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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>1. Introduction — 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="2. Lexical analysis" href="lexical_analysis.html" /> <link rel="prev" title="The Python Language Reference" href="index.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="file:///usr/share/doc/python2.7/html/reference/introduction.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div id="outdated-warning" style="padding: .5em; text-align: center; background-color: #FFBABA; color: #6A0E0E;"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/reference/introduction.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="lexical_analysis.html" title="2. Lexical analysis" accesskey="N">next</a> |</li> <li class="right" > <a href="index.html" title="The Python Language Reference" 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">The Python Language Reference</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="introduction"> <span id="id1"></span><h1>1. Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h1> <p>This reference manual describes the Python programming language. It is not intended as a tutorial.</p> <p>While I am trying to be as precise as possible, I chose to use English rather than formal specifications for everything except syntax and lexical analysis. This should make the document more understandable to the average reader, but will leave room for ambiguities. Consequently, if you were coming from Mars and tried to re-implement Python from this document alone, you might have to guess things and in fact you would probably end up implementing quite a different language. On the other hand, if you are using Python and wonder what the precise rules about a particular area of the language are, you should definitely be able to find them here. If you would like to see a more formal definition of the language, maybe you could volunteer your time — or invent a cloning machine :-).</p> <p>It is dangerous to add too many implementation details to a language reference document — the implementation may change, and other implementations of the same language may work differently. On the other hand, there is currently only one Python implementation in widespread use (although alternate implementations exist), and its particular quirks are sometimes worth being mentioned, especially where the implementation imposes additional limitations. Therefore, you’ll find short “implementation notes” sprinkled throughout the text.</p> <p>Every Python implementation comes with a number of built-in and standard modules. These are documented in <a class="reference internal" href="../library/index.html#library-index"><span class="std std-ref">The Python Standard Library</span></a>. A few built-in modules are mentioned when they interact in a significant way with the language definition.</p> <div class="section" id="alternate-implementations"> <span id="implementations"></span><h2>1.1. Alternate Implementations<a class="headerlink" href="#alternate-implementations" title="Permalink to this headline">¶</a></h2> <p>Though there is one Python implementation which is by far the most popular, there are some alternate implementations which are of particular interest to different audiences.</p> <p>Known implementations include:</p> <dl class="docutils"> <dt>CPython</dt> <dd>This is the original and most-maintained implementation of Python, written in C. New language features generally appear here first.</dd> <dt>Jython</dt> <dd>Python implemented in Java. This implementation can be used as a scripting language for Java applications, or can be used to create applications using the Java class libraries. It is also often used to create tests for Java libraries. More information can be found at <a class="reference external" href="http://www.jython.org/">the Jython website</a>.</dd> <dt>Python for .NET</dt> <dd>This implementation actually uses the CPython implementation, but is a managed .NET application and makes .NET libraries available. It was created by Brian Lloyd. For more information, see the <a class="reference external" href="https://pythonnet.github.io/">Python for .NET home page</a>.</dd> <dt>IronPython</dt> <dd>An alternate Python for .NET. Unlike Python.NET, this is a complete Python implementation that generates IL, and compiles Python code directly to .NET assemblies. It was created by Jim Hugunin, the original creator of Jython. For more information, see <a class="reference external" href="http://ironpython.net/">the IronPython website</a>.</dd> <dt>PyPy</dt> <dd>An implementation of Python written completely in Python. It supports several advanced features not found in other implementations like stackless support and a Just in Time compiler. One of the goals of the project is to encourage experimentation with the language itself by making it easier to modify the interpreter (since it is written in Python). Additional information is available on <a class="reference external" href="http://pypy.org/">the PyPy project’s home page</a>.</dd> </dl> <p>Each of these implementations varies in some way from the language as documented in this manual, or introduces specific information beyond what’s covered in the standard Python documentation. Please refer to the implementation-specific documentation to determine what else you need to know about the specific implementation you’re using.</p> </div> <div class="section" id="notation"> <span id="id2"></span><h2>1.2. Notation<a class="headerlink" href="#notation" title="Permalink to this headline">¶</a></h2> <p id="index-0">The descriptions of lexical analysis and syntax use a modified BNF grammar notation. This uses the following style of definition:</p> <pre> <strong id="grammar-token-name">name </strong> ::= <a class="reference internal" href="#grammar-token-lc-letter"><code class="xref docutils literal notranslate"><span class="pre">lc_letter</span></code></a> (<a class="reference internal" href="#grammar-token-lc-letter"><code class="xref docutils literal notranslate"><span class="pre">lc_letter</span></code></a> | "_")* <strong id="grammar-token-lc-letter">lc_letter</strong> ::= "a"..."z" </pre> <p>The first line says that a <code class="docutils literal notranslate"><span class="pre">name</span></code> is an <code class="docutils literal notranslate"><span class="pre">lc_letter</span></code> followed by a sequence of zero or more <code class="docutils literal notranslate"><span class="pre">lc_letter</span></code>s and underscores. An <code class="docutils literal notranslate"><span class="pre">lc_letter</span></code> in turn is any of the single characters <code class="docutils literal notranslate"><span class="pre">'a'</span></code> through <code class="docutils literal notranslate"><span class="pre">'z'</span></code>. (This rule is actually adhered to for the names defined in lexical and grammar rules in this document.)</p> <p>Each rule begins with a name (which is the name defined by the rule) and <code class="docutils literal notranslate"><span class="pre">::=</span></code>. A vertical bar (<code class="docutils literal notranslate"><span class="pre">|</span></code>) is used to separate alternatives; it is the least binding operator in this notation. A star (<code class="docutils literal notranslate"><span class="pre">*</span></code>) means zero or more repetitions of the preceding item; likewise, a plus (<code class="docutils literal notranslate"><span class="pre">+</span></code>) means one or more repetitions, and a phrase enclosed in square brackets (<code class="docutils literal notranslate"><span class="pre">[</span> <span class="pre">]</span></code>) means zero or one occurrences (in other words, the enclosed phrase is optional). The <code class="docutils literal notranslate"><span class="pre">*</span></code> and <code class="docutils literal notranslate"><span class="pre">+</span></code> operators bind as tightly as possible; parentheses are used for grouping. Literal strings are enclosed in quotes. White space is only meaningful to separate tokens. Rules are normally contained on a single line; rules with many alternatives may be formatted alternatively with each line after the first beginning with a vertical bar.</p> <p id="index-1">In lexical definitions (as the example above), two more conventions are used: Two literal characters separated by three dots mean a choice of any single character in the given (inclusive) range of ASCII characters. A phrase between angular brackets (<code class="docutils literal notranslate"><span class="pre"><...></span></code>) gives an informal description of the symbol defined; e.g., this could be used to describe the notion of ‘control character’ if needed.</p> <p>Even though the notation used is almost the same, there is a big difference between the meaning of lexical and syntactic definitions: a lexical definition operates on the individual characters of the input source, while a syntax definition operates on the stream of tokens generated by the lexical analysis. All uses of BNF in the next chapter (“Lexical Analysis”) are lexical definitions; uses in subsequent chapters are syntactic definitions.</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="#">1. Introduction</a><ul> <li><a class="reference internal" href="#alternate-implementations">1.1. Alternate Implementations</a></li> <li><a class="reference internal" href="#notation">1.2. Notation</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="index.html" title="previous chapter">The Python Language Reference</a></p> <h4>Next topic</h4> <p class="topless"><a href="lexical_analysis.html" title="next chapter">2. 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