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<div class="section" id="introduction">
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<span id="id1"></span><h1>1. Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h1>
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<p>This reference manual describes the Python programming language. It is not
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intended as a tutorial.</p>
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<p>While I am trying to be as precise as possible, I chose to use English rather
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than formal specifications for everything except syntax and lexical analysis.
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This should make the document more understandable to the average reader, but
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will leave room for ambiguities. Consequently, if you were coming from Mars and
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tried to re-implement Python from this document alone, you might have to guess
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things and in fact you would probably end up implementing quite a different
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language. On the other hand, if you are using Python and wonder what the precise
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rules about a particular area of the language are, you should definitely be able
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to find them here. If you would like to see a more formal definition of the
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language, maybe you could volunteer your time — or invent a cloning machine
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:-).</p>
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<p>It is dangerous to add too many implementation details to a language reference
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document — the implementation may change, and other implementations of the
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same language may work differently. On the other hand, CPython is the one
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Python implementation in widespread use (although alternate implementations
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continue to gain support), and its particular quirks are sometimes worth being
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mentioned, especially where the implementation imposes additional limitations.
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Therefore, you’ll find short “implementation notes” sprinkled throughout the
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text.</p>
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<p>Every Python implementation comes with a number of built-in and standard
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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
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are mentioned when they interact in a significant way with the language
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definition.</p>
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<div class="section" id="alternate-implementations">
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<span id="implementations"></span><h2>1.1. Alternate Implementations<a class="headerlink" href="#alternate-implementations" title="Permalink to this headline">¶</a></h2>
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<p>Though there is one Python implementation which is by far the most popular,
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there are some alternate implementations which are of particular interest to
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different audiences.</p>
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<p>Known implementations include:</p>
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<dl class="simple">
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<dt>CPython</dt><dd><p>This is the original and most-maintained implementation of Python, written in C.
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New language features generally appear here first.</p>
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</dd>
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<dt>Jython</dt><dd><p>Python implemented in Java. This implementation can be used as a scripting
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language for Java applications, or can be used to create applications using the
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Java class libraries. It is also often used to create tests for Java libraries.
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More information can be found at <a class="reference external" href="http://www.jython.org/">the Jython website</a>.</p>
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</dd>
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<dt>Python for .NET</dt><dd><p>This implementation actually uses the CPython implementation, but is a managed
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.NET application and makes .NET libraries available. It was created by Brian
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Lloyd. For more information, see the <a class="reference external" href="https://pythonnet.github.io/">Python for .NET home page</a>.</p>
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</dd>
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<dt>IronPython</dt><dd><p>An alternate Python for .NET. Unlike Python.NET, this is a complete Python
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implementation that generates IL, and compiles Python code directly to .NET
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assemblies. It was created by Jim Hugunin, the original creator of Jython. For
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more information, see <a class="reference external" href="http://ironpython.net/">the IronPython website</a>.</p>
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</dd>
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<dt>PyPy</dt><dd><p>An implementation of Python written completely in Python. It supports several
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advanced features not found in other implementations like stackless support
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and a Just in Time compiler. One of the goals of the project is to encourage
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experimentation with the language itself by making it easier to modify the
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interpreter (since it is written in Python). Additional information is
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available on <a class="reference external" href="http://pypy.org/">the PyPy project’s home page</a>.</p>
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</dd>
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</dl>
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<p>Each of these implementations varies in some way from the language as documented
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in this manual, or introduces specific information beyond what’s covered in the
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standard Python documentation. Please refer to the implementation-specific
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documentation to determine what else you need to know about the specific
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implementation you’re using.</p>
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</div>
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<div class="section" id="notation">
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<span id="id2"></span><h2>1.2. Notation<a class="headerlink" href="#notation" title="Permalink to this headline">¶</a></h2>
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<p id="index-0">The descriptions of lexical analysis and syntax use a modified BNF grammar
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notation. This uses the following style of definition:</p>
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<pre>
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<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> | "_")*
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<strong id="grammar-token-lc-letter">lc_letter</strong> ::= "a"..."z"
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</pre>
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<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
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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
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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
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adhered to for the names defined in lexical and grammar rules in this document.)</p>
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<p>Each rule begins with a name (which is the name defined by the rule) and
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<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
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least binding operator in this notation. A star (<code class="docutils literal notranslate"><span class="pre">*</span></code>) means zero or more
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repetitions of the preceding item; likewise, a plus (<code class="docutils literal notranslate"><span class="pre">+</span></code>) means one or more
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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
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one occurrences (in other words, the enclosed phrase is optional). The <code class="docutils literal notranslate"><span class="pre">*</span></code>
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and <code class="docutils literal notranslate"><span class="pre">+</span></code> operators bind as tightly as possible; parentheses are used for
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grouping. Literal strings are enclosed in quotes. White space is only
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meaningful to separate tokens. Rules are normally contained on a single line;
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rules with many alternatives may be formatted alternatively with each line after
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the first beginning with a vertical bar.</p>
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<p id="index-1">In lexical definitions (as the example above), two more conventions are used:
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Two literal characters separated by three dots mean a choice of any single
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character in the given (inclusive) range of ASCII characters. A phrase between
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angular brackets (<code class="docutils literal notranslate"><span class="pre"><...></span></code>) gives an informal description of the symbol
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defined; e.g., this could be used to describe the notion of ‘control character’
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if needed.</p>
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<p>Even though the notation used is almost the same, there is a big difference
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between the meaning of lexical and syntactic definitions: a lexical definition
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operates on the individual characters of the input source, while a syntax
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definition operates on the stream of tokens generated by the lexical analysis.
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All uses of BNF in the next chapter (“Lexical Analysis”) are lexical
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definitions; uses in subsequent chapters are syntactic definitions.</p>
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</div>
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<h3><a href="../contents.html">Table of Contents</a></h3>
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<ul>
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<li><a class="reference internal" href="#">1. Introduction</a><ul>
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<li><a class="reference internal" href="#alternate-implementations">1.1. Alternate Implementations</a></li>
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<li><a class="reference internal" href="#notation">1.2. Notation</a></li>
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