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<section id="introduction">
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<span id="id1"></span><h1><span class="section-number">1. </span>Introduction<a class="headerlink" href="#introduction" title="Link to this heading">¶</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
|
||
are mentioned when they interact in a significant way with the language
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||
definition.</p>
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||
<section id="alternate-implementations">
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||
<span id="implementations"></span><h2><span class="section-number">1.1. </span>Alternate Implementations<a class="headerlink" href="#alternate-implementations" title="Link to this heading">¶</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="simple">
|
||
<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>
|
||
</dd>
|
||
<dt>Jython</dt><dd><p>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="https://www.jython.org/">the Jython website</a>.</p>
|
||
</dd>
|
||
<dt>Python for .NET</dt><dd><p>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>.</p>
|
||
</dd>
|
||
<dt>IronPython</dt><dd><p>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="https://ironpython.net/">the IronPython website</a>.</p>
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||
</dd>
|
||
<dt>PyPy</dt><dd><p>An implementation of Python written completely in Python. It supports several
|
||
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
|
||
experimentation with the language itself by making it easier to modify the
|
||
interpreter (since it is written in Python). Additional information is
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||
available on <a class="reference external" href="https://pypy.org/">the PyPy project’s home page</a>.</p>
|
||
</dd>
|
||
</dl>
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||
<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
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||
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>
|
||
</section>
|
||
<section id="notation">
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<span id="id2"></span><h2><span class="section-number">1.2. </span>Notation<a class="headerlink" href="#notation" title="Link to this heading">¶</a></h2>
|
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<p id="index-0">The descriptions of lexical analysis and syntax use a modified
|
||
<a class="reference external" href="https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_form">Backus–Naur form (BNF)</a> grammar
|
||
notation. This uses the following style of definition:</p>
|
||
<pre>
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<strong id="grammar-token-notation-name">name</strong> ::= <a class="reference internal" href="#grammar-token-notation-lc_letter"><code class="xref docutils literal notranslate"><span class="pre">lc_letter</span></code></a> (<a class="reference internal" href="#grammar-token-notation-lc_letter"><code class="xref docutils literal notranslate"><span class="pre">lc_letter</span></code></a> | "_")*
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<strong id="grammar-token-notation-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
|
||
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
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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
|
||
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
|
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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:
|
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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>
|
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</section>
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<li><a class="reference internal" href="#alternate-implementations">1.1. Alternate Implementations</a></li>
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