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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>Extending/Embedding 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="Python on Windows FAQ" href="windows.html" /> <link rel="prev" title="Library and Extension FAQ" href="library.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="file:///usr/share/doc/python2.7/html/faq/extending.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div id="outdated-warning" style="padding: .5em; text-align: center; background-color: #FFBABA; color: #6A0E0E;"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/extending.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="windows.html" title="Python on Windows FAQ" accesskey="N">next</a> |</li> <li class="right" > <a href="library.html" title="Library and Extension 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="extending-embedding-faq"> <h1><a class="toc-backref" href="#id2">Extending/Embedding FAQ</a><a class="headerlink" href="#extending-embedding-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="#extending-embedding-faq" id="id2">Extending/Embedding FAQ</a><ul> <li><a class="reference internal" href="#can-i-create-my-own-functions-in-c" id="id3">Can I create my own functions in C?</a></li> <li><a class="reference internal" href="#id1" id="id4">Can I create my own functions in C++?</a></li> <li><a class="reference internal" href="#writing-c-is-hard-are-there-any-alternatives" id="id5">Writing C is hard; are there any alternatives?</a></li> <li><a class="reference internal" href="#how-can-i-execute-arbitrary-python-statements-from-c" id="id6">How can I execute arbitrary Python statements from C?</a></li> <li><a class="reference internal" href="#how-can-i-evaluate-an-arbitrary-python-expression-from-c" id="id7">How can I evaluate an arbitrary Python expression from C?</a></li> <li><a class="reference internal" href="#how-do-i-extract-c-values-from-a-python-object" id="id8">How do I extract C values from a Python object?</a></li> <li><a class="reference internal" href="#how-do-i-use-py-buildvalue-to-create-a-tuple-of-arbitrary-length" id="id9">How do I use Py_BuildValue() to create a tuple of arbitrary length?</a></li> <li><a class="reference internal" href="#how-do-i-call-an-object-s-method-from-c" id="id10">How do I call an object’s method from C?</a></li> <li><a class="reference internal" href="#how-do-i-catch-the-output-from-pyerr-print-or-anything-that-prints-to-stdout-stderr" id="id11">How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?</a></li> <li><a class="reference internal" href="#how-do-i-access-a-module-written-in-python-from-c" id="id12">How do I access a module written in Python from C?</a></li> <li><a class="reference internal" href="#how-do-i-interface-to-c-objects-from-python" id="id13">How do I interface to C++ objects from Python?</a></li> <li><a class="reference internal" href="#i-added-a-module-using-the-setup-file-and-the-make-fails-why" id="id14">I added a module using the Setup file and the make fails; why?</a></li> <li><a class="reference internal" href="#how-do-i-debug-an-extension" id="id15">How do I debug an extension?</a></li> <li><a class="reference internal" href="#i-want-to-compile-a-python-module-on-my-linux-system-but-some-files-are-missing-why" id="id16">I want to compile a Python module on my Linux system, but some files are missing. Why?</a></li> <li><a class="reference internal" href="#what-does-systemerror-pyimport-fixupextension-module-yourmodule-not-loaded-mean" id="id17">What does “SystemError: _PyImport_FixupExtension: module yourmodule not loaded” mean?</a></li> <li><a class="reference internal" href="#how-do-i-tell-incomplete-input-from-invalid-input" id="id18">How do I tell “incomplete input” from “invalid input”?</a></li> <li><a class="reference internal" href="#how-do-i-find-undefined-g-symbols-builtin-new-or-pure-virtual" id="id19">How do I find undefined g++ symbols __builtin_new or __pure_virtual?</a></li> <li><a class="reference internal" href="#can-i-create-an-object-class-with-some-methods-implemented-in-c-and-others-in-python-e-g-through-inheritance" id="id20">Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?</a></li> <li><a class="reference internal" href="#when-importing-module-x-why-do-i-get-undefined-symbol-pyunicodeucs2" id="id21">When importing module X, why do I get “undefined symbol: PyUnicodeUCS2*”?</a></li> </ul> </li> </ul> </div> <div class="section" id="can-i-create-my-own-functions-in-c"> <h2><a class="toc-backref" href="#id3">Can I create my own functions in C?</a><a class="headerlink" href="#can-i-create-my-own-functions-in-c" title="Permalink to this headline">¶</a></h2> <p>Yes, you can create built-in modules containing functions, variables, exceptions and even new types in C. This is explained in the document <a class="reference internal" href="../extending/index.html#extending-index"><span class="std std-ref">Extending and Embedding the Python Interpreter</span></a>.</p> <p>Most intermediate or advanced Python books will also cover this topic.</p> </div> <div class="section" id="id1"> <h2><a class="toc-backref" href="#id4">Can I create my own functions in C++?</a><a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h2> <p>Yes, using the C compatibility features found in C++. Place <code class="docutils literal notranslate"><span class="pre">extern</span> <span class="pre">"C"</span> <span class="pre">{</span> <span class="pre">...</span> <span class="pre">}</span></code> around the Python include files and put <code class="docutils literal notranslate"><span class="pre">extern</span> <span class="pre">"C"</span></code> before each function that is going to be called by the Python interpreter. Global or static C++ objects with constructors are probably not a good idea.</p> </div> <div class="section" id="writing-c-is-hard-are-there-any-alternatives"> <span id="c-wrapper-software"></span><h2><a class="toc-backref" href="#id5">Writing C is hard; are there any alternatives?</a><a class="headerlink" href="#writing-c-is-hard-are-there-any-alternatives" title="Permalink to this headline">¶</a></h2> <p>There are a number of alternatives to writing your own C extensions, depending on what you’re trying to do.</p> <p>If you need more speed, <a class="reference external" href="http://psyco.sourceforge.net/">Psyco</a> generates x86 assembly code from Python bytecode. You can use Psyco to compile the most time-critical functions in your code, and gain a significant improvement with very little effort, as long as you’re running on a machine with an x86-compatible processor.</p> <p><a class="reference external" href="http://cython.org">Cython</a> and its relative <a class="reference external" href="https://www.cosc.canterbury.ac.nz/greg.ewing/python/Pyrex/">Pyrex</a> are compilers that accept a slightly modified form of Python and generate the corresponding C code. Pyrex makes it possible to write an extension without having to learn Python’s C API.</p> <p>If you need to interface to some C or C++ library for which no Python extension currently exists, you can try wrapping the library’s data types and functions with a tool such as <a class="reference external" href="http://www.swig.org">SWIG</a>. <a class="reference external" href="https://riverbankcomputing.com/software/sip/intro">SIP</a>, <a class="reference external" href="http://cxx.sourceforge.net/">CXX</a> <a class="reference external" href="http://www.boost.org/libs/python/doc/index.html">Boost</a>, or <a class="reference external" href="https://github.com/scipy/weave">Weave</a> are also alternatives for wrapping C++ libraries.</p> </div> <div class="section" id="how-can-i-execute-arbitrary-python-statements-from-c"> <h2><a class="toc-backref" href="#id6">How can I execute arbitrary Python statements from C?</a><a class="headerlink" href="#how-can-i-execute-arbitrary-python-statements-from-c" title="Permalink to this headline">¶</a></h2> <p>The highest-level function to do this is <a class="reference internal" href="../c-api/veryhigh.html#c.PyRun_SimpleString" title="PyRun_SimpleString"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyRun_SimpleString()</span></code></a> which takes a single string argument to be executed in the context of the module <code class="docutils literal notranslate"><span class="pre">__main__</span></code> and returns 0 for success and -1 when an exception occurred (including <code class="docutils literal notranslate"><span class="pre">SyntaxError</span></code>). If you want more control, use <a class="reference internal" href="../c-api/veryhigh.html#c.PyRun_String" title="PyRun_String"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyRun_String()</span></code></a>; see the source for <a class="reference internal" href="../c-api/veryhigh.html#c.PyRun_SimpleString" title="PyRun_SimpleString"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyRun_SimpleString()</span></code></a> in <code class="docutils literal notranslate"><span class="pre">Python/pythonrun.c</span></code>.</p> </div> <div class="section" id="how-can-i-evaluate-an-arbitrary-python-expression-from-c"> <h2><a class="toc-backref" href="#id7">How can I evaluate an arbitrary Python expression from C?</a><a class="headerlink" href="#how-can-i-evaluate-an-arbitrary-python-expression-from-c" title="Permalink to this headline">¶</a></h2> <p>Call the function <a class="reference internal" href="../c-api/veryhigh.html#c.PyRun_String" title="PyRun_String"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyRun_String()</span></code></a> from the previous question with the start symbol <a class="reference internal" href="../c-api/veryhigh.html#c.Py_eval_input" title="Py_eval_input"><code class="xref c c-data docutils literal notranslate"><span class="pre">Py_eval_input</span></code></a>; it parses an expression, evaluates it and returns its value.</p> </div> <div class="section" id="how-do-i-extract-c-values-from-a-python-object"> <h2><a class="toc-backref" href="#id8">How do I extract C values from a Python object?</a><a class="headerlink" href="#how-do-i-extract-c-values-from-a-python-object" title="Permalink to this headline">¶</a></h2> <p>That depends on the object’s type. If it’s a tuple, <a class="reference internal" href="../c-api/tuple.html#c.PyTuple_Size" title="PyTuple_Size"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyTuple_Size()</span></code></a> returns its length and <a class="reference internal" href="../c-api/tuple.html#c.PyTuple_GetItem" title="PyTuple_GetItem"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyTuple_GetItem()</span></code></a> returns the item at a specified index. Lists have similar functions, <code class="xref c c-func docutils literal notranslate"><span class="pre">PyListSize()</span></code> and <a class="reference internal" href="../c-api/list.html#c.PyList_GetItem" title="PyList_GetItem"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyList_GetItem()</span></code></a>.</p> <p>For strings, <a class="reference internal" href="../c-api/string.html#c.PyString_Size" title="PyString_Size"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyString_Size()</span></code></a> returns its length and <a class="reference internal" href="../c-api/string.html#c.PyString_AsString" title="PyString_AsString"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyString_AsString()</span></code></a> a pointer to its value. Note that Python strings may contain null bytes so C’s <code class="xref c c-func docutils literal notranslate"><span class="pre">strlen()</span></code> should not be used.</p> <p>To test the type of an object, first make sure it isn’t <em>NULL</em>, and then use <a class="reference internal" href="../c-api/string.html#c.PyString_Check" title="PyString_Check"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyString_Check()</span></code></a>, <a class="reference internal" href="../c-api/tuple.html#c.PyTuple_Check" title="PyTuple_Check"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyTuple_Check()</span></code></a>, <a class="reference internal" href="../c-api/list.html#c.PyList_Check" title="PyList_Check"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyList_Check()</span></code></a>, etc.</p> <p>There is also a high-level API to Python objects which is provided by the so-called ‘abstract’ interface – read <code class="docutils literal notranslate"><span class="pre">Include/abstract.h</span></code> for further details. It allows interfacing with any kind of Python sequence using calls like <a class="reference internal" href="../c-api/sequence.html#c.PySequence_Length" title="PySequence_Length"><code class="xref c c-func docutils literal notranslate"><span class="pre">PySequence_Length()</span></code></a>, <a class="reference internal" href="../c-api/sequence.html#c.PySequence_GetItem" title="PySequence_GetItem"><code class="xref c c-func docutils literal notranslate"><span class="pre">PySequence_GetItem()</span></code></a>, etc.) as well as many other useful protocols.</p> </div> <div class="section" id="how-do-i-use-py-buildvalue-to-create-a-tuple-of-arbitrary-length"> <h2><a class="toc-backref" href="#id9">How do I use Py_BuildValue() to create a tuple of arbitrary length?</a><a class="headerlink" href="#how-do-i-use-py-buildvalue-to-create-a-tuple-of-arbitrary-length" title="Permalink to this headline">¶</a></h2> <p>You can’t. Use <code class="docutils literal notranslate"><span class="pre">t</span> <span class="pre">=</span> <span class="pre">PyTuple_New(n)</span></code> instead, and fill it with objects using <code class="docutils literal notranslate"><span class="pre">PyTuple_SetItem(t,</span> <span class="pre">i,</span> <span class="pre">o)</span></code> – note that this “eats” a reference count of <code class="docutils literal notranslate"><span class="pre">o</span></code>, so you have to <a class="reference internal" href="../c-api/refcounting.html#c.Py_INCREF" title="Py_INCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_INCREF()</span></code></a> it. Lists have similar functions <code class="docutils literal notranslate"><span class="pre">PyList_New(n)</span></code> and <code class="docutils literal notranslate"><span class="pre">PyList_SetItem(l,</span> <span class="pre">i,</span> <span class="pre">o)</span></code>. Note that you <em>must</em> set all the tuple items to some value before you pass the tuple to Python code – <code class="docutils literal notranslate"><span class="pre">PyTuple_New(n)</span></code> initializes them to NULL, which isn’t a valid Python value.</p> </div> <div class="section" id="how-do-i-call-an-object-s-method-from-c"> <h2><a class="toc-backref" href="#id10">How do I call an object’s method from C?</a><a class="headerlink" href="#how-do-i-call-an-object-s-method-from-c" title="Permalink to this headline">¶</a></h2> <p>The <a class="reference internal" href="../c-api/object.html#c.PyObject_CallMethod" title="PyObject_CallMethod"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_CallMethod()</span></code></a> function can be used to call an arbitrary method of an object. The parameters are the object, the name of the method to call, a format string like that used with <a class="reference internal" href="../c-api/arg.html#c.Py_BuildValue" title="Py_BuildValue"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_BuildValue()</span></code></a>, and the argument values:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">PyObject</span> <span class="o">*</span> <span class="nf">PyObject_CallMethod</span><span class="p">(</span><span class="n">PyObject</span> <span class="o">*</span><span class="n">object</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">method_name</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">arg_format</span><span class="p">,</span> <span class="p">...);</span> </pre></div> </div> <p>This works for any object that has methods – whether built-in or user-defined. You are responsible for eventually <a class="reference internal" href="../c-api/refcounting.html#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a>’ing the return value.</p> <p>To call, e.g., a file object’s “seek” method with arguments 10, 0 (assuming the file object pointer is “f”):</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">res</span> <span class="o">=</span> <span class="n">PyObject_CallMethod</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="s">"seek"</span><span class="p">,</span> <span class="s">"(ii)"</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">res</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="p">{</span> <span class="p">...</span> <span class="n">an</span> <span class="n">exception</span> <span class="n">occurred</span> <span class="p">...</span> <span class="p">}</span> <span class="k">else</span> <span class="p">{</span> <span class="n">Py_DECREF</span><span class="p">(</span><span class="n">res</span><span class="p">);</span> <span class="p">}</span> </pre></div> </div> <p>Note that since <a class="reference internal" href="../c-api/object.html#c.PyObject_CallObject" title="PyObject_CallObject"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_CallObject()</span></code></a> <em>always</em> wants a tuple for the argument list, to call a function without arguments, pass “()” for the format, and to call a function with one argument, surround the argument in parentheses, e.g. “(i)”.</p> </div> <div class="section" id="how-do-i-catch-the-output-from-pyerr-print-or-anything-that-prints-to-stdout-stderr"> <h2><a class="toc-backref" href="#id11">How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?</a><a class="headerlink" href="#how-do-i-catch-the-output-from-pyerr-print-or-anything-that-prints-to-stdout-stderr" title="Permalink to this headline">¶</a></h2> <p>In Python code, define an object that supports the <code class="docutils literal notranslate"><span class="pre">write()</span></code> method. Assign this object to <a class="reference internal" href="../library/sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdout</span></code></a> and <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>. Call print_error, or just allow the standard traceback mechanism to work. Then, the output will go wherever your <code class="docutils literal notranslate"><span class="pre">write()</span></code> method sends it.</p> <p>The easiest way to do this is to use the StringIO class in the standard library.</p> <p>Sample code and use for catching stdout:</p> <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">class</span> <span class="nc">StdoutCatcher</span><span class="p">:</span> <span class="gp">... </span> <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">data</span> <span class="o">=</span> <span class="s1">''</span> <span class="gp">... </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">stuff</span><span class="p">):</span> <span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">data</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">data</span> <span class="o">+</span> <span class="n">stuff</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="kn">import</span> <span class="nn">sys</span> <span class="gp">>>> </span><span class="n">sys</span><span class="o">.</span><span class="n">stdout</span> <span class="o">=</span> <span class="n">StdoutCatcher</span><span class="p">()</span> <span class="gp">>>> </span><span class="k">print</span> <span class="s1">'foo'</span> <span class="gp">>>> </span><span class="k">print</span> <span class="s1">'hello world!'</span> <span class="gp">>>> </span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="o">.</span><span class="n">data</span><span class="p">)</span> <span class="go">foo</span> <span class="go">hello world!</span> </pre></div> </div> </div> <div class="section" id="how-do-i-access-a-module-written-in-python-from-c"> <h2><a class="toc-backref" href="#id12">How do I access a module written in Python from C?</a><a class="headerlink" href="#how-do-i-access-a-module-written-in-python-from-c" title="Permalink to this headline">¶</a></h2> <p>You can get a pointer to the module object as follows:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">module</span> <span class="o">=</span> <span class="n">PyImport_ImportModule</span><span class="p">(</span><span class="s">"<modulename>"</span><span class="p">);</span> </pre></div> </div> <p>If the module hasn’t been imported yet (i.e. it is not yet present 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>), this initializes the module; otherwise it simply returns the value of <code class="docutils literal notranslate"><span class="pre">sys.modules["<modulename>"]</span></code>. Note that it doesn’t enter the module into any namespace – it only ensures it has been initialized and is stored 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> <p>You can then access the module’s attributes (i.e. any name defined in the module) as follows:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">attr</span> <span class="o">=</span> <span class="n">PyObject_GetAttrString</span><span class="p">(</span><span class="n">module</span><span class="p">,</span> <span class="s">"<attrname>"</span><span class="p">);</span> </pre></div> </div> <p>Calling <a class="reference internal" href="../c-api/object.html#c.PyObject_SetAttrString" title="PyObject_SetAttrString"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_SetAttrString()</span></code></a> to assign to variables in the module also works.</p> </div> <div class="section" id="how-do-i-interface-to-c-objects-from-python"> <h2><a class="toc-backref" href="#id13">How do I interface to C++ objects from Python?</a><a class="headerlink" href="#how-do-i-interface-to-c-objects-from-python" title="Permalink to this headline">¶</a></h2> <p>Depending on your requirements, there are many approaches. To do this manually, begin by reading <a class="reference internal" href="../extending/index.html#extending-index"><span class="std std-ref">the “Extending and Embedding” document</span></a>. Realize that for the Python run-time system, there isn’t a whole lot of difference between C and C++ – so the strategy of building a new Python type around a C structure (pointer) type will also work for C++ objects.</p> <p>For C++ libraries, see <a class="reference internal" href="#c-wrapper-software"><span class="std std-ref">Writing C is hard; are there any alternatives?</span></a>.</p> </div> <div class="section" id="i-added-a-module-using-the-setup-file-and-the-make-fails-why"> <h2><a class="toc-backref" href="#id14">I added a module using the Setup file and the make fails; why?</a><a class="headerlink" href="#i-added-a-module-using-the-setup-file-and-the-make-fails-why" title="Permalink to this headline">¶</a></h2> <p>Setup must end in a newline, if there is no newline there, the build process fails. (Fixing this requires some ugly shell script hackery, and this bug is so minor that it doesn’t seem worth the effort.)</p> </div> <div class="section" id="how-do-i-debug-an-extension"> <h2><a class="toc-backref" href="#id15">How do I debug an extension?</a><a class="headerlink" href="#how-do-i-debug-an-extension" title="Permalink to this headline">¶</a></h2> <p>When using GDB with dynamically loaded extensions, you can’t set a breakpoint in your extension until your extension is loaded.</p> <p>In your <code class="docutils literal notranslate"><span class="pre">.gdbinit</span></code> file (or interactively), add the command:</p> <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>br _PyImport_LoadDynamicModule </pre></div> </div> <p>Then, when you run GDB:</p> <div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="gp">$</span> gdb /local/bin/python <span class="go">gdb) run myscript.py</span> <span class="go">gdb) continue # repeat until your extension is loaded</span> <span class="go">gdb) finish # so that your extension is loaded</span> <span class="go">gdb) br myfunction.c:50</span> <span class="go">gdb) continue</span> </pre></div> </div> </div> <div class="section" id="i-want-to-compile-a-python-module-on-my-linux-system-but-some-files-are-missing-why"> <h2><a class="toc-backref" href="#id16">I want to compile a Python module on my Linux system, but some files are missing. Why?</a><a class="headerlink" href="#i-want-to-compile-a-python-module-on-my-linux-system-but-some-files-are-missing-why" title="Permalink to this headline">¶</a></h2> <p>Most packaged versions of Python don’t include the <code class="file docutils literal notranslate"><span class="pre">/usr/lib/python2.</span><em><span class="pre">x</span></em><span class="pre">/config/</span></code> directory, which contains various files required for compiling Python extensions.</p> <p>For Red Hat, install the python-devel RPM to get the necessary files.</p> <p>For Debian, run <code class="docutils literal notranslate"><span class="pre">apt-get</span> <span class="pre">install</span> <span class="pre">python-dev</span></code>.</p> </div> <div class="section" id="what-does-systemerror-pyimport-fixupextension-module-yourmodule-not-loaded-mean"> <h2><a class="toc-backref" href="#id17">What does “SystemError: _PyImport_FixupExtension: module yourmodule not loaded” mean?</a><a class="headerlink" href="#what-does-systemerror-pyimport-fixupextension-module-yourmodule-not-loaded-mean" title="Permalink to this headline">¶</a></h2> <p>This means that you have created an extension module named “yourmodule”, but your module init function does not initialize with that name.</p> <p>Every module init function will have a line similar to:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">module</span> <span class="o">=</span> <span class="n">Py_InitModule</span><span class="p">(</span><span class="s">"yourmodule"</span><span class="p">,</span> <span class="n">yourmodule_functions</span><span class="p">);</span> </pre></div> </div> <p>If the string passed to this function is not the same name as your extension module, the <a class="reference internal" href="../library/exceptions.html#exceptions.SystemError" title="exceptions.SystemError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemError</span></code></a> exception will be raised.</p> </div> <div class="section" id="how-do-i-tell-incomplete-input-from-invalid-input"> <h2><a class="toc-backref" href="#id18">How do I tell “incomplete input” from “invalid input”?</a><a class="headerlink" href="#how-do-i-tell-incomplete-input-from-invalid-input" title="Permalink to this headline">¶</a></h2> <p>Sometimes you want to emulate the Python interactive interpreter’s behavior, where it gives you a continuation prompt when the input is incomplete (e.g. you typed the start of an “if” statement or you didn’t close your parentheses or triple string quotes), but it gives you a syntax error message immediately when the input is invalid.</p> <p>In Python you can use the <a class="reference internal" href="../library/codeop.html#module-codeop" title="codeop: Compile (possibly incomplete) Python code."><code class="xref py py-mod docutils literal notranslate"><span class="pre">codeop</span></code></a> module, which approximates the parser’s behavior sufficiently. IDLE uses this, for example.</p> <p>The easiest way to do it in C is to call <a class="reference internal" href="../c-api/veryhigh.html#c.PyRun_InteractiveLoop" title="PyRun_InteractiveLoop"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyRun_InteractiveLoop()</span></code></a> (perhaps in a separate thread) and let the Python interpreter handle the input for you. You can also set the <code class="xref c c-func docutils literal notranslate"><span class="pre">PyOS_ReadlineFunctionPointer()</span></code> to point at your custom input function. See <code class="docutils literal notranslate"><span class="pre">Modules/readline.c</span></code> and <code class="docutils literal notranslate"><span class="pre">Parser/myreadline.c</span></code> for more hints.</p> <p>However sometimes you have to run the embedded Python interpreter in the same thread as your rest application and you can’t allow the <a class="reference internal" href="../c-api/veryhigh.html#c.PyRun_InteractiveLoop" title="PyRun_InteractiveLoop"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyRun_InteractiveLoop()</span></code></a> to stop while waiting for user input. The one solution then is to call <code class="xref c c-func docutils literal notranslate"><span class="pre">PyParser_ParseString()</span></code> and test for <code class="docutils literal notranslate"><span class="pre">e.error</span></code> equal to <code class="docutils literal notranslate"><span class="pre">E_EOF</span></code>, which means the input is incomplete. Here’s a sample code fragment, untested, inspired by code from Alex Farber:</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#include</span> <span class="cpf"><Python.h></span><span class="cp"></span> <span class="cp">#include</span> <span class="cpf"><node.h></span><span class="cp"></span> <span class="cp">#include</span> <span class="cpf"><errcode.h></span><span class="cp"></span> <span class="cp">#include</span> <span class="cpf"><grammar.h></span><span class="cp"></span> <span class="cp">#include</span> <span class="cpf"><parsetok.h></span><span class="cp"></span> <span class="cp">#include</span> <span class="cpf"><compile.h></span><span class="cp"></span> <span class="kt">int</span> <span class="nf">testcomplete</span><span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="n">code</span><span class="p">)</span> <span class="cm">/* code should end in \n */</span> <span class="cm">/* return -1 for error, 0 for incomplete, 1 for complete */</span> <span class="p">{</span> <span class="n">node</span> <span class="o">*</span><span class="n">n</span><span class="p">;</span> <span class="n">perrdetail</span> <span class="n">e</span><span class="p">;</span> <span class="n">n</span> <span class="o">=</span> <span class="n">PyParser_ParseString</span><span class="p">(</span><span class="n">code</span><span class="p">,</span> <span class="o">&</span><span class="n">_PyParser_Grammar</span><span class="p">,</span> <span class="n">Py_file_input</span><span class="p">,</span> <span class="o">&</span><span class="n">e</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">n</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="p">{</span> <span class="k">if</span> <span class="p">(</span><span class="n">e</span><span class="p">.</span><span class="n">error</span> <span class="o">==</span> <span class="n">E_EOF</span><span class="p">)</span> <span class="k">return</span> <span class="mi">0</span><span class="p">;</span> <span class="k">return</span> <span class="o">-</span><span class="mi">1</span><span class="p">;</span> <span class="p">}</span> <span class="n">PyNode_Free</span><span class="p">(</span><span class="n">n</span><span class="p">);</span> <span class="k">return</span> <span class="mi">1</span><span class="p">;</span> <span class="p">}</span> </pre></div> </div> <p>Another solution is trying to compile the received string with <a class="reference internal" href="../c-api/veryhigh.html#c.Py_CompileString" title="Py_CompileString"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_CompileString()</span></code></a>. If it compiles without errors, try to execute the returned code object by calling <a class="reference internal" href="../c-api/veryhigh.html#c.PyEval_EvalCode" title="PyEval_EvalCode"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyEval_EvalCode()</span></code></a>. Otherwise save the input for later. If the compilation fails, find out if it’s an error or just more input is required - by extracting the message string from the exception tuple and comparing it to the string “unexpected EOF while parsing”. Here is a complete example using the GNU readline library (you may want to ignore <strong>SIGINT</strong> while calling readline()):</p> <div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#include</span> <span class="cpf"><stdio.h></span><span class="cp"></span> <span class="cp">#include</span> <span class="cpf"><readline.h></span><span class="cp"></span> <span class="cp">#include</span> <span class="cpf"><Python.h></span><span class="cp"></span> <span class="cp">#include</span> <span class="cpf"><object.h></span><span class="cp"></span> <span class="cp">#include</span> <span class="cpf"><compile.h></span><span class="cp"></span> <span class="cp">#include</span> <span class="cpf"><eval.h></span><span class="cp"></span> <span class="kt">int</span> <span class="nf">main</span> <span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span><span class="o">*</span> <span class="n">argv</span><span class="p">[])</span> <span class="p">{</span> <span class="kt">int</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span><span class="p">,</span> <span class="n">done</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="cm">/* lengths of line, code */</span> <span class="kt">char</span> <span class="n">ps1</span><span class="p">[]</span> <span class="o">=</span> <span class="s">">>> "</span><span class="p">;</span> <span class="kt">char</span> <span class="n">ps2</span><span class="p">[]</span> <span class="o">=</span> <span class="s">"... "</span><span class="p">;</span> <span class="kt">char</span> <span class="o">*</span><span class="n">prompt</span> <span class="o">=</span> <span class="n">ps1</span><span class="p">;</span> <span class="kt">char</span> <span class="o">*</span><span class="n">msg</span><span class="p">,</span> <span class="o">*</span><span class="n">line</span><span class="p">,</span> <span class="o">*</span><span class="n">code</span> <span class="o">=</span> <span class="nb">NULL</span><span class="p">;</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">src</span><span class="p">,</span> <span class="o">*</span><span class="n">glb</span><span class="p">,</span> <span class="o">*</span><span class="n">loc</span><span class="p">;</span> <span class="n">PyObject</span> <span class="o">*</span><span class="n">exc</span><span class="p">,</span> <span class="o">*</span><span class="n">val</span><span class="p">,</span> <span class="o">*</span><span class="n">trb</span><span class="p">,</span> <span class="o">*</span><span class="n">obj</span><span class="p">,</span> <span class="o">*</span><span class="n">dum</span><span class="p">;</span> <span class="n">Py_Initialize</span> <span class="p">();</span> <span class="n">loc</span> <span class="o">=</span> <span class="n">PyDict_New</span> <span class="p">();</span> <span class="n">glb</span> <span class="o">=</span> <span class="n">PyDict_New</span> <span class="p">();</span> <span class="n">PyDict_SetItemString</span> <span class="p">(</span><span class="n">glb</span><span class="p">,</span> <span class="s">"__builtins__"</span><span class="p">,</span> <span class="n">PyEval_GetBuiltins</span> <span class="p">());</span> <span class="k">while</span> <span class="p">(</span><span class="o">!</span><span class="n">done</span><span class="p">)</span> <span class="p">{</span> <span class="n">line</span> <span class="o">=</span> <span class="n">readline</span> <span class="p">(</span><span class="n">prompt</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="nb">NULL</span> <span class="o">==</span> <span class="n">line</span><span class="p">)</span> <span class="cm">/* Ctrl-D pressed */</span> <span class="p">{</span> <span class="n">done</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span> <span class="p">}</span> <span class="k">else</span> <span class="p">{</span> <span class="n">i</span> <span class="o">=</span> <span class="n">strlen</span> <span class="p">(</span><span class="n">line</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="n">i</span> <span class="o">></span> <span class="mi">0</span><span class="p">)</span> <span class="n">add_history</span> <span class="p">(</span><span class="n">line</span><span class="p">);</span> <span class="cm">/* save non-empty lines */</span> <span class="k">if</span> <span class="p">(</span><span class="nb">NULL</span> <span class="o">==</span> <span class="n">code</span><span class="p">)</span> <span class="cm">/* nothing in code yet */</span> <span class="n">j</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="k">else</span> <span class="n">j</span> <span class="o">=</span> <span class="n">strlen</span> <span class="p">(</span><span class="n">code</span><span class="p">);</span> <span class="n">code</span> <span class="o">=</span> <span class="n">realloc</span> <span class="p">(</span><span class="n">code</span><span class="p">,</span> <span class="n">i</span> <span class="o">+</span> <span class="n">j</span> <span class="o">+</span> <span class="mi">2</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="nb">NULL</span> <span class="o">==</span> <span class="n">code</span><span class="p">)</span> <span class="cm">/* out of memory */</span> <span class="n">exit</span> <span class="p">(</span><span class="mi">1</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="mi">0</span> <span class="o">==</span> <span class="n">j</span><span class="p">)</span> <span class="cm">/* code was empty, so */</span> <span class="n">code</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="sc">'\0'</span><span class="p">;</span> <span class="cm">/* keep strncat happy */</span> <span class="n">strncat</span> <span class="p">(</span><span class="n">code</span><span class="p">,</span> <span class="n">line</span><span class="p">,</span> <span class="n">i</span><span class="p">);</span> <span class="cm">/* append line to code */</span> <span class="n">code</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="sc">'\n'</span><span class="p">;</span> <span class="cm">/* append '\n' to code */</span> <span class="n">code</span><span class="p">[</span><span class="n">i</span> <span class="o">+</span> <span class="n">j</span> <span class="o">+</span> <span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="sc">'\0'</span><span class="p">;</span> <span class="n">src</span> <span class="o">=</span> <span class="n">Py_CompileString</span> <span class="p">(</span><span class="n">code</span><span class="p">,</span> <span class="s">"<stdin>"</span><span class="p">,</span> <span class="n">Py_single_input</span><span class="p">);</span> <span class="k">if</span> <span class="p">(</span><span class="nb">NULL</span> <span class="o">!=</span> <span class="n">src</span><span class="p">)</span> <span class="cm">/* compiled just fine - */</span> <span class="p">{</span> <span class="k">if</span> <span class="p">(</span><span class="n">ps1</span> <span class="o">==</span> <span class="n">prompt</span> <span class="o">||</span> <span class="cm">/* ">>> " or */</span> <span class="sc">'\n'</span> <span class="o">==</span> <span class="n">code</span><span class="p">[</span><span class="n">i</span> <span class="o">+</span> <span class="n">j</span> <span class="o">-</span> <span class="mi">1</span><span class="p">])</span> <span class="cm">/* "... " and double '\n' */</span> <span class="p">{</span> <span class="cm">/* so execute it */</span> <span class="n">dum</span> <span class="o">=</span> <span class="n">PyEval_EvalCode</span> <span class="p">((</span><span class="n">PyCodeObject</span> <span class="o">*</span><span class="p">)</span><span class="n">src</span><span class="p">,</span> <span class="n">glb</span><span class="p">,</span> <span class="n">loc</span><span class="p">);</span> <span class="n">Py_XDECREF</span> <span class="p">(</span><span class="n">dum</span><span class="p">);</span> <span class="n">Py_XDECREF</span> <span class="p">(</span><span class="n">src</span><span class="p">);</span> <span class="n">free</span> <span class="p">(</span><span class="n">code</span><span class="p">);</span> <span class="n">code</span> <span class="o">=</span> <span class="nb">NULL</span><span class="p">;</span> <span class="k">if</span> <span class="p">(</span><span class="n">PyErr_Occurred</span> <span class="p">())</span> <span class="n">PyErr_Print</span> <span class="p">();</span> <span class="n">prompt</span> <span class="o">=</span> <span class="n">ps1</span><span class="p">;</span> <span class="p">}</span> <span class="p">}</span> <span class="cm">/* syntax error or E_EOF? */</span> <span class="k">else</span> <span class="k">if</span> <span class="p">(</span><span class="n">PyErr_ExceptionMatches</span> <span class="p">(</span><span class="n">PyExc_SyntaxError</span><span class="p">))</span> <span class="p">{</span> <span class="n">PyErr_Fetch</span> <span class="p">(</span><span class="o">&</span><span class="n">exc</span><span class="p">,</span> <span class="o">&</span><span class="n">val</span><span class="p">,</span> <span class="o">&</span><span class="n">trb</span><span class="p">);</span> <span class="cm">/* clears exception! */</span> <span class="k">if</span> <span class="p">(</span><span class="n">PyArg_ParseTuple</span> <span class="p">(</span><span class="n">val</span><span class="p">,</span> <span class="s">"sO"</span><span class="p">,</span> <span class="o">&</span><span class="n">msg</span><span class="p">,</span> <span class="o">&</span><span class="n">obj</span><span class="p">)</span> <span class="o">&&</span> <span class="o">!</span><span class="n">strcmp</span> <span class="p">(</span><span class="n">msg</span><span class="p">,</span> <span class="s">"unexpected EOF while parsing"</span><span class="p">))</span> <span class="cm">/* E_EOF */</span> <span class="p">{</span> <span class="n">Py_XDECREF</span> <span class="p">(</span><span class="n">exc</span><span class="p">);</span> <span class="n">Py_XDECREF</span> <span class="p">(</span><span class="n">val</span><span class="p">);</span> <span class="n">Py_XDECREF</span> <span class="p">(</span><span class="n">trb</span><span class="p">);</span> <span class="n">prompt</span> <span class="o">=</span> <span class="n">ps2</span><span class="p">;</span> <span class="p">}</span> <span class="k">else</span> <span class="cm">/* some other syntax error */</span> <span class="p">{</span> <span class="n">PyErr_Restore</span> <span class="p">(</span><span class="n">exc</span><span class="p">,</span> <span class="n">val</span><span class="p">,</span> <span class="n">trb</span><span class="p">);</span> <span class="n">PyErr_Print</span> <span class="p">();</span> <span class="n">free</span> <span class="p">(</span><span class="n">code</span><span class="p">);</span> <span class="n">code</span> <span class="o">=</span> <span class="nb">NULL</span><span class="p">;</span> <span class="n">prompt</span> <span class="o">=</span> <span class="n">ps1</span><span class="p">;</span> <span class="p">}</span> <span class="p">}</span> <span class="k">else</span> <span class="cm">/* some non-syntax error */</span> <span class="p">{</span> <span class="n">PyErr_Print</span> <span class="p">();</span> <span class="n">free</span> <span class="p">(</span><span class="n">code</span><span class="p">);</span> <span class="n">code</span> <span class="o">=</span> <span class="nb">NULL</span><span class="p">;</span> <span class="n">prompt</span> <span class="o">=</span> <span class="n">ps1</span><span class="p">;</span> <span class="p">}</span> <span class="n">free</span> <span class="p">(</span><span class="n">line</span><span class="p">);</span> <span class="p">}</span> <span class="p">}</span> <span class="n">Py_XDECREF</span><span class="p">(</span><span class="n">glb</span><span class="p">);</span> <span class="n">Py_XDECREF</span><span class="p">(</span><span class="n">loc</span><span class="p">);</span> <span class="n">Py_Finalize</span><span class="p">();</span> <span class="n">exit</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span> <span class="p">}</span> </pre></div> </div> </div> <div class="section" id="how-do-i-find-undefined-g-symbols-builtin-new-or-pure-virtual"> <h2><a class="toc-backref" href="#id19">How do I find undefined g++ symbols __builtin_new or __pure_virtual?</a><a class="headerlink" href="#how-do-i-find-undefined-g-symbols-builtin-new-or-pure-virtual" title="Permalink to this headline">¶</a></h2> <p>To dynamically load g++ extension modules, you must recompile Python, relink it using g++ (change LINKCC in the Python Modules Makefile), and link your extension module using g++ (e.g., <code class="docutils literal notranslate"><span class="pre">g++</span> <span class="pre">-shared</span> <span class="pre">-o</span> <span class="pre">mymodule.so</span> <span class="pre">mymodule.o</span></code>).</p> </div> <div class="section" id="can-i-create-an-object-class-with-some-methods-implemented-in-c-and-others-in-python-e-g-through-inheritance"> <h2><a class="toc-backref" href="#id20">Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?</a><a class="headerlink" href="#can-i-create-an-object-class-with-some-methods-implemented-in-c-and-others-in-python-e-g-through-inheritance" title="Permalink to this headline">¶</a></h2> <p>Yes, you can inherit from built-in classes such as <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>, <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>, etc.</p> <p>The Boost Python Library (BPL, <a class="reference external" href="http://www.boost.org/libs/python/doc/index.html">http://www.boost.org/libs/python/doc/index.html</a>) provides a way of doing this from C++ (i.e. you can inherit from an extension class written in C++ using the BPL).</p> </div> <div class="section" id="when-importing-module-x-why-do-i-get-undefined-symbol-pyunicodeucs2"> <h2><a class="toc-backref" href="#id21">When importing module X, why do I get “undefined symbol: PyUnicodeUCS2*”?</a><a class="headerlink" href="#when-importing-module-x-why-do-i-get-undefined-symbol-pyunicodeucs2" title="Permalink to this headline">¶</a></h2> <p>You are using a version of Python that uses a 4-byte representation for Unicode characters, but some C extension module you are importing was compiled using a Python that uses a 2-byte representation for Unicode characters (the default).</p> <p>If instead the name of the undefined symbol starts with <code class="docutils literal notranslate"><span class="pre">PyUnicodeUCS4</span></code>, the problem is the reverse: Python was built using 2-byte Unicode characters, and the extension module was compiled using a Python with 4-byte Unicode characters.</p> <p>This can easily occur when using pre-built extension packages. RedHat Linux 7.x, in particular, provided a “python2” binary that is compiled with 4-byte Unicode. This only causes the link failure if the extension uses any of the <code class="docutils literal notranslate"><span class="pre">PyUnicode_*()</span></code> functions. It is also a problem if an extension uses any of the Unicode-related format specifiers for <a class="reference internal" href="../c-api/arg.html#c.Py_BuildValue" title="Py_BuildValue"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_BuildValue()</span></code></a> (or similar) or parameter specifications for <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a>.</p> <p>You can check the size of the Unicode character a Python interpreter is using by checking the value of sys.maxunicode:</p> <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">sys</span> <span class="gp">>>> </span><span class="k">if</span> <span class="n">sys</span><span class="o">.</span><span class="n">maxunicode</span> <span class="o">></span> <span class="mi">65535</span><span class="p">:</span> <span class="gp">... </span> <span class="k">print</span> <span class="s1">'UCS4 build'</span> <span class="gp">... </span><span class="k">else</span><span class="p">:</span> <span class="gp">... </span> <span class="k">print</span> <span class="s1">'UCS2 build'</span> </pre></div> </div> <p>The only way to solve this problem is to use extension modules compiled with a Python binary built using the same size for Unicode characters.</p> </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="#">Extending/Embedding FAQ</a><ul> <li><a class="reference internal" href="#can-i-create-my-own-functions-in-c">Can I create my own functions in C?</a></li> <li><a class="reference internal" href="#id1">Can I create my own functions in C++?</a></li> <li><a class="reference internal" href="#writing-c-is-hard-are-there-any-alternatives">Writing C is hard; are there any alternatives?</a></li> <li><a class="reference internal" href="#how-can-i-execute-arbitrary-python-statements-from-c">How can I execute arbitrary Python statements from C?</a></li> <li><a class="reference internal" href="#how-can-i-evaluate-an-arbitrary-python-expression-from-c">How can I evaluate an arbitrary Python expression from C?</a></li> <li><a class="reference internal" href="#how-do-i-extract-c-values-from-a-python-object">How do I extract C values from a Python object?</a></li> <li><a class="reference internal" href="#how-do-i-use-py-buildvalue-to-create-a-tuple-of-arbitrary-length">How do I use Py_BuildValue() to create a tuple of arbitrary length?</a></li> <li><a class="reference internal" href="#how-do-i-call-an-object-s-method-from-c">How do I call an object’s method from C?</a></li> <li><a class="reference internal" href="#how-do-i-catch-the-output-from-pyerr-print-or-anything-that-prints-to-stdout-stderr">How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?</a></li> <li><a class="reference internal" href="#how-do-i-access-a-module-written-in-python-from-c">How do I access a module written in Python from C?</a></li> <li><a class="reference internal" href="#how-do-i-interface-to-c-objects-from-python">How do I interface to C++ objects from Python?</a></li> <li><a class="reference internal" href="#i-added-a-module-using-the-setup-file-and-the-make-fails-why">I added a module using the Setup file and the make fails; why?</a></li> <li><a class="reference internal" href="#how-do-i-debug-an-extension">How do I debug an extension?</a></li> <li><a class="reference internal" href="#i-want-to-compile-a-python-module-on-my-linux-system-but-some-files-are-missing-why">I want to compile a Python module on my Linux system, but some files are missing. Why?</a></li> <li><a class="reference internal" href="#what-does-systemerror-pyimport-fixupextension-module-yourmodule-not-loaded-mean">What does “SystemError: _PyImport_FixupExtension: module yourmodule not loaded” mean?</a></li> <li><a class="reference internal" href="#how-do-i-tell-incomplete-input-from-invalid-input">How do I tell “incomplete input” from “invalid input”?</a></li> <li><a class="reference internal" href="#how-do-i-find-undefined-g-symbols-builtin-new-or-pure-virtual">How do I find undefined g++ symbols __builtin_new or __pure_virtual?</a></li> <li><a class="reference internal" href="#can-i-create-an-object-class-with-some-methods-implemented-in-c-and-others-in-python-e-g-through-inheritance">Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?</a></li> <li><a class="reference internal" href="#when-importing-module-x-why-do-i-get-undefined-symbol-pyunicodeucs2">When importing module X, why do I get “undefined symbol: PyUnicodeUCS2*”?</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="library.html" title="previous chapter">Library and Extension FAQ</a></p> <h4>Next topic</h4> <p class="topless"><a href="windows.html" title="next chapter">Python on Windows FAQ</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/faq/extending.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="windows.html" title="Python on Windows FAQ" >next</a> |</li> <li class="right" > <a href="library.html" title="Library and Extension 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>