span class="nf">down</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">-</span> <span class="mi">1</span> <span class="n">count</span> <span class="o">=</span> <span class="n">counter</span><span class="p">()</span> <span class="n">inc</span><span class="p">,</span> <span class="n">dec</span><span class="p">,</span> <span class="n">reset</span> <span class="o">=</span> <span class="n">count</span><span class="o">.</span><span class="n">up</span><span class="p">,</span> <span class="n">count</span><span class="o">.</span><span class="n">down</span><span class="p">,</span> <span class="n">count</span><span class="o">.</span><span class="n">set</span> </pre></div> </div> <p>Here <code class="docutils literal notranslate"><span class="pre">inc()</span></code>, <code class="docutils literal notranslate"><span class="pre">dec()</span></code> and <code class="docutils literal notranslate"><span class="pre">reset()</span></code> act like functions which share the same counting variable.</p> </div> <div class="section" id="how-do-i-copy-an-object-in-python"> <h3><a class="toc-backref" href="#id20">How do I copy an object in Python?</a><a class="headerlink" href="#how-do-i-copy-an-object-in-python" title="Permalink to this headline">¶</a></h3> <p>In general, try <a class="reference internal" href="../library/copy.html#copy.copy" title="copy.copy"><code class="xref py py-func docutils literal notranslate"><span class="pre">copy.copy()</span></code></a> or <a class="reference internal" href="../library/copy.html#copy.deepcopy" title="copy.deepcopy"><code class="xref py py-func docutils literal notranslate"><span class="pre">copy.deepcopy()</span></code></a> for the general case. Not all objects can be copied, but most can.</p> <p>Some objects can be copied more easily. Dictionaries have a <a class="reference internal" href="../library/stdtypes.html#dict.copy" title="dict.copy"><code class="xref py py-meth docutils literal notranslate"><span class="pre">copy()</span></code></a> method:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">newdict</span> <span class="o">=</span> <span class="n">olddict</span><span class="o">.</span><span class="n">copy</span><span class="p">()</span> </pre></div> </div> <p>Sequences can be copied by slicing:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">new_l</span> <span class="o">=</span> <span class="n">l</span><span class="p">[:]</span> </pre></div> </div> </div> <div class="section" id="how-can-i-find-the-methods-or-attributes-of-an-object"> <h3><a class="toc-backref" href="#id21">How can I find the methods or attributes of an object?</a><a class="headerlink" href="#how-can-i-find-the-methods-or-attributes-of-an-object" title="Permalink to this headline">¶</a></h3> <p>For an instance x of a user-defined class, <code class="docutils literal notranslate"><span class="pre">dir(x)</span></code> returns an alphabetized list of the names containing the instance attributes and methods and attributes defined by its class.</p> </div> <div class="section" id="how-can-my-code-discover-the-name-of-an-object"> <h3><a class="toc-backref" href="#id22">How can my code discover the name of an object?</a><a class="headerlink" href="#how-can-my-code-discover-the-name-of-an-object" title="Permalink to this headline">¶</a></h3> <p>Generally speaking, it can’t, because objects don’t really have names. Essentially, assignment always binds a name to a value; The same is true of <code class="docutils literal notranslate"><span class="pre">def</span></code> and <code class="docutils literal notranslate"><span class="pre">class</span></code> statements, but in that case the value is a callable. Consider the following code:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">A</span><span class="p">:</span> <span class="gp">... </span> <span class="k">pass</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">B</span> <span class="o">=</span> <span class="n">A</span> <span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="n">B</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">b</span> <span class="o">=</span> <span class="n">a</span> <span class="gp">>>> </span><span class="nb">print</span> <span class="n">b</span> <span class="go"><__main__.A instance at 0x16D07CC></span> <span class="gp">>>> </span><span class="nb">print</span> <span class="n">a</span> <span class="go"><__main__.A instance at 0x16D07CC></span> </pre></div> </div> <p>Arguably the class has a name: even though it is bound to two names and invoked through the name B the created instance is still reported as an instance of class A. However, it is impossible to say whether the instance’s name is a or b, since both names are bound to the same value.</p> <p>Generally speaking it should not be necessary for your code to “know the names” of particular values. Unless you are deliberately writing introspective programs, this is usually an indication that a change of approach might be beneficial.</p> <p>In comp.lang.python, Fredrik Lundh once gave an excellent analogy in answer to this question:</p> <blockquote> <div><p>The same way as you get the name of that cat you found on your porch: the cat (object) itself cannot tell you its name, and it doesn’t really care – so the only way to find out what it’s called is to ask all your neighbours (namespaces) if it’s their cat (object)…</p> <p>….and don’t be surprised if you’ll find that it’s known by many names, or no name at all!</p> </div></blockquote> </div> <div class="section" id="what-s-up-with-the-comma-operator-s-precedence"> <h3><a class="toc-backref" href="#id23">What’s up with the comma operator’s precedence?</a><a class="headerlink" href="#what-s-up-with-the-comma-operator-s-precedence" title="Permalink to this headline">¶</a></h3> <p>Comma is not an operator in Python. Consider this session:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="s2">"a"</span> <span class="ow">in</span> <span class="s2">"b"</span><span class="p">,</span> <span class="s2">"a"</span> <span class="go">(False, 'a')</span> </pre></div> </div> <p>Since the comma is not an operator, but a separator between expressions the above is evaluated as if you had entered:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="s2">"a"</span> <span class="ow">in</span> <span class="s2">"b"</span><span class="p">),</span> <span class="s2">"a"</span> </pre></div> </div> <p>not:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="s2">"a"</span> <span class="ow">in</span> <span class="p">(</span><span class="s2">"b"</span><span class="p">,</span> <span class="s2">"a"</span><span class="p">)</span> </pre></div> </div> <p>The same is true of the various assignment operators (<code class="docutils literal notranslate"><span class="pre">=</span></code>, <code class="docutils literal notranslate"><span class="pre">+=</span></code> etc). They are not truly operators but syntactic delimiters in assignment statements.</p> </div> <div class="section" id="is-there-an-equivalent-of-c-s-ternary-operator"> <h3><a class="toc-backref" href="#id24">Is there an equivalent of C’s “?:” ternary operator?</a><a class="headerlink" href="#is-there-an-equivalent-of-c-s-ternary-operator" title="Permalink to this headline">¶</a></h3> <p>Yes, this feature was added in Python 2.5. The syntax would be as follows:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">on_true</span><span class="p">]</span> <span class="k">if</span> <span class="p">[</span><span class="n">expression</span><span class="p">]</span> <span class="k">else</span> <span class="p">[</span><span class="n">on_false</span><span class="p">]</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="o">=</span> <span class="mi">50</span><span class="p">,</span> <span class="mi">25</span> <span class="n">small</span> <span class="o">=</span> <span class="n">x</span> <span class="k">if</span> <span class="n">x</span> <span class="o"><</span> <span class="n">y</span> <span class="k">else</span> <span class="n">y</span> </pre></div> </div> <p>For versions previous to 2.5 the answer would be ‘No’.</p> </div> <div class="section" id="is-it-possible-to-write-obfuscated-one-liners-in-python"> <h3><a class="toc-backref" href="#id25">Is it possible to write obfuscated one-liners in Python?</a><a class="headerlink" href="#is-it-possible-to-write-obfuscated-one-liners-in-python" title="Permalink to this headline">¶</a></h3> <p>Yes. Usually this is done by nesting <a class="reference internal" href="../reference/expressions.html#lambda"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">lambda</span></code></a> within <a class="reference internal" href="../reference/expressions.html#lambda"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">lambda</span></code></a>. See the following three examples, due to Ulf Bartelt:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># Primes < 1000</span> <span class="nb">print</span> <span class="nb">filter</span><span class="p">(</span><span class="kc">None</span><span class="p">,</span><span class="nb">map</span><span class="p">(</span><span class="k">lambda</span> <span class="n">y</span><span class="p">:</span><span class="n">y</span><span class="o">*</span><span class="n">reduce</span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">:</span><span class="n">x</span><span class="o">*</span><span class="n">y</span><span class="o">!=</span><span class="mi">0</span><span class="p">,</span> <span class="nb">map</span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="o">=</span><span class="n">y</span><span class="p">:</span><span class="n">y</span><span class="o">%</span><span class="n">x</span><span class="p">,</span><span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span><span class="nb">int</span><span class="p">(</span><span class="nb">pow</span><span class="p">(</span><span class="n">y</span><span class="p">,</span><span class="mf">0.5</span><span class="p">)</span><span class="o">+</span><span class="mi">1</span><span class="p">))),</span><span class="mi">1</span><span class="p">),</span><span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span><span class="mi">1000</span><span class="p">)))</span> <span class="c1"># First 10 Fibonacci numbers</span> <span class="nb">print</span> <span class="nb">map</span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="p">,</span><span class="n">f</span><span class="o">=</span><span class="k">lambda</span> <span class="n">x</span><span class="p">,</span><span class="n">f</span><span class="p">:(</span><span class="n">f</span><span class="p">(</span><span class="n">x</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span><span class="n">f</span><span class="p">)</span><span class="o">+</span><span class="n">f</span><span class="p">(</span><span class="n">x</span><span class="o">-</span><span class="mi">2</span><span class="p">,</span><span class="n">f</span><span class="p">))</span> <span class="k">if</span> <span class="n">x</span><span class="o">></span><span class="mi">1</span> <span class="k">else</span> <span class="mi">1</span><span class="p">:</span> <span class="n">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="n">f</span><span class="p">),</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">))</span> <span class="c1"># Mandelbrot set</span> <span class="nb">print</span> <span class="p">(</span><span class="k">lambda</span> <span class="n">Ru</span><span class="p">,</span><span class="n">Ro</span><span class="p">,</span><span class="n">Iu</span><span class="p">,</span><span class="n">Io</span><span class="p">,</span><span class="n">IM</span><span class="p">,</span><span class="n">Sx</span><span class="p">,</span><span class="n">Sy</span><span class="p">:</span><span class="n">reduce</span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">:</span><span class="n">x</span><span class="o">+</span><span class="n">y</span><span class="p">,</span><span class="nb">map</span><span class="p">(</span><span class="k">lambda</span> <span class="n">y</span><span class="p">,</span> <span class="n">Iu</span><span class="o">=</span><span class="n">Iu</span><span class="p">,</span><span class="n">Io</span><span class="o">=</span><span class="n">Io</span><span class="p">,</span><span class="n">Ru</span><span class="o">=</span><span class="n">Ru</span><span class="p">,</span><span class="n">Ro</span><span class="o">=</span><span class="n">Ro</span><span class="p">,</span><span class="n">Sy</span><span class="o">=</span><span class="n">Sy</span><span class="p">,</span><span class="n">L</span><span class="o">=</span><span class="k">lambda</span> <span class="n">yc</span><span class="p">,</span><span class="n">Iu</span><span class="o">=</span><span class="n">Iu</span><span class="p">,</span><span class="n">Io</span><span class="o">=</span><span class="n">Io</span><span class="p">,</span><span class="n">Ru</span><span class="o">=</span><span class="n">Ru</span><span class="p">,</span><span class="n">Ro</span><span class="o">=</span><span class="n">Ro</span><span class="p">,</span><span class="n">i</span><span class="o">=</span><span class="n">IM</span><span class="p">,</span> <span class="n">Sx</span><span class="o">=</span><span class="n">Sx</span><span class="p">,</span><span class="n">Sy</span><span class="o">=</span><span class="n">Sy</span><span class="p">:</span><span class="n">reduce</span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">:</span><span class="n">x</span><span class="o">+</span><span class="n">y</span><span class="p">,</span><span class="nb">map</span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="p">,</span><span class="n">xc</span><span class="o">=</span><span class="n">Ru</span><span class="p">,</span><span class="n">yc</span><span class="o">=</span><span class="n">yc</span><span class="p">,</span><span class="n">Ru</span><span class="o">=</span><span class="n">Ru</span><span class="p">,</span><span class="n">Ro</span><span class="o">=</span><span class="n">Ro</span><span class="p">,</span> <span class="n">i</span><span class="o">=</span><span class="n">i</span><span class="p">,</span><span class="n">Sx</span><span class="o">=</span><span class="n">Sx</span><span class="p">,</span><span class="n">F</span><span class="o">=</span><span class="k">lambda</span> <span class="n">xc</span><span class="p">,</span><span class="n">yc</span><span class="p">,</span><span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">,</span><span class="n">k</span><span class="p">,</span><span class="n">f</span><span class="o">=</span><span class="k">lambda</span> <span class="n">xc</span><span class="p">,</span><span class="n">yc</span><span class="p">,</span><span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">,</span><span class="n">k</span><span class="p">,</span><span class="n">f</span><span class="p">:(</span><span class="n">k</span><span class="o"><=</span><span class="mi">0</span><span class="p">)</span><span class="ow">or</span> <span class="p">(</span><span class="n">x</span><span class="o">*</span><span class="n">x</span><span class="o">+</span><span class="n">y</span><span class="o">*</span><span class="n">y</span> <span class="o">>=</span><span class="mf">4.0</span><span class="p">)</span> <span class="ow">or</span> <span class="mi">1</span><span class="o">+</span><span class="n">f</span><span class="p">(</span><span class="n">xc</span><span class="p">,</span><span class="n">yc</span><span class="p">,</span><span class="n">x</span><span class="o">*</span><span class="n">x</span><span class="o">-</span><span class="n">y</span><span class="o">*</span><span class="n">y</span><span class="o">+</span><span class="n">xc</span><span class="p">,</span><span class="mf">2.0</span><span class="o">*</span><span class="n">x</span><span class="o">*</span><span class="n">y</span><span class="o">+</span><span class="n">yc</span><span class="p">,</span><span class="n">k</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span><span class="n">f</span><span class="p">):</span><span class="n">f</span><span class="p">(</span><span class="n">xc</span><span class="p">,</span><span class="n">yc</span><span class="p">,</span><span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">,</span><span class="n">k</span><span class="p">,</span><span class="n">f</span><span class="p">):</span><span class="nb">chr</span><span class="p">(</span> <span class="mi">64</span><span class="o">+</span><span class="n">F</span><span class="p">(</span><span class="n">Ru</span><span class="o">+</span><span class="n">x</span><span class="o">*</span><span class="p">(</span><span class="n">Ro</span><span class="o">-</span><span class="n">Ru</span><span class="p">)</span><span class="o">/</span><span class="n">Sx</span><span class="p">,</span><span class="n">yc</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="n">i</span><span class="p">)),</span><span class="nb">range</span><span class="p">(</span><span class="n">Sx</span><span class="p">))):</span><span class="n">L</span><span class="p">(</span><span class="n">Iu</span><span class="o">+</span><span class="n">y</span><span class="o">*</span><span class="p">(</span><span class="n">Io</span><span class="o">-</span><span class="n">Iu</span><span class="p">)</span><span class="o">/</span><span class="n">Sy</span><span class="p">),</span><span class="nb">range</span><span class="p">(</span><span class="n">Sy</span> <span class="p">))))(</span><span class="o">-</span><span class="mf">2.1</span><span class="p">,</span> <span class="mf">0.7</span><span class="p">,</span> <span class="o">-</span><span class="mf">1.2</span><span class="p">,</span> <span class="mf">1.2</span><span class="p">,</span> <span class="mi">30</span><span class="p">,</span> <span class="mi">80</span><span class="p">,</span> <span class="mi">24</span><span class="p">)</span> <span class="c1"># \___ ___/ \___ ___/ | | |__ lines on screen</span> <span class="c1"># V V | |______ columns on screen</span> <span class="c1"># | | |__________ maximum of "iterations"</span> <span class="c1"># | |_________________ range on y axis</span> <span class="c1"># |____________________________ range on x axis</span> </pre></div> </div> <p>Don’t try this at home, kids!</p> </div> </div> <div class="section" id="numbers-and-strings"> <h2><a class="toc-backref" href="#id26">Numbers and strings</a><a class="headerlink" href="#numbers-and-strings" title="Permalink to this headline">¶</a></h2> <div class="section" id="how-do-i-specify-hexadecimal-and-octal-integers"> <h3><a class="toc-backref" href="#id27">How do I specify hexadecimal and octal integers?</a><a class="headerlink" href="#how-do-i-specify-hexadecimal-and-octal-integers" title="Permalink to this headline">¶</a></h3> <p>To specify an octal digit, precede the octal value with a zero, and then a lower or uppercase “o”. For example, to set the variable “a” to the octal value “10” (8 in decimal), type:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="mo">0o10</span> <span class="gp">>>> </span><span class="n">a</span> <span class="go">8</span> </pre></div> </div> <p>Hexadecimal is just as easy. Simply precede the hexadecimal number with a zero, and then a lower or uppercase “x”. Hexadecimal digits can be specified in lower or uppercase. For example, in the Python interpreter:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="mh">0xa5</span> <span class="gp">>>> </span><span class="n">a</span> <span class="go">165</span> <span class="gp">>>> </span><span class="n">b</span> <span class="o">=</span> <span class="mh">0XB2</span> <span class="gp">>>> </span><span class="n">b</span> <span class="go">178</span> </pre></div> </div> </div> <div class="section" id="why-does-22-10-return-3"> <h3><a class="toc-backref" href="#id28">Why does -22 // 10 return -3?</a><a class="headerlink" href="#why-does-22-10-return-3" title="Permalink to this headline">¶</a></h3> <p>It’s primarily driven by the desire that <code class="docutils literal notranslate"><span class="pre">i</span> <span class="pre">%</span> <span class="pre">j</span></code> have the same sign as <code class="docutils literal notranslate"><span class="pre">j</span></code>. If you want that, and also want:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">i</span> <span class="o">==</span> <span class="p">(</span><span class="n">i</span> <span class="o">//</span> <span class="n">j</span><span class="p">)</span> <span class="o">*</span> <span class="n">j</span> <span class="o">+</span> <span class="p">(</span><span class="n">i</span> <span class="o">%</span> <span class="n">j</span><span class="p">)</span> </pre></div> </div> <p>then integer division has to return the floor. C also requires that identity to hold, and then compilers that truncate <code class="docutils literal notranslate"><span class="pre">i</span> <span class="pre">//</span> <span class="pre">j</span></code> need to make <code class="docutils literal notranslate"><span class="pre">i</span> <span class="pre">%</span> <span class="pre">j</span></code> have the same sign as <code class="docutils literal notranslate"><span class="pre">i</span></code>.</p> <p>There are few real use cases for <code class="docutils literal notranslate"><span class="pre">i</span> <span class="pre">%</span> <span class="pre">j</span></code> when <code class="docutils literal notranslate"><span class="pre">j</span></code> is negative. When <code class="docutils literal notranslate"><span class="pre">j</span></code> is positive, there are many, and in virtually all of them it’s more useful for <code class="docutils literal notranslate"><span class="pre">i</span> <span class="pre">%</span> <span class="pre">j</span></code> to be <code class="docutils literal notranslate"><span class="pre">>=</span> <span class="pre">0</span></code>. If the clock says 10 now, what did it say 200 hours ago? <code class="docutils literal notranslate"><span class="pre">-190</span> <span class="pre">%</span> <span class="pre">12</span> <span class="pre">==</span> <span class="pre">2</span></code> is useful; <code class="docutils literal notranslate"><span class="pre">-190</span> <span class="pre">%</span> <span class="pre">12</span> <span class="pre">==</span> <span class="pre">-10</span></code> is a bug waiting to bite.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">On Python 2, <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">/</span> <span class="pre">b</span></code> returns the same as <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">//</span> <span class="pre">b</span></code> if <code class="docutils literal notranslate"><span class="pre">__future__.division</span></code> is not in effect. This is also known as “classic” division.</p> </div> </div> <div class="section" id="how-do-i-convert-a-string-to-a-number"> <h3><a class="toc-backref" href="#id29">How do I convert a string to a number?</a><a class="headerlink" href="#how-do-i-convert-a-string-to-a-number" title="Permalink to this headline">¶</a></h3> <p>For integers, use the built-in <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate"><span class="pre">int()</span></code></a> type constructor, e.g. <code class="docutils literal notranslate"><span class="pre">int('144')</span> <span class="pre">==</span> <span class="pre">144</span></code>. Similarly, <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-func docutils literal notranslate"><span class="pre">float()</span></code></a> converts to floating-point, e.g. <code class="docutils literal notranslate"><span class="pre">float('144')</span> <span class="pre">==</span> <span class="pre">144.0</span></code>.</p> <p>By default, these interpret the number as decimal, so that <code class="docutils literal notranslate"><span class="pre">int('0144')</span> <span class="pre">==</span> <span class="pre">144</span></code> and <code class="docutils literal notranslate"><span class="pre">int('0x144')</span></code> raises <a class="reference internal" href="../library/exceptions.html#exceptions.ValueError" title="exceptions.ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a>. <code class="docutils literal notranslate"><span class="pre">int(string,</span> <span class="pre">base)</span></code> takes the base to convert from as a second optional argument, so <code class="docutils literal notranslate"><span class="pre">int('0x144',</span> <span class="pre">16)</span> <span class="pre">==</span> <span class="pre">324</span></code>. If the base is specified as 0, the number is interpreted using Python’s rules: a leading ‘0’ indicates octal, and ‘0x’ indicates a hex number.</p> <p>Do not use the built-in function <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> if all you need is to convert strings to numbers. <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> will be significantly slower and it presents a security risk: someone could pass you a Python expression that might have unwanted side effects. For example, someone could pass <code class="docutils literal notranslate"><span class="pre">__import__('os').system("rm</span> <span class="pre">-rf</span> <span class="pre">$HOME")</span></code> which would erase your home directory.</p> <p><a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> also has the effect of interpreting numbers as Python expressions, so that e.g. <code class="docutils literal notranslate"><span class="pre">eval('09')</span></code> gives a syntax error because Python regards numbers starting with ‘0’ as octal (base 8).</p> </div> <div class="section" id="how-do-i-convert-a-number-to-a-string"> <h3><a class="toc-backref" href="#id30">How do I convert a number to a string?</a><a class="headerlink" href="#how-do-i-convert-a-number-to-a-string" title="Permalink to this headline">¶</a></h3> <p>To convert, e.g., the number 144 to the string ‘144’, use the built-in type constructor <a class="reference internal" href="../library/functions.html#str" title="str"><code class="xref py py-func docutils literal notranslate"><span class="pre">str()</span></code></a>. If you want a hexadecimal or octal representation, use the built-in functions <a class="reference internal" href="../library/functions.html#hex" title="hex"><code class="xref py py-func docutils literal notranslate"><span class="pre">hex()</span></code></a> or <a class="reference internal" href="../library/functions.html#oct" title="oct"><code class="xref py py-func docutils literal notranslate"><span class="pre">oct()</span></code></a>. For fancy formatting, see the <a class="reference internal" href="../library/string.html#formatstrings"><span class="std std-ref">Format String Syntax</span></a> section, e.g. <code class="docutils literal notranslate"><span class="pre">"{:04d}".format(144)</span></code> yields <code class="docutils literal notranslate"><span class="pre">'0144'</span></code> and <code class="docutils literal notranslate"><span class="pre">"{:.3f}".format(1.0/3.0)</span></code> yields <code class="docutils literal notranslate"><span class="pre">'0.333'</span></code>. In Python 2, the division (/) operator returns the floor of the mathematical result of division if the arguments are ints or longs, but it returns a reasonable approximation of the division result if the arguments are floats or complex:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="si">{:.3f}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="mi">1</span><span class="o">/</span><span class="mi">3</span><span class="p">))</span> <span class="go">0.000</span> <span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="si">{:.3f}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="mf">1.0</span><span class="o">/</span><span class="mi">3</span><span class="p">))</span> <span class="go">0.333</span> </pre></div> </div> <p>In Python 3, the default behaviour of the division operator (see <span class="target" id="index-2"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0238"><strong>PEP 238</strong></a>) has been changed but you can have the same behaviour in Python 2 if you import <code class="docutils literal notranslate"><span class="pre">division</span></code> from <a class="reference internal" href="../library/__future__.html#module-__future__" title="__future__: Future statement definitions"><code class="xref py py-mod docutils literal notranslate"><span class="pre">__future__</span></code></a>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">__future__</span> <span class="k">import</span> <span class="n">division</span> <span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="si">{:.3f}</span><span class="s1">'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="mi">1</span><span class="o">/</span><span class="mi">3</span><span class="p">))</span> <span class="go">0.333</span> </pre></div> </div> <p>You may also use <a class="reference internal" href="../library/stdtypes.html#string-formatting"><span class="std std-ref">the % operator</span></a> on strings. See the library reference manual for details.</p> </div> <div class="section" id="how-do-i-modify-a-string-in-place"> <h3><a class="toc-backref" href="#id31">How do I modify a string in place?</a><a class="headerlink" href="#how-do-i-modify-a-string-in-place" title="Permalink to this headline">¶</a></h3> <p>You can’t, because strings are immutable. If you need an object with this ability, try converting the string to a list or use the array module:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">io</span> <span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="s2">"Hello, world"</span> <span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="n">s</span><span class="p">)</span> <span class="gp">>>> </span><span class="nb">print</span> <span class="n">a</span> <span class="go">['H', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd']</span> <span class="gp">>>> </span><span class="n">a</span><span class="p">[</span><span class="mi">7</span><span class="p">:]</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="s2">"there!"</span><span class="p">)</span> <span class="gp">>>> </span><span class="s1">''</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">a</span><span class="p">)</span> <span class="go">'Hello, there!'</span> <span class="gp">>>> </span><span class="kn">import</span> <span class="nn">array</span> <span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="n">array</span><span class="o">.</span><span class="n">array</span><span class="p">(</span><span class="s1">'c'</span><span class="p">,</span> <span class="n">s</span><span class="p">)</span> <span class="gp">>>> </span><span class="nb">print</span> <span class="n">a</span> <span class="go">array('c', 'Hello, world')</span> <span class="gp">>>> </span><span class="n">a</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'y'</span><span class="p">;</span> <span class="nb">print</span> <span class="n">a</span> <span class="go">array('c', 'yello, world')</span> <span class="gp">>>> </span><span class="n">a</span><span class="o">.</span><span class="n">tostring</span><span class="p">()</span> <span class="go">'yello, world'</span> </pre></div> </div> </div> <div class="section" id="how-do-i-use-strings-to-call-functions-methods"> <h3><a class="toc-backref" href="#id32">How do I use strings to call functions/methods?</a><a class="headerlink" href="#how-do-i-use-strings-to-call-functions-methods" title="Permalink to this headline">¶</a></h3> <p>There are various techniques.</p> <ul> <li><p class="first">The best is to use a dictionary that maps strings to functions. The primary advantage of this technique is that the strings do not need to match the names of the functions. This is also the primary technique used to emulate a case construct:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">a</span><span class="p">():</span> <span class="k">pass</span> <span class="k">def</span> <span class="nf">b</span><span class="p">():</span> <span class="k">pass</span> <span class="n">dispatch</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'go'</span><span class="p">:</span> <span class="n">a</span><span class="p">,</span> <span class="s1">'stop'</span><span class="p">:</span> <span class="n">b</span><span class="p">}</span> <span class="c1"># Note lack of parens for funcs</span> <span class="n">dispatch</span><span class="p">[</span><span class="n">get_input</span><span class="p">()]()</span> <span class="c1"># Note trailing parens to call function</span> </pre></div> </div> </li> <li><p class="first">Use the built-in function <a class="reference internal" href="../library/functions.html#getattr" title="getattr"><code class="xref py py-func docutils literal notranslate"><span class="pre">getattr()</span></code></a>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">foo</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">foo</span><span class="p">,</span> <span class="s1">'bar'</span><span class="p">)()</span> </pre></div> </div> <p>Note that <a class="reference internal" href="../library/functions.html#getattr" title="getattr"><code class="xref py py-func docutils literal notranslate"><span class="pre">getattr()</span></code></a> works on any object, including classes, class instances, modules, and so on.</p> <p>This is used in several places in the standard library, like this:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Foo</span><span class="p">:</span> <span class="k">def</span> <span class="nf">do_foo</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="o">...</span> <span class="k">def</span> <span class="nf">do_bar</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="o">...</span> <span class="n">f</span> <span class="o">=</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">foo_instance</span><span class="p">,</span> <span class="s1">'do_'</span> <span class="o">+</span> <span class="n">opname</span><span class="p">)</span> <span class="n">f</span><span class="p">()</span> </pre></div> </div> </li> <li><p class="first">Use <a class="reference internal" href="../library/functions.html#locals" title="locals"><code class="xref py py-func docutils literal notranslate"><span class="pre">locals()</span></code></a> or <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> to resolve the function name:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">myFunc</span><span class="p">():</span> <span class="nb">print</span> <span class="s2">"hello"</span> <span class="n">fname</span> <span class="o">=</span> <span class="s2">"myFunc"</span> <span class="n">f</span> <span class="o">=</span> <span class="nb">locals</span><span class="p">()[</span><span class="n">fname</span><span class="p">]</span> <span class="n">f</span><span class="p">()</span> <span class="n">f</span> <span class="o">=</span> <span class="nb">eval</span><span class="p">(</span><span class="n">fname</span><span class="p">)</span> <span class="n">f</span><span class="p">()</span> </pre></div> </div> <p>Note: Using <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> is slow and dangerous. If you don’t have absolute control over the contents of the string, someone could pass a string that resulted in an arbitrary function being executed.</p> </li> </ul> </div> <div class="section" id="is-there-an-equivalent-to-perl-s-chomp-for-removing-trailing-newlines-from-strings"> <h3><a class="toc-backref" href="#id33">Is there an equivalent to Perl’s chomp() for removing trailing newlines from strings?</a><a class="headerlink" href="#is-there-an-equivalent-to-perl-s-chomp-for-removing-trailing-newlines-from-strings" title="Permalink to this headline">¶</a></h3> <p>Starting with Python 2.2, you can use <code class="docutils literal notranslate"><span class="pre">S.rstrip("\r\n")</span></code> to remove all occurrences of any line terminator from the end of the string <code class="docutils literal notranslate"><span class="pre">S</span></code> without removing other trailing whitespace. If the string <code class="docutils literal notranslate"><span class="pre">S</span></code> represents more than one line, with several empty lines at the end, the line terminators for all the blank lines will be removed:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">lines</span> <span class="o">=</span> <span class="p">(</span><span class="s2">"line 1 </span><span class="se">\r\n</span><span class="s2">"</span> <span class="gp">... </span> <span class="s2">"</span><span class="se">\r\n</span><span class="s2">"</span> <span class="gp">... </span> <span class="s2">"</span><span class="se">\r\n</span><span class="s2">"</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">lines</span><span class="o">.</span><span class="n">rstrip</span><span class="p">(</span><span class="s2">"</span><span class="se">\n\r</span><span class="s2">"</span><span class="p">)</span> <span class="go">'line 1 '</span> </pre></div> </div> <p>Since this is typically only desired when reading text one line at a time, using <code class="docutils literal notranslate"><span class="pre">S.rstrip()</span></code> this way works well.</p> <p>For older versions of Python, there are two partial substitutes:</p> <ul class="simple"> <li>If you want to remove all trailing whitespace, use the <code class="docutils literal notranslate"><span class="pre">rstrip()</span></code> method of string objects. This removes all trailing whitespace, not just a single newline.</li> <li>Otherwise, if there is only one line in the string <code class="docutils literal notranslate"><span class="pre">S</span></code>, use <code class="docutils literal notranslate"><span class="pre">S.splitlines()[0]</span></code>.</li> </ul> </div> <div class="section" id="is-there-a-scanf-or-sscanf-equivalent"> <h3><a class="toc-backref" href="#id34">Is there a scanf() or sscanf() equivalent?</a><a class="headerlink" href="#is-there-a-scanf-or-sscanf-equivalent" title="Permalink to this headline">¶</a></h3> <p>Not as such.</p> <p>For simple input parsing, the easiest approach is usually to split the line into whitespace-delimited words using the <a class="reference internal" href="../library/stdtypes.html#str.split" title="str.split"><code class="xref py py-meth docutils literal notranslate"><span class="pre">split()</span></code></a> method of string objects and then convert decimal strings to numeric values using <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate"><span class="pre">int()</span></code></a> or <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-func docutils literal notranslate"><span class="pre">float()</span></code></a>. <code class="docutils literal notranslate"><span class="pre">split()</span></code> supports an optional “sep” parameter which is useful if the line uses something other than whitespace as a separator.</p> <p>For more complicated input parsing, regular expressions are more powerful than C’s <code class="xref c c-func docutils literal notranslate"><span class="pre">sscanf()</span></code> and better suited for the task.</p> </div> <div class="section" id="what-does-unicodeerror-ascii-decoding-encoding-error-ordinal-not-in-range-128-mean"> <h3><a class="toc-backref" href="#id35">What does ‘UnicodeError: ASCII [decoding,encoding] error: ordinal not in range(128)’ mean?</a><a class="headerlink" href="#what-does-unicodeerror-ascii-decoding-encoding-error-ordinal-not-in-range-128-mean" title="Permalink to this headline">¶</a></h3> <p>This error indicates that your Python installation can handle only 7-bit ASCII strings. There are a couple ways to fix or work around the problem.</p> <p>If your programs must handle data in arbitrary character set encodings, the environment the application runs in will generally identify the encoding of the data it is handing you. You need to convert the input to Unicode data using that encoding. For example, a program that handles email or web input will typically find character set encoding information in Content-Type headers. This can then be used to properly convert input data to Unicode. Assuming the string referred to by <code class="docutils literal notranslate"><span class="pre">value</span></code> is encoded as UTF-8:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">value</span> <span class="o">=</span> <span class="n">unicode</span><span class="p">(</span><span class="n">value</span><span class="p">,</span> <span class="s2">"utf-8"</span><span class="p">)</span> </pre></div> </div> <p>will return a Unicode object. If the data is not correctly encoded as UTF-8, the above call will raise a <a class="reference internal" href="../library/exceptions.html#exceptions.UnicodeError" title="exceptions.UnicodeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">UnicodeError</span></code></a> exception.</p> <p>If you only want strings converted to Unicode which have non-ASCII data, you can try converting them first assuming an ASCII encoding, and then generate Unicode objects if that fails:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span> <span class="n">x</span> <span class="o">=</span> <span class="n">unicode</span><span class="p">(</span><span class="n">value</span><span class="p">,</span> <span class="s2">"ascii"</span><span class="p">)</span> <span class="k">except</span> <span class="ne">UnicodeError</span><span class="p">:</span> <span class="n">value</span> <span class="o">=</span> <span class="n">unicode</span><span class="p">(</span><span class="n">value</span><span class="p">,</span> <span class="s2">"utf-8"</span><span class="p">)</span> <span class="k">else</span><span class="p">:</span> <span class="c1"># value was valid ASCII data</span> <span class="k">pass</span> </pre></div> </div> <p>It’s possible to set a default encoding in a file called <code class="docutils literal notranslate"><span class="pre">sitecustomize.py</span></code> that’s part of the Python library. However, this isn’t recommended because changing the Python-wide default encoding may cause third-party extension modules to fail.</p> <p>Note that on Windows, there is an encoding known as “mbcs”, which uses an encoding specific to your current locale. In many cases, and particularly when working with COM, this may be an appropriate default encoding to use.</p> </div> </div> <div class="section" id="sequences-tuples-lists"> <h2><a class="toc-backref" href="#id36">Sequences (Tuples/Lists)</a><a class="headerlink" href="#sequences-tuples-lists" title="Permalink to this headline">¶</a></h2> <div class="section" id="how-do-i-convert-between-tuples-and-lists"> <h3><a class="toc-backref" href="#id37">How do I convert between tuples and lists?</a><a class="headerlink" href="#how-do-i-convert-between-tuples-and-lists" title="Permalink to this headline">¶</a></h3> <p>The type constructor <code class="docutils literal notranslate"><span class="pre">tuple(seq)</span></code> converts any sequence (actually, any iterable) into a tuple with the same items in the same order.</p> <p>For example, <code class="docutils literal notranslate"><span class="pre">tuple([1,</span> <span class="pre">2,</span> <span class="pre">3])</span></code> yields <code class="docutils literal notranslate"><span class="pre">(1,</span> <span class="pre">2,</span> <span class="pre">3)</span></code> and <code class="docutils literal notranslate"><span class="pre">tuple('abc')</span></code> yields <code class="docutils literal notranslate"><span class="pre">('a',</span> <span class="pre">'b',</span> <span class="pre">'c')</span></code>. If the argument is a tuple, it does not make a copy but returns the same object, so it is cheap to call <a class="reference internal" href="../library/functions.html#tuple" title="tuple"><code class="xref py py-func docutils literal notranslate"><span class="pre">tuple()</span></code></a> when you aren’t sure that an object is already a tuple.</p> <p>The type constructor <code class="docutils literal notranslate"><span class="pre">list(seq)</span></code> converts any sequence or iterable into a list with the same items in the same order. For example, <code class="docutils literal notranslate"><span class="pre">list((1,</span> <span class="pre">2,</span> <span class="pre">3))</span></code> yields <code class="docutils literal notranslate"><span class="pre">[1,</span> <span class="pre">2,</span> <span class="pre">3]</span></code> and <code class="docutils literal notranslate"><span class="pre">list('abc')</span></code> yields <code class="docutils literal notranslate"><span class="pre">['a',</span> <span class="pre">'b',</span> <span class="pre">'c']</span></code>. If the argument is a list, it makes a copy just like <code class="docutils literal notranslate"><span class="pre">seq[:]</span></code> would.</p> </div> <div class="section" id="what-s-a-negative-index"> <h3><a class="toc-backref" href="#id38">What’s a negative index?</a><a class="headerlink" href="#what-s-a-negative-index" title="Permalink to this headline">¶</a></h3> <p>Python sequences are indexed with positive numbers and negative numbers. For positive numbers 0 is the first index 1 is the second index and so forth. For negative indices -1 is the last index and -2 is the penultimate (next to last) index and so forth. Think of <code class="docutils literal notranslate"><span class="pre">seq[-n]</span></code> as the same as <code class="docutils literal notranslate"><span class="pre">seq[len(seq)-n]</span></code>.</p> <p>Using negative indices can be very convenient. For example <code class="docutils literal notranslate"><span class="pre">S[:-1]</span></code> is all of the string except for its last character, which is useful for removing the trailing newline from a string.</p> </div> <div class="section" id="how-do-i-iterate-over-a-sequence-in-reverse-order"> <h3><a class="toc-backref" href="#id39">How do I iterate over a sequence in reverse order?</a><a class="headerlink" href="#how-do-i-iterate-over-a-sequence-in-reverse-order" title="Permalink to this headline">¶</a></h3> <p>Use the <a class="reference internal" href="../library/functions.html#reversed" title="reversed"><code class="xref py py-func docutils literal notranslate"><span class="pre">reversed()</span></code></a> built-in function, which is new in Python 2.4:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">reversed</span><span class="p">(</span><span class="n">sequence</span><span class="p">):</span> <span class="o">...</span> <span class="c1"># do something with x ...</span> </pre></div> </div> <p>This won’t touch your original sequence, but build a new copy with reversed order to iterate over.</p> <p>With Python 2.3, you can use an extended slice syntax:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">sequence</span><span class="p">[::</span><span class="o">-</span><span class="mi">1</span><span class="p">]:</span> <span class="o">...</span> <span class="c1"># do something with x ...</span> </pre></div> </div> </div> <div class="section" id="how-do-you-remove-duplicates-from-a-list"> <h3><a class="toc-backref" href="#id40">How do you remove duplicates from a list?</a><a class="headerlink" href="#how-do-you-remove-duplicates-from-a-list" title="Permalink to this headline">¶</a></h3> <p>See the Python Cookbook for a long discussion of many ways to do this:</p> <blockquote> <div><a class="reference external" href="https://code.activestate.com/recipes/52560/">https://code.activestate.com/recipes/52560/</a></div></blockquote> <p>If you don’t mind reordering the list, sort it and then scan from the end of the list, deleting duplicates as you go:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">mylist</span><span class="p">:</span> <span class="n">mylist</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span> <span class="n">last</span> <span class="o">=</span> <span class="n">mylist</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">mylist</span><span class="p">)</span><span class="o">-</span><span class="mi">2</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">):</span> <span class="k">if</span> <span class="n">last</span> <span class="o">==</span> <span class="n">mylist</span><span class="p">[</span><span class="n">i</span><span class="p">]:</span> <span class="k">del</span> <span class="n">mylist</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="k">else</span><span class="p">:</span> <span class="n">last</span> <span class="o">=</span> <span class="n">mylist</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> </pre></div> </div> <p>If all elements of the list may be used as dictionary keys (i.e. they are all hashable) this is often faster</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">d</span> <span class="o">=</span> <span class="p">{}</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">mylist</span><span class="p">:</span> <span class="n">d</span><span class="p">[</span><span class="n">x</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span> <span class="n">mylist</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="n">d</span><span class="o">.</span><span class="n">keys</span><span class="p">())</span> </pre></div> </div> <p>In Python 2.5 and later, the following is possible instead:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">mylist</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="nb">set</span><span class="p">(</span><span class="n">mylist</span><span class="p">))</span> </pre></div> </div> <p>This converts the list into a set, thereby removing duplicates, and then back into a list.</p> </div> <div class="section" id="how-do-you-make-an-array-in-python"> <h3><a class="toc-backref" href="#id41">How do you make an array in Python?</a><a class="headerlink" href="#how-do-you-make-an-array-in-python" title="Permalink to this headline">¶</a></h3> <p>Use a list:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="s2">"this"</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="s2">"is"</span><span class="p">,</span> <span class="s2">"an"</span><span class="p">,</span> <span class="s2">"array"</span><span class="p">]</span> </pre></div> </div> <p>Lists are equivalent to C or Pascal arrays in their time complexity; the primary difference is that a Python list can contain objects of many different types.</p> <p>The <code class="docutils literal notranslate"><span class="pre">array</span></code> module also provides methods for creating arrays of fixed types with compact representations, but they are slower to index than lists. Also note that the Numeric extensions and others define array-like structures with various characteristics as well.</p> <p>To get Lisp-style linked lists, you can emulate cons cells using tuples:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">lisp_list</span> <span class="o">=</span> <span class="p">(</span><span class="s2">"like"</span><span class="p">,</span> <span class="p">(</span><span class="s2">"this"</span><span class="p">,</span> <span class="p">(</span><span class="s2">"example"</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span> <span class="p">)</span> <span class="p">)</span> </pre></div> </div> <p>If mutability is desired, you could use lists instead of tuples. Here the analogue of lisp car is <code class="docutils literal notranslate"><span class="pre">lisp_list[0]</span></code> and the analogue of cdr is <code class="docutils literal notranslate"><span class="pre">lisp_list[1]</span></code>. Only do this if you’re sure you really need to, because it’s usually a lot slower than using Python lists.</p> </div> <div class="section" id="how-do-i-create-a-multidimensional-list"> <span id="faq-multidimensional-list"></span><h3><a class="toc-backref" href="#id42">How do I create a multidimensional list?</a><a class="headerlink" href="#how-do-i-create-a-multidimensional-list" title="Permalink to this headline">¶</a></h3> <p>You probably tried to make a multidimensional array like this:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">A</span> <span class="o">=</span> <span class="p">[[</span><span class="kc">None</span><span class="p">]</span> <span class="o">*</span> <span class="mi">2</span><span class="p">]</span> <span class="o">*</span> <span class="mi">3</span> </pre></div> </div> <p>This looks correct if you print it:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">A</span> <span class="go">[[None, None], [None, None], [None, None]]</span> </pre></div> </div> <p>But when you assign a value, it shows up in multiple places:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">A</span><span class="p">[</span><span class="mi">0</span><span class="p">][</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">5</span> <span class="gp">>>> </span><span class="n">A</span> <span class="go">[[5, None], [5, None], [5, None]]</span> </pre></div> </div> <p>The reason is that replicating a list with <code class="docutils literal notranslate"><span class="pre">*</span></code> doesn’t create copies, it only creates references to the existing objects. The <code class="docutils literal notranslate"><span class="pre">*3</span></code> creates a list containing 3 references to the same list of length two. Changes to one row will show in all rows, which is almost certainly not what you want.</p> <p>The suggested approach is to create a list of the desired length first and then fill in each element with a newly created list:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">A</span> <span class="o">=</span> <span class="p">[</span><span class="kc">None</span><span class="p">]</span> <span class="o">*</span> <span class="mi">3</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span> <span class="n">A</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">=</span> <span class="p">[</span><span class="kc">None</span><span class="p">]</span> <span class="o">*</span> <span class="mi">2</span> </pre></div> </div> <p>This generates a list containing 3 different lists of length two. You can also use a list comprehension:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">w</span><span class="p">,</span> <span class="n">h</span> <span class="o">=</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span> <span class="n">A</span> <span class="o">=</span> <span class="p">[[</span><span class="kc">None</span><span class="p">]</span> <span class="o">*</span> <span class="n">w</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">h</span><span class="p">)]</span> </pre></div> </div> <p>Or, you can use an extension that provides a matrix datatype; <a class="reference external" href="http://www.numpy.org/">NumPy</a> is the best known.</p> </div> <div class="section" id="how-do-i-apply-a-method-to-a-sequence-of-objects"> <h3><a class="toc-backref" href="#id43">How do I apply a method to a sequence of objects?</a><a class="headerlink" href="#how-do-i-apply-a-method-to-a-sequence-of-objects" title="Permalink to this headline">¶</a></h3> <p>Use a list comprehension:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">result</span> <span class="o">=</span> <span class="p">[</span><span class="n">obj</span><span class="o">.</span><span class="n">method</span><span class="p">()</span> <span class="k">for</span> <span class="n">obj</span> <span class="ow">in</span> <span class="n">mylist</span><span class="p">]</span> </pre></div> </div> <p>More generically, you can try the following function:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">method_map</span><span class="p">(</span><span class="n">objects</span><span class="p">,</span> <span class="n">method</span><span class="p">,</span> <span class="n">arguments</span><span class="p">):</span> <span class="sd">"""method_map([a,b], "meth", (1,2)) gives [a.meth(1,2), b.meth(1,2)]"""</span> <span class="n">nobjects</span> <span class="o">=</span> <span class="nb">len</span><span class="p">(</span><span class="n">objects</span><span class="p">)</span> <span class="n">methods</span> <span class="o">=</span> <span class="nb">map</span><span class="p">(</span><span class="nb">getattr</span><span class="p">,</span> <span class="n">objects</span><span class="p">,</span> <span class="p">[</span><span class="n">method</span><span class="p">]</span><span class="o">*</span><span class="n">nobjects</span><span class="p">)</span> <span class="k">return</span> <span class="nb">map</span><span class="p">(</span><span class="n">apply</span><span class="p">,</span> <span class="n">methods</span><span class="p">,</span> <span class="p">[</span><span class="n">arguments</span><span class="p">]</span><span class="o">*</span><span class="n">nobjects</span><span class="p">)</span> </pre></div> </div> </div> <div class="section" id="why-does-a-tuple-i-item-raise-an-exception-when-the-addition-works"> <h3><a class="toc-backref" href="#id44">Why does a_tuple[i] += [‘item’] raise an exception when the addition works?</a><a class="headerlink" href="#why-does-a-tuple-i-item-raise-an-exception-when-the-addition-works" title="Permalink to this headline">¶</a></h3> <p>This is because of a combination of the fact that augmented assignment operators are <em>assignment</em> operators, and the difference between mutable and immutable objects in Python.</p> <p>This discussion applies in general when augmented assignment operators are applied to elements of a tuple that point to mutable objects, but we’ll use a <code class="docutils literal notranslate"><span class="pre">list</span></code> and <code class="docutils literal notranslate"><span class="pre">+=</span></code> as our exemplar.</p> <p>If you wrote:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a_tuple</span> <span class="o">=</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">a_tuple</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">+=</span> <span class="mi">1</span> <span class="gt">Traceback (most recent call last):</span> <span class="c">...</span> <span class="gr">TypeError</span>: <span class="n">'tuple' object does not support item assignment</span> </pre></div> </div> <p>The reason for the exception should be immediately clear: <code class="docutils literal notranslate"><span class="pre">1</span></code> is added to the object <code class="docutils literal notranslate"><span class="pre">a_tuple[0]</span></code> points to (<code class="docutils literal notranslate"><span class="pre">1</span></code>), producing the result object, <code class="docutils literal notranslate"><span class="pre">2</span></code>, but when we attempt to assign the result of the computation, <code class="docutils literal notranslate"><span class="pre">2</span></code>, to element <code class="docutils literal notranslate"><span class="pre">0</span></code> of the tuple, we get an error because we can’t change what an element of a tuple points to.</p> <p>Under the covers, what this augmented assignment statement is doing is approximately this:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">result</span> <span class="o">=</span> <span class="n">a_tuple</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">+</span> <span class="mi">1</span> <span class="gp">>>> </span><span class="n">a_tuple</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="n">result</span> <span class="gt">Traceback (most recent call last):</span> <span class="c">...</span> <span class="gr">TypeError</span>: <span class="n">'tuple' object does not support item assignment</span> </pre></div> </div> <p>It is the assignment part of the operation that produces the error, since a tuple is immutable.</p> <p>When you write something like:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a_tuple</span> <span class="o">=</span> <span class="p">([</span><span class="s1">'foo'</span><span class="p">],</span> <span class="s1">'bar'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">a_tuple</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">+=</span> <span class="p">[</span><span class="s1">'item'</span><span class="p">]</span> <span class="gt">Traceback (most recent call last):</span> <span class="c">...</span> <span class="gr">TypeError</span>: <span class="n">'tuple' object does not support item assignment</span> </pre></div> </div> <p>The exception is a bit more surprising, and even more surprising is the fact that even though there was an error, the append worked:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a_tuple</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="go">['foo', 'item']</span> </pre></div> </div> <p>To see why this happens, you need to know that (a) if an object implements an <code class="docutils literal notranslate"><span class="pre">__iadd__</span></code> magic method, it gets called when the <code class="docutils literal notranslate"><span class="pre">+=</span></code> augmented assignment is executed, and its return value is what gets used in the assignment statement; and (b) for lists, <code class="docutils literal notranslate"><span class="pre">__iadd__</span></code> is equivalent to calling <code class="docutils literal notranslate"><span class="pre">extend</span></code> on the list and returning the list. That’s why we say that for lists, <code class="docutils literal notranslate"><span class="pre">+=</span></code> is a “shorthand” for <code class="docutils literal notranslate"><span class="pre">list.extend</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a_list</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">>>> </span><span class="n">a_list</span> <span class="o">+=</span> <span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">a_list</span> <span class="go">[1]</span> </pre></div> </div> <p>This is equivalent to:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">result</span> <span class="o">=</span> <span class="n">a_list</span><span class="o">.</span><span class="fm">__iadd__</span><span class="p">([</span><span class="mi">1</span><span class="p">])</span> <span class="gp">>>> </span><span class="n">a_list</span> <span class="o">=</span> <span class="n">result</span> </pre></div> </div> <p>The object pointed to by a_list has been mutated, and the pointer to the mutated object is assigned back to <code class="docutils literal notranslate"><span class="pre">a_list</span></code>. The end result of the assignment is a no-op, since it is a pointer to the same object that <code class="docutils literal notranslate"><span class="pre">a_list</span></code> was previously pointing to, but the assignment still happens.</p> <p>Thus, in our tuple example what is happening is equivalent to:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">result</span> <span class="o">=</span> <span class="n">a_tuple</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="fm">__iadd__</span><span class="p">([</span><span class="s1">'item'</span><span class="p">])</span> <span class="gp">>>> </span><span class="n">a_tuple</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="n">result</span> <span class="gt">Traceback (most recent call last):</span> <span class="c">...</span> <span class="gr">TypeError</span>: <span class="n">'tuple' object does not support item assignment</span> </pre></div> </div> <p>The <code class="docutils literal notranslate"><span class="pre">__iadd__</span></code> succeeds, and thus the list is extended, but even though <code class="docutils literal notranslate"><span class="pre">result</span></code> points to the same object that <code class="docutils literal notranslate"><span class="pre">a_tuple[0]</span></code> already points to, that final assignment still results in an error, because tuples are immutable.</p> </div> </div> <div class="section" id="dictionaries"> <h2><a class="toc-backref" href="#id45">Dictionaries</a><a class="headerlink" href="#dictionaries" title="Permalink to this headline">¶</a></h2> <div class="section" id="how-can-i-get-a-dictionary-to-display-its-keys-in-a-consistent-order"> <h3><a class="toc-backref" href="#id46">How can I get a dictionary to display its keys in a consistent order?</a><a class="headerlink" href="#how-can-i-get-a-dictionary-to-display-its-keys-in-a-consistent-order" title="Permalink to this headline">¶</a></h3> <p>You can’t. Dictionaries store their keys in an unpredictable order, so the display order of a dictionary’s elements will be similarly unpredictable.</p> <p>This can be frustrating if you want to save a printable version to a file, make some changes and then compare it with some other printed dictionary. In this case, use the <code class="docutils literal notranslate"><span class="pre">pprint</span></code> module to pretty-print the dictionary; the items will be presented in order sorted by the key.</p> <p>A more complicated solution is to subclass <code class="docutils literal notranslate"><span class="pre">dict</span></code> to create a <code class="docutils literal notranslate"><span class="pre">SortedDict</span></code> class that prints itself in a predictable order. Here’s one simpleminded implementation of such a class:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">SortedDict</span><span class="p">(</span><span class="nb">dict</span><span class="p">):</span> <span class="k">def</span> <span class="nf">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="n">keys</span> <span class="o">=</span> <span class="nb">sorted</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">keys</span><span class="p">())</span> <span class="n">result</span> <span class="o">=</span> <span class="p">(</span><span class="s2">"</span><span class="si">{!r}</span><span class="s2">: </span><span class="si">{!r}</span><span class="s2">"</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">k</span><span class="p">,</span> <span class="bp">self</span><span class="p">[</span><span class="n">k</span><span class="p">])</span> <span class="k">for</span> <span class="n">k</span> <span class="ow">in</span> <span class="n">keys</span><span class="p">)</span> <span class="k">return</span> <span class="s2">"{{</span><span class="si">{}</span><span class="s2">}}"</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="s2">", "</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">result</span><span class="p">))</span> <span class="fm">__str__</span> <span class="o">=</span> <span class="fm">__repr__</span> </pre></div> </div> <p>This will work for many common situations you might encounter, though it’s far from a perfect solution. The largest flaw is that if some values in the dictionary are also dictionaries, their values won’t be presented in any particular order.</p> </div> <div class="section" id="i-want-to-do-a-complicated-sort-can-you-do-a-schwartzian-transform-in-python"> <h3><a class="toc-backref" href="#id47">I want to do a complicated sort: can you do a Schwartzian Transform in Python?</a><a class="headerlink" href="#i-want-to-do-a-complicated-sort-can-you-do-a-schwartzian-transform-in-python" title="Permalink to this headline">¶</a></h3> <p>The technique, attributed to Randal Schwartz of the Perl community, sorts the elements of a list by a metric which maps each element to its “sort value”. In Python, use the <code class="docutils literal notranslate"><span class="pre">key</span></code> argument for the <code class="xref py py-func docutils literal notranslate"><span class="pre">sort()</span></code> function:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">Isorted</span> <span class="o">=</span> <span class="n">L</span><span class="p">[:]</span> <span class="n">Isorted</span><span class="o">.</span><span class="n">sort</span><span class="p">(</span><span class="n">key</span><span class="o">=</span><span class="k">lambda</span> <span class="n">s</span><span class="p">:</span> <span class="nb">int</span><span class="p">(</span><span class="n">s</span><span class="p">[</span><span class="mi">10</span><span class="p">:</span><span class="mi">15</span><span class="p">]))</span> </pre></div> </div> </div> <div class="section" id="how-can-i-sort-one-list-by-values-from-another-list"> <h3><a class="toc-backref" href="#id48">How can I sort one list by values from another list?</a><a class="headerlink" href="#how-can-i-sort-one-list-by-values-from-another-list" title="Permalink to this headline">¶</a></h3> <p>Merge them into a single list of tuples, sort the resulting list, and then pick out the element you want.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">list1</span> <span class="o">=</span> <span class="p">[</span><span class="s2">"what"</span><span class="p">,</span> <span class="s2">"I'm"</span><span class="p">,</span> <span class="s2">"sorting"</span><span class="p">,</span> <span class="s2">"by"</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">list2</span> <span class="o">=</span> <span class="p">[</span><span class="s2">"something"</span><span class="p">,</span> <span class="s2">"else"</span><span class="p">,</span> <span class="s2">"to"</span><span class="p">,</span> <span class="s2">"sort"</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">pairs</span> <span class="o">=</span> <span class="nb">zip</span><span class="p">(</span><span class="n">list1</span><span class="p">,</span> <span class="n">list2</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">pairs</span> <span class="go">[('what', 'something'), ("I'm", 'else'), ('sorting', 'to'), ('by', 'sort')]</span> <span class="gp">>>> </span><span class="n">pairs</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">result</span> <span class="o">=</span> <span class="p">[</span> <span class="n">x</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">pairs</span> <span class="p">]</span> <span class="gp">>>> </span><span class="n">result</span> <span class="go">['else', 'sort', 'to', 'something']</span> </pre></div> </div> <p>An alternative for the last step is:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">result</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">>>> </span><span class="k">for</span> <span class="n">p</span> <span class="ow">in</span> <span class="n">pairs</span><span class="p">:</span> <span class="n">result</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">p</span><span class="p">[</span><span class="mi">1</span><span class="p">])</span> </pre></div> </div> <p>If you find this more legible, you might prefer to use this instead of the final list comprehension. However, it is almost twice as slow for long lists. Why? First, the <code class="docutils literal notranslate"><span class="pre">append()</span></code> operation has to reallocate memory, and while it uses some tricks to avoid doing that each time, it still has to do it occasionally, and that costs quite a bit. Second, the expression “result.append” requires an extra attribute lookup, and third, there’s a speed reduction from having to make all those function calls.</p> </div> </div> <div class="section" id="objects"> <h2><a class="toc-backref" href="#id49">Objects</a><a class="headerlink" href="#objects" title="Permalink to this headline">¶</a></h2> <div class="section" id="what-is-a-class"> <h3><a class="toc-backref" href="#id50">What is a class?</a><a class="headerlink" href="#what-is-a-class" title="Permalink to this headline">¶</a></h3> <p>A class is the particular object type created by executing a class statement. Class objects are used as templates to create instance objects, which embody both the data (attributes) and code (methods) specific to a datatype.</p> <p>A class can be based on one or more other classes, called its base class(es). It then inherits the attributes and methods of its base classes. This allows an object model to be successively refined by inheritance. You might have a generic <code class="docutils literal notranslate"><span class="pre">Mailbox</span></code> class that provides basic accessor methods for a mailbox, and subclasses such as <code class="docutils literal notranslate"><span class="pre">MboxMailbox</span></code>, <code class="docutils literal notranslate"><span class="pre">MaildirMailbox</span></code>, <code class="docutils literal notranslate"><span class="pre">OutlookMailbox</span></code> that handle various specific mailbox formats.</p> </div> <div class="section" id="what-is-a-method"> <h3><a class="toc-backref" href="#id51">What is a method?</a><a class="headerlink" href="#what-is-a-method" title="Permalink to this headline">¶</a></h3> <p>A method is a function on some object <code class="docutils literal notranslate"><span class="pre">x</span></code> that you normally call as <code class="docutils literal notranslate"><span class="pre">x.name(arguments...)</span></code>. Methods are defined as functions inside the class definition:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span> <span class="k">def</span> <span class="nf">meth</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">arg</span><span class="p">):</span> <span class="k">return</span> <span class="n">arg</span> <span class="o">*</span> <span class="mi">2</span> <span class="o">+</span> <span class="bp">self</span><span class="o">.</span><span class="n">attribute</span> </pre></div> </div> </div> <div class="section" id="what-is-self"> <h3><a class="toc-backref" href="#id52">What is self?</a><a class="headerlink" href="#what-is-self" title="Permalink to this headline">¶</a></h3> <p>Self is merely a conventional name for the first argument of a method. A method defined as <code class="docutils literal notranslate"><span class="pre">meth(self,</span> <span class="pre">a,</span> <span class="pre">b,</span> <span class="pre">c)</span></code> should be called as <code class="docutils literal notranslate"><span class="pre">x.meth(a,</span> <span class="pre">b,</span> <span class="pre">c)</span></code> for some instance <code class="docutils literal notranslate"><span class="pre">x</span></code> of the class in which the definition occurs; the called method will think it is called as <code class="docutils literal notranslate"><span class="pre">meth(x,</span> <span class="pre">a,</span> <span class="pre">b,</span> <span class="pre">c)</span></code>.</p> <p>See also <a class="reference internal" href="design.html#why-self"><span class="std std-ref">Why must ‘self’ be used explicitly in method definitions and calls?</span></a>.</p> </div> <div class="section" id="how-do-i-check-if-an-object-is-an-instance-of-a-given-class-or-of-a-subclass-of-it"> <h3><a class="toc-backref" href="#id53">How do I check if an object is an instance of a given class or of a subclass of it?</a><a class="headerlink" href="#how-do-i-check-if-an-object-is-an-instance-of-a-given-class-or-of-a-subclass-of-it" title="Permalink to this headline">¶</a></h3> <p>Use the built-in function <code class="docutils literal notranslate"><span class="pre">isinstance(obj,</span> <span class="pre">cls)</span></code>. You can check if an object is an instance of any of a number of classes by providing a tuple instead of a single class, e.g. <code class="docutils literal notranslate"><span class="pre">isinstance(obj,</span> <span class="pre">(class1,</span> <span class="pre">class2,</span> <span class="pre">...))</span></code>, and can also check whether an object is one of Python’s built-in types, e.g. <code class="docutils literal notranslate"><span class="pre">isinstance(obj,</span> <span class="pre">str)</span></code> or <code class="docutils literal notranslate"><span class="pre">isinstance(obj,</span> <span class="pre">(int,</span> <span class="pre">long,</span> <span class="pre">float,</span> <span class="pre">complex))</span></code>.</p> <p>Note that most programs do not use <a class="reference internal" href="../library/functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal notranslate"><span class="pre">isinstance()</span></code></a> on user-defined classes very often. If you are developing the classes yourself, a more proper object-oriented style is to define methods on the classes that encapsulate a particular behaviour, instead of checking the object’s class and doing a different thing based on what class it is. For example, if you have a function that does something:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">search</span><span class="p">(</span><span class="n">obj</span><span class="p">):</span> <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="n">Mailbox</span><span class="p">):</span> <span class="o">...</span> <span class="c1"># code to search a mailbox</span> <span class="k">elif</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="n">Document</span><span class="p">):</span> <span class="o">...</span> <span class="c1"># code to search a document</span> <span class="k">elif</span> <span class="o">...</span> </pre></div> </div> <p>A better approach is to define a <code class="docutils literal notranslate"><span class="pre">search()</span></code> method on all the classes and just call it:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Mailbox</span><span class="p">:</span> <span class="k">def</span> <span class="nf">search</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="o">...</span> <span class="c1"># code to search a mailbox</span> <span class="k">class</span> <span class="nc">Document</span><span class="p">:</span> <span class="k">def</span> <span class="nf">search</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="o">...</span> <span class="c1"># code to search a document</span> <span class="n">obj</span><span class="o">.</span><span class="n">search</span><span class="p">()</span> </pre></div> </div> </div> <div class="section" id="what-is-delegation"> <h3><a class="toc-backref" href="#id54">What is delegation?</a><a class="headerlink" href="#what-is-delegation" title="Permalink to this headline">¶</a></h3> <p>Delegation is an object oriented technique (also called a design pattern). Let’s say you have an object <code class="docutils literal notranslate"><span class="pre">x</span></code> and want to change the behaviour of just one of its methods. You can create a new class that provides a new implementation of the method you’re interested in changing and delegates all other methods to the corresponding method of <code class="docutils literal notranslate"><span class="pre">x</span></code>.</p> <p>Python programmers can easily implement delegation. For example, the following class implements a class that behaves like a file but converts all written data to uppercase:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">UpperOut</span><span class="p">:</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">outfile</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">_outfile</span> <span class="o">=</span> <span class="n">outfile</span> <span class="k">def</span> <span class="nf">write</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">s</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">_outfile</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">s</span><span class="o">.</span><span class="n">upper</span><span class="p">())</span> <span class="k">def</span> <span class="nf">__getattr__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span> <span class="k">return</span> <span class="nb">getattr</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">_outfile</span><span class="p">,</span> <span class="n">name</span><span class="p">)</span> </pre></div> </div> <p>Here the <code class="docutils literal notranslate"><span class="pre">UpperOut</span></code> class redefines the <code class="docutils literal notranslate"><span class="pre">write()</span></code> method to convert the argument string to uppercase before calling the underlying <code class="docutils literal notranslate"><span class="pre">self.__outfile.write()</span></code> method. All other methods are delegated to the underlying <code class="docutils literal notranslate"><span class="pre">self.__outfile</span></code> object. The delegation is accomplished via the <code class="docutils literal notranslate"><span class="pre">__getattr__</span></code> method; consult <a class="reference internal" href="../reference/datamodel.html#attribute-access"><span class="std std-ref">the language reference</span></a> for more information about controlling attribute access.</p> <p>Note that for more general cases delegation can get trickier. When attributes must be set as well as retrieved, the class must define a <a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> method too, and it must do so carefully. The basic implementation of <a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> is roughly equivalent to the following:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">X</span><span class="p">:</span> <span class="o">...</span> <span class="k">def</span> <span class="nf">__setattr__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="vm">__dict__</span><span class="p">[</span><span class="n">name</span><span class="p">]</span> <span class="o">=</span> <span class="n">value</span> <span class="o">...</span> </pre></div> </div> <p>Most <a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__setattr__()</span></code></a> implementations must modify <code class="docutils literal notranslate"><span class="pre">self.__dict__</span></code> to store local state for self without causing an infinite recursion.</p> </div> <div class="section" id="how-do-i-call-a-method-defined-in-a-base-class-from-a-derived-class-that-overrides-it"> <h3><a class="toc-backref" href="#id55">How do I call a method defined in a base class from a derived class that overrides it?</a><a class="headerlink" href="#how-do-i-call-a-method-defined-in-a-base-class-from-a-derived-class-that-overrides-it" title="Permalink to this headline">¶</a></h3> <p>If you’re using new-style classes, use the built-in <a class="reference internal" href="../library/functions.html#super" title="super"><code class="xref py py-func docutils literal notranslate"><span class="pre">super()</span></code></a> function:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Derived</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span> <span class="k">def</span> <span class="nf">meth</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="nb">super</span><span class="p">(</span><span class="n">Derived</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">meth</span><span class="p">()</span> </pre></div> </div> <p>If you’re using classic classes: For a class definition such as <code class="docutils literal notranslate"><span class="pre">class</span> <span class="pre">Derived(Base):</span> <span class="pre">...</span></code> you can call method <code class="docutils literal notranslate"><span class="pre">meth()</span></code> defined in <code class="docutils literal notranslate"><span class="pre">Base</span></code> (or one of <code class="docutils literal notranslate"><span class="pre">Base</span></code>’s base classes) as <code class="docutils literal notranslate"><span class="pre">Base.meth(self,</span> <span class="pre">arguments...)</span></code>. Here, <code class="docutils literal notranslate"><span class="pre">Base.meth</span></code> is an unbound method, so you need to provide the <code class="docutils literal notranslate"><span class="pre">self</span></code> argument.</p> </div> <div class="section" id="how-can-i-organize-my-code-to-make-it-easier-to-change-the-base-class"> <h3><a class="toc-backref" href="#id56">How can I organize my code to make it easier to change the base class?</a><a class="headerlink" href="#how-can-i-organize-my-code-to-make-it-easier-to-change-the-base-class" title="Permalink to this headline">¶</a></h3> <p>You could define an alias for the base class, assign the real base class to it before your class definition, and use the alias throughout your class. Then all you have to change is the value assigned to the alias. Incidentally, this trick is also handy if you want to decide dynamically (e.g. depending on availability of resources) which base class to use. Example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">BaseAlias</span> <span class="o">=</span> <span class="o"><</span><span class="n">real</span> <span class="n">base</span> <span class="n">class</span><span class="o">></span> <span class="k">class</span> <span class="nc">Derived</span><span class="p">(</span><span class="n">BaseAlias</span><span class="p">):</span> <span class="k">def</span> <span class="nf">meth</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="n">BaseAlias</span><span class="o">.</span><span class="n">meth</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span> <span class="o">...</span> </pre></div> </div> </div> <div class="section" id="how-do-i-create-static-class-data-and-static-class-methods"> <h3><a class="toc-backref" href="#id57">How do I create static class data and static class methods?</a><a class="headerlink" href="#how-do-i-create-static-class-data-and-static-class-methods" title="Permalink to this headline">¶</a></h3> <p>Both static data and static methods (in the sense of C++ or Java) are supported in Python.</p> <p>For static data, simply define a class attribute. To assign a new value to the attribute, you have to explicitly use the class name in the assignment:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span> <span class="n">count</span> <span class="o">=</span> <span class="mi">0</span> <span class="c1"># number of times C.__init__ called</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="n">C</span><span class="o">.</span><span class="n">count</span> <span class="o">=</span> <span class="n">C</span><span class="o">.</span><span class="n">count</span> <span class="o">+</span> <span class="mi">1</span> <span class="k">def</span> <span class="nf">getcount</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="k">return</span> <span class="n">C</span><span class="o">.</span><span class="n">count</span> <span class="c1"># or return self.count</span> </pre></div> </div> <p><code class="docutils literal notranslate"><span class="pre">c.count</span></code> also refers to <code class="docutils literal notranslate"><span class="pre">C.count</span></code> for any <code class="docutils literal notranslate"><span class="pre">c</span></code> such that <code class="docutils literal notranslate"><span class="pre">isinstance(c,</span> <span class="pre">C)</span></code> holds, unless overridden by <code class="docutils literal notranslate"><span class="pre">c</span></code> itself or by some class on the base-class search path from <code class="docutils literal notranslate"><span class="pre">c.__class__</span></code> back to <code class="docutils literal notranslate"><span class="pre">C</span></code>.</p> <p>Caution: within a method of C, an assignment like <code class="docutils literal notranslate"><span class="pre">self.count</span> <span class="pre">=</span> <span class="pre">42</span></code> creates a new and unrelated instance named “count” in <code class="docutils literal notranslate"><span class="pre">self</span></code>’s own dict. Rebinding of a class-static data name must always specify the class whether inside a method or not:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">C</span><span class="o">.</span><span class="n">count</span> <span class="o">=</span> <span class="mi">314</span> </pre></div> </div> <p>Static methods are possible since Python 2.2:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span> <span class="k">def</span> <span class="nf">static</span><span class="p">(</span><span class="n">arg1</span><span class="p">,</span> <span class="n">arg2</span><span class="p">,</span> <span class="n">arg3</span><span class="p">):</span> <span class="c1"># No 'self' parameter!</span> <span class="o">...</span> <span class="n">static</span> <span class="o">=</span> <span class="nb">staticmethod</span><span class="p">(</span><span class="n">static</span><span class="p">)</span> </pre></div> </div> <p>With Python 2.4’s decorators, this can also be written as</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span> <span class="nd">@staticmethod</span> <span class="k">def</span> <span class="nf">static</span><span class="p">(</span><span class="n">arg1</span><span class="p">,</span> <span class="n">arg2</span><span class="p">,</span> <span class="n">arg3</span><span class="p">):</span> <span class="c1"># No 'self' parameter!</span> <span class="o">...</span> </pre></div> </div> <p>However, a far more straightforward way to get the effect of a static method is via a simple module-level function:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">getcount</span><span class="p">():</span> <span class="k">return</span> <span class="n">C</span><span class="o">.</span><span class="n">count</span> </pre></div> </div> <p>If your code is structured so as to define one class (or tightly related class hierarchy) per module, this supplies the desired encapsulation.</p> </div> <div class="section" id="how-can-i-overload-constructors-or-methods-in-python"> <h3><a class="toc-backref" href="#id58">How can I overload constructors (or methods) in Python?</a><a class="headerlink" href="#how-can-i-overload-constructors-or-methods-in-python" title="Permalink to this headline">¶</a></h3> <p>This answer actually applies to all methods, but the question usually comes up first in the context of constructors.</p> <p>In C++ you’d write</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">class</span> <span class="n">C</span> <span class="p">{</span> <span class="n">C</span><span class="p">()</span> <span class="p">{</span> <span class="n">cout</span> <span class="o"><<</span> <span class="s">"No arguments</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span> <span class="p">}</span> <span class="n">C</span><span class="p">(</span><span class="kt">int</span> <span class="n">i</span><span class="p">)</span> <span class="p">{</span> <span class="n">cout</span> <span class="o"><<</span> <span class="s">"Argument is "</span> <span class="o"><<</span> <span class="n">i</span> <span class="o"><<</span> <span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span> <span class="p">}</span> <span class="p">}</span> </pre></div> </div> <p>In Python you have to write a single constructor that catches all cases using default arguments. For example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">i</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span> <span class="k">if</span> <span class="n">i</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span> <span class="nb">print</span> <span class="s2">"No arguments"</span> <span class="k">else</span><span class="p">:</span> <span class="nb">print</span> <span class="s2">"Argument is"</span><span class="p">,</span> <span class="n">i</span> </pre></div> </div> <p>This is not entirely equivalent, but close enough in practice.</p> <p>You could also try a variable-length argument list, e.g.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">):</span> <span class="o">...</span> </pre></div> </div> <p>The same approach works for all method definitions.</p> </div> <div class="section" id="i-try-to-use-spam-and-i-get-an-error-about-someclassname-spam"> <h3><a class="toc-backref" href="#id59">I try to use __spam and I get an error about _SomeClassName__spam.</a><a class="headerlink" href="#i-try-to-use-spam-and-i-get-an-error-about-someclassname-spam" title="Permalink to this headline">¶</a></h3> <p>Variable names with double leading underscores are “mangled” to provide a simple but effective way to define class private variables. Any identifier of the form <code class="docutils literal notranslate"><span class="pre">__spam</span></code> (at least two leading underscores, at most one trailing underscore) is textually replaced with <code class="docutils literal notranslate"><span class="pre">_classname__spam</span></code>, where <code class="docutils literal notranslate"><span class="pre">classname</span></code> is the current class name with any leading underscores stripped.</p> <p>This doesn’t guarantee privacy: an outside user can still deliberately access the “_classname__spam” attribute, and private values are visible in the object’s <code class="docutils literal notranslate"><span class="pre">__dict__</span></code>. Many Python programmers never bother to use private variable names at all.</p> </div> <div class="section" id="my-class-defines-del-but-it-is-not-called-when-i-delete-the-object"> <h3><a class="toc-backref" href="#id60">My class defines __del__ but it is not called when I delete the object.</a><a class="headerlink" href="#my-class-defines-del-but-it-is-not-called-when-i-delete-the-object" title="Permalink to this headline">¶</a></h3> <p>There are several possible reasons for this.</p> <p>The del statement does not necessarily call <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> – it simply decrements the object’s reference count, and if this reaches zero <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> is called.</p> <p>If your data structures contain circular links (e.g. a tree where each child has a parent reference and each parent has a list of children) the reference counts will never go back to zero. Once in a while Python runs an algorithm to detect such cycles, but the garbage collector might run some time after the last reference to your data structure vanishes, so your <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method may be called at an inconvenient and random time. This is inconvenient if you’re trying to reproduce a problem. Worse, the order in which object’s <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> methods are executed is arbitrary. You can run <a class="reference internal" href="../library/gc.html#gc.collect" title="gc.collect"><code class="xref py py-func docutils literal notranslate"><span class="pre">gc.collect()</span></code></a> to force a collection, but there <em>are</em> pathological cases where objects will never be collected.</p> <p>Despite the cycle collector, it’s still a good idea to define an explicit <code class="docutils literal notranslate"><span class="pre">close()</span></code> method on objects to be called whenever you’re done with them. The <code class="docutils literal notranslate"><span class="pre">close()</span></code> method can then remove attributes that refer to subobjecs. Don’t call <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> directly – <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> should call <code class="docutils literal notranslate"><span class="pre">close()</span></code> and <code class="docutils literal notranslate"><span class="pre">close()</span></code> should make sure that it can be called more than once for the same object.</p> <p>Another way to avoid cyclical references is to use the <a class="reference internal" href="../library/weakref.html#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal notranslate"><span class="pre">weakref</span></code></a> module, which allows you to point to objects without incrementing their reference count. Tree data structures, for instance, should use weak references for their parent and sibling references (if they need them!).</p> <p>If the object has ever been a local variable in a function that caught an expression in an except clause, chances are that a reference to the object still exists in that function’s stack frame as contained in the stack trace. Normally, calling <a class="reference internal" href="../library/sys.html#sys.exc_clear" title="sys.exc_clear"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.exc_clear()</span></code></a> will take care of this by clearing the last recorded exception.</p> <p>Finally, if your <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method raises an exception, a warning message is printed to <a class="reference internal" href="../library/sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stderr</span></code></a>.</p> </div> <div class="section" id="how-do-i-get-a-list-of-all-instances-of-a-given-class"> <h3><a class="toc-backref" href="#id61">How do I get a list of all instances of a given class?</a><a class="headerlink" href="#how-do-i-get-a-list-of-all-instances-of-a-given-class" title="Permalink to this headline">¶</a></h3> <p>Python does not keep track of all instances of a class (or of a built-in type). You can program the class’s constructor to keep track of all instances by keeping a list of weak references to each instance.</p> </div> <div class="section" id="why-does-the-result-of-id-appear-to-be-not-unique"> <h3><a class="toc-backref" href="#id62">Why does the result of <code class="docutils literal notranslate"><span class="pre">id()</span></code> appear to be not unique?</a><a class="headerlink" href="#why-does-the-result-of-id-appear-to-be-not-unique" title="Permalink to this headline">¶</a></h3> <p>The <a class="reference internal" href="../library/functions.html#id" title="id"><code class="xref py py-func docutils literal notranslate"><span class="pre">id()</span></code></a> builtin returns an integer that is guaranteed to be unique during the lifetime of the object. Since in CPython, this is the object’s memory address, it happens frequently that after an object is deleted from memory, the next freshly created object is allocated at the same position in memory. This is illustrated by this example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">id</span><span class="p">(</span><span class="mi">1000</span><span class="p">)</span> <span class="go">13901272</span> <span class="gp">>>> </span><span class="nb">id</span><span class="p">(</span><span class="mi">2000</span><span class="p">)</span> <span class="go">13901272</span> </pre></div> </div> <p>The two ids belong to different integer objects that are created before, and deleted immediately after execution of the <code class="docutils literal notranslate"><span class="pre">id()</span></code> call. To be sure that objects whose id you want to examine are still alive, create another reference to the object:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="mi">1000</span><span class="p">;</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">2000</span> <span class="gp">>>> </span><span class="nb">id</span><span class="p">(</span><span class="n">a</span><span class="p">)</span> <span class="go">13901272</span> <span class="gp">>>> </span><span class="nb">id</span><span class="p">(</span><span class="n">b</span><span class="p">)</span> <span class="go">13891296</span> </pre></div> </div> </div> </div> <div class="section" id="modules"> <h2><a class="toc-backref" href="#id63">Modules</a><a class="headerlink" href="#modules" title="Permalink to this headline">¶</a></h2> <div class="section" id="how-do-i-create-a-pyc-file"> <h3><a class="toc-backref" href="#id64">How do I create a .pyc file?</a><a class="headerlink" href="#how-do-i-create-a-pyc-file" title="Permalink to this headline">¶</a></h3> <p>When a module is imported for the first time (or when the source is more recent than the current compiled file) a <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> file containing the compiled code should be created in the same directory as the <code class="docutils literal notranslate"><span class="pre">.py</span></code> file.</p> <p>One reason that a <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> file may not be created is permissions problems with the directory. This can happen, for example, if you develop as one user but run as another, such as if you are testing with a web server. Creation of a .pyc file is automatic if you’re importing a module and Python has the ability (permissions, free space, etc…) to write the compiled module back to the directory.</p> <p>Running Python on a top level script is not considered an import and no <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> will be created. For example, if you have a top-level module <code class="docutils literal notranslate"><span class="pre">foo.py</span></code> that imports another module <code class="docutils literal notranslate"><span class="pre">xyz.py</span></code>, when you run <code class="docutils literal notranslate"><span class="pre">foo</span></code>, <code class="docutils literal notranslate"><span class="pre">xyz.pyc</span></code> will be created since <code class="docutils literal notranslate"><span class="pre">xyz</span></code> is imported, but no <code class="docutils literal notranslate"><span class="pre">foo.pyc</span></code> file will be created since <code class="docutils literal notranslate"><span class="pre">foo.py</span></code> isn’t being imported.</p> <p>If you need to create <code class="docutils literal notranslate"><span class="pre">foo.pyc</span></code> – that is, to create a <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> file for a module that is not imported – you can, using the <a class="reference internal" href="../library/py_compile.html#module-py_compile" title="py_compile: Generate byte-code files from Python source files."><code class="xref py py-mod docutils literal notranslate"><span class="pre">py_compile</span></code></a> and <a class="reference internal" href="../library/compileall.html#module-compileall" title="compileall: Tools for byte-compiling all Python source files in a directory tree."><code class="xref py py-mod docutils literal notranslate"><span class="pre">compileall</span></code></a> modules.</p> <p>The <a class="reference internal" href="../library/py_compile.html#module-py_compile" title="py_compile: Generate byte-code files from Python source files."><code class="xref py py-mod docutils literal notranslate"><span class="pre">py_compile</span></code></a> module can manually compile any module. One way is to use the <code class="docutils literal notranslate"><span class="pre">compile()</span></code> function in that module interactively:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">py_compile</span> <span class="gp">>>> </span><span class="n">py_compile</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="s1">'foo.py'</span><span class="p">)</span> </pre></div> </div> <p>This will write the <code class="docutils literal notranslate"><span class="pre">.pyc</span></code> to the same location as <code class="docutils literal notranslate"><span class="pre">foo.py</span></code> (or you can override that with the optional parameter <code class="docutils literal notranslate"><span class="pre">cfile</span></code>).</p> <p>You can also automatically compile all files in a directory or directories using the <a class="reference internal" href="../library/compileall.html#module-compileall" title="compileall: Tools for byte-compiling all Python source files in a directory tree."><code class="xref py py-mod docutils literal notranslate"><span class="pre">compileall</span></code></a> module. You can do it from the shell prompt by running <code class="docutils literal notranslate"><span class="pre">compileall.py</span></code> and providing the path of a directory containing Python files to compile:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">python</span> <span class="o">-</span><span class="n">m</span> <span class="n">compileall</span> <span class="o">.</span> </pre></div> </div> </div> <div class="section" id="how-do-i-find-the-current-module-name"> <h3><a class="toc-backref" href="#id65">How do I find the current module name?</a><a class="headerlink" href="#how-do-i-find-the-current-module-name" title="Permalink to this headline">¶</a></h3> <p>A module can find out its own module name by looking at the predefined global variable <code class="docutils literal notranslate"><span class="pre">__name__</span></code>. If this has the value <code class="docutils literal notranslate"><span class="pre">'__main__'</span></code>, the program is running as a script. Many modules that are usually used by importing them also provide a command-line interface or a self-test, and only execute this code after checking <code class="docutils literal notranslate"><span class="pre">__name__</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">main</span><span class="p">():</span> <span class="nb">print</span> <span class="s1">'Running test...'</span> <span class="o">...</span> <span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span> <span class="n">main</span><span class="p">()</span> </pre></div> </div> </div> <div class="section" id="how-can-i-have-modules-that-mutually-import-each-other"> <h3><a class="toc-backref" href="#id66">How can I have modules that mutually import each other?</a><a class="headerlink" href="#how-can-i-have-modules-that-mutually-import-each-other" title="Permalink to this headline">¶</a></h3> <p>Suppose you have the following modules:</p> <p>foo.py:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">bar</span> <span class="k">import</span> <span class="n">bar_var</span> <span class="n">foo_var</span> <span class="o">=</span> <span class="mi">1</span> </pre></div> </div> <p>bar.py:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">foo</span> <span class="k">import</span> <span class="n">foo_var</span> <span class="n">bar_var</span> <span class="o">=</span> <span class="mi">2</span> </pre></div> </div> <p>The problem is that the interpreter will perform the following steps:</p> <ul class="simple"> <li>main imports foo</li> <li>Empty globals for foo are created</li> <li>foo is compiled and starts executing</li> <li>foo imports bar</li> <li>Empty globals for bar are created</li> <li>bar is compiled and starts executing</li> <li>bar imports foo (which is a no-op since there already is a module named foo)</li> <li>bar.foo_var = foo.foo_var</li> </ul> <p>The last step fails, because Python isn’t done with interpreting <code class="docutils literal notranslate"><span class="pre">foo</span></code> yet and the global symbol dictionary for <code class="docutils literal notranslate"><span class="pre">foo</span></code> is still empty.</p> <p>The same thing happens when you use <code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">foo</span></code>, and then try to access <code class="docutils literal notranslate"><span class="pre">foo.foo_var</span></code> in global code.</p> <p>There are (at least) three possible workarounds for this problem.</p> <p>Guido van Rossum recommends avoiding all uses of <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre"><module></span> <span class="pre">import</span> <span class="pre">...</span></code>, and placing all code inside functions. Initializations of global variables and class variables should use constants or built-in functions only. This means everything from an imported module is referenced as <code class="docutils literal notranslate"><span class="pre"><module>.<name></span></code>.</p> <p>Jim Roskind suggests performing steps in the following order in each module:</p> <ul class="simple"> <li>exports (globals, functions, and classes that don’t need imported base classes)</li> <li><code class="docutils literal notranslate"><span class="pre">import</span></code> statements</li> <li>active code (including globals that are initialized from imported values).</li> </ul> <p>van Rossum doesn’t like this approach much because the imports appear in a strange place, but it does work.</p> <p>Matthias Urlichs recommends restructuring your code so that the recursive import is not necessary in the first place.</p> <p>These solutions are not mutually exclusive.</p> </div> <div class="section" id="import-x-y-z-returns-module-x-how-do-i-get-z"> <h3><a class="toc-backref" href="#id67">__import__(‘x.y.z’) returns <module ‘x’>; how do I get z?</a><a class="headerlink" href="#import-x-y-z-returns-module-x-how-do-i-get-z" title="Permalink to this headline">¶</a></h3> <p>Consider using the convenience function <a class="reference internal" href="../library/importlib.html#importlib.import_module" title="importlib.import_module"><code class="xref py py-func docutils literal notranslate"><span class="pre">import_module()</span></code></a> from <a class="reference internal" href="../library/importlib.html#module-importlib" title="importlib: Convenience wrappers for __import__"><code class="xref py py-mod docutils literal notranslate"><span class="pre">importlib</span></code></a> instead:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">z</span> <span class="o">=</span> <span class="n">importlib</span><span class="o">.</span><span class="n">import_module</span><span class="p">(</span><span class="s1">'x.y.z'</span><span class="p">)</span> </pre></div> </div> </div> <div class="section" id="when-i-edit-an-imported-module-and-reimport-it-the-changes-don-t-show-up-why-does-this-happen"> <h3><a class="toc-backref" href="#id68">When I edit an imported module and reimport it, the changes don’t show up. Why does this happen?</a><a class="headerlink" href="#when-i-edit-an-imported-module-and-reimport-it-the-changes-don-t-show-up-why-does-this-happen" title="Permalink to this headline">¶</a></h3> <p>For reasons of efficiency as well as consistency, Python only reads the module file on the first time a module is imported. If it didn’t, in a program consisting of many modules where each one imports the same basic module, the basic module would be parsed and re-parsed many times. To force rereading of a changed module, do this:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">modname</span> <span class="n">reload</span><span class="p">(</span><span class="n">modname</span><span class="p">)</span> </pre></div> </div> <p>Warning: this technique is not 100% fool-proof. In particular, modules containing statements like</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">modname</span> <span class="k">import</span> <span class="n">some_objects</span> </pre></div> </div> <p>will continue to work with the old version of the imported objects. If the module contains class definitions, existing class instances will <em>not</em> be updated to use the new class definition. This can result in the following paradoxical behaviour:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">cls</span> <span class="gp">>>> </span><span class="n">c</span> <span class="o">=</span> <span class="bp">cls</span><span class="o">.</span><span class="n">C</span><span class="p">()</span> <span class="c1"># Create an instance of C</span> <span class="gp">>>> </span><span class="n">reload</span><span class="p">(</span><span class="bp">cls</span><span class="p">)</span> <span class="go"><module 'cls' from 'cls.pyc'></span> <span class="gp">>>> </span><span class="nb">isinstance</span><span class="p">(</span><span class="n">c</span><span class="p">,</span> <span class="bp">cls</span><span class="o">.</span><span class="n">C</span><span class="p">)</span> <span class="c1"># isinstance is false?!?</span> <span class="go">False</span> </pre></div> </div> <p>The nature of the problem is made clear if you print out the class objects:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">c</span><span class="o">.</span><span class="vm">__class__</span> <span class="go"><class cls.C at 0x7352a0></span> <span class="gp">>>> </span><span class="bp">cls</span><span class="o">.</span><span class="n">C</span> <span class="go"><class cls.C at 0x4198d0></span> </pre></div> </div> </div> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table of Contents</a></h3> <ul> <li><a class="reference internal" href="#">Programming FAQ</a><ul> <li><a class="reference internal" href="#general-questions">General Questions</a></li> <li><a class="reference internal" href="#core-language">Core Language</a></li> <li><a class="reference internal" href="#numbers-and-strings">Numbers and strings</a></li> <li><a class="reference internal" href="#sequences-tuples-lists">Sequences (Tuples/Lists)</a></li> <li><a class="reference internal" href="#dictionaries">Dictionaries</a></li> <li><a class="reference internal" href="#objects">Objects</a></li> <li><a class="reference internal" href="#modules">Modules</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="general.html" title="previous chapter">General Python FAQ</a></p> <h4>Next topic</h4> <p class="topless"><a href="design.html" title="next chapter">Design and History FAQ</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/faq/programming.rst.txt" rel="nofollow">Show Source</a></li> </ul> </div> <div id="searchbox" style="display: none" role="search"> <h3>Quick search</h3> <div class="searchformwrapper"> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="design.html" title="Design and History FAQ" >next</a> |</li> <li class="right" > <a href="general.html" title="General Python FAQ" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.18 documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >Python Frequently Asked Questions</a> »</li> </ul> </div> <div class="footer"> © <a href="../copyright.html">Copyright</a> 1990-2024, Python Software Foundation. <br /> The Python Software Foundation is a non-profit corporation. <a href="https://www.python.org/psf/donations/">Please donate.</a> <br /> Last updated on December 09, 2024. <a href="../bugs.html">Found a bug</a>? <br /> Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.8.5. </div> </body> </html>