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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>8. Compound statements — Python 3.6.7 documentation</title> <link rel="stylesheet" href="../_static/pydoctheme.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/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 3.6.7 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="9. Top-level components" href="toplevel_components.html" /> <link rel="prev" title="7. Simple statements" href="simple_stmts.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/reference/compound_stmts.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <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="toplevel_components.html" title="9. Top-level components" accesskey="N">next</a> |</li> <li class="right" > <a href="simple_stmts.html" title="7. Simple statements" 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">3.6.7 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">The Python Language Reference</a> »</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" 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> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="compound-statements"> <span id="compound"></span><h1>8. Compound statements<a class="headerlink" href="#compound-statements" title="Permalink to this headline">¶</a></h1> <p id="index-0">Compound statements contain (groups of) other statements; they affect or control the execution of those other statements in some way. In general, compound statements span multiple lines, although in simple incarnations a whole compound statement may be contained in one line.</p> <p>The <a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a>, <a class="reference internal" href="#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a> and <a class="reference internal" href="#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> statements implement traditional control flow constructs. <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> specifies exception handlers and/or cleanup code for a group of statements, while the <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement allows the execution of initialization and finalization code around a block of code. Function and class definitions are also syntactically compound statements.</p> <p id="index-1">A compound statement consists of one or more ‘clauses.’ A clause consists of a header and a ‘suite.’ The clause headers of a particular compound statement are all at the same indentation level. Each clause header begins with a uniquely identifying keyword and ends with a colon. A suite is a group of statements controlled by a clause. A suite can be one or more semicolon-separated simple statements on the same line as the header, following the header’s colon, or it can be one or more indented statements on subsequent lines. Only the latter form of a suite can contain nested compound statements; the following is illegal, mostly because it wouldn’t be clear to which <a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> clause a following <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> clause would belong:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">test1</span><span class="p">:</span> <span class="k">if</span> <span class="n">test2</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> </pre></div> </div> <p>Also note that the semicolon binds tighter than the colon in this context, so that in the following example, either all or none of the <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate"><span class="pre">print()</span></code></a> calls are executed:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">x</span> <span class="o"><</span> <span class="n">y</span> <span class="o"><</span> <span class="n">z</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">);</span> <span class="nb">print</span><span class="p">(</span><span class="n">y</span><span class="p">);</span> <span class="nb">print</span><span class="p">(</span><span class="n">z</span><span class="p">)</span> </pre></div> </div> <p>Summarizing:</p> <pre> <strong id="grammar-token-compound_stmt">compound_stmt</strong> ::= <a class="reference internal" href="#grammar-token-if_stmt"><code class="xref docutils literal notranslate"><span class="pre">if_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-while_stmt"><code class="xref docutils literal notranslate"><span class="pre">while_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-for_stmt"><code class="xref docutils literal notranslate"><span class="pre">for_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-try_stmt"><code class="xref docutils literal notranslate"><span class="pre">try_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-with_stmt"><code class="xref docutils literal notranslate"><span class="pre">with_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-funcdef"><code class="xref docutils literal notranslate"><span class="pre">funcdef</span></code></a> | <a class="reference internal" href="#grammar-token-classdef"><code class="xref docutils literal notranslate"><span class="pre">classdef</span></code></a> | <a class="reference internal" href="#grammar-token-async_with_stmt"><code class="xref docutils literal notranslate"><span class="pre">async_with_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-async_for_stmt"><code class="xref docutils literal notranslate"><span class="pre">async_for_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-async_funcdef"><code class="xref docutils literal notranslate"><span class="pre">async_funcdef</span></code></a> <strong id="grammar-token-suite">suite </strong> ::= <a class="reference internal" href="#grammar-token-stmt_list"><code class="xref docutils literal notranslate"><span class="pre">stmt_list</span></code></a> NEWLINE | NEWLINE INDENT <a class="reference internal" href="#grammar-token-statement"><code class="xref docutils literal notranslate"><span class="pre">statement</span></code></a>+ DEDENT <strong id="grammar-token-statement">statement </strong> ::= <a class="reference internal" href="#grammar-token-stmt_list"><code class="xref docutils literal notranslate"><span class="pre">stmt_list</span></code></a> NEWLINE | <a class="reference internal" href="#grammar-token-compound_stmt"><code class="xref docutils literal notranslate"><span class="pre">compound_stmt</span></code></a> <strong id="grammar-token-stmt_list">stmt_list </strong> ::= <a class="reference internal" href="simple_stmts.html#grammar-token-simple_stmt"><code class="xref docutils literal notranslate"><span class="pre">simple_stmt</span></code></a> (";" <a class="reference internal" href="simple_stmts.html#grammar-token-simple_stmt"><code class="xref docutils literal notranslate"><span class="pre">simple_stmt</span></code></a>)* [";"] </pre> <p id="index-2">Note that statements always end in a <code class="docutils literal notranslate"><span class="pre">NEWLINE</span></code> possibly followed by a <code class="docutils literal notranslate"><span class="pre">DEDENT</span></code>. Also note that optional continuation clauses always begin with a keyword that cannot start a statement, thus there are no ambiguities (the ‘dangling <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a>’ problem is solved in Python by requiring nested <a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> statements to be indented).</p> <p>The formatting of the grammar rules in the following sections places each clause on a separate line for clarity.</p> <div class="section" id="the-if-statement"> <span id="else"></span><span id="elif"></span><span id="if"></span><h2>8.1. The <a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> statement<a class="headerlink" href="#the-if-statement" title="Permalink to this headline">¶</a></h2> <p id="index-3">The <a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> statement is used for conditional execution:</p> <pre> <strong id="grammar-token-if_stmt">if_stmt</strong> ::= "if" <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a> ("elif" <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a>)* ["else" ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a>] </pre> <p>It selects exactly one of the suites by evaluating the expressions one by one until one is found to be true (see section <a class="reference internal" href="expressions.html#booleans"><span class="std std-ref">Boolean operations</span></a> for the definition of true and false); then that suite is executed (and no other part of the <a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> statement is executed or evaluated). If all expressions are false, the suite of the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> clause, if present, is executed.</p> </div> <div class="section" id="the-while-statement"> <span id="while"></span><h2>8.2. The <a class="reference internal" href="#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a> statement<a class="headerlink" href="#the-while-statement" title="Permalink to this headline">¶</a></h2> <p id="index-4">The <a class="reference internal" href="#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a> statement is used for repeated execution as long as an expression is true:</p> <pre> <strong id="grammar-token-while_stmt">while_stmt</strong> ::= "while" <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a> ["else" ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a>] </pre> <p>This repeatedly tests the expression and, if it is true, executes the first suite; if the expression is false (which may be the first time it is tested) the suite of the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> clause, if present, is executed and the loop terminates.</p> <p id="index-5">A <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> statement executed in the first suite terminates the loop without executing the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> clause’s suite. A <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> statement executed in the first suite skips the rest of the suite and goes back to testing the expression.</p> </div> <div class="section" id="the-for-statement"> <span id="for"></span><h2>8.3. The <a class="reference internal" href="#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> statement<a class="headerlink" href="#the-for-statement" title="Permalink to this headline">¶</a></h2> <p id="index-6">The <a class="reference internal" href="#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> statement is used to iterate over the elements of a sequence (such as a string, tuple or list) or other iterable object:</p> <pre> <strong id="grammar-token-for_stmt">for_stmt</strong> ::= "for" <a class="reference internal" href="simple_stmts.html#grammar-token-target_list"><code class="xref docutils literal notranslate"><span class="pre">target_list</span></code></a> "in" <a class="reference internal" href="expressions.html#grammar-token-expression_list"><code class="xref docutils literal notranslate"><span class="pre">expression_list</span></code></a> ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a> ["else" ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a>] </pre> <p>The expression list is evaluated once; it should yield an iterable object. An iterator is created for the result of the <code class="docutils literal notranslate"><span class="pre">expression_list</span></code>. The suite is then executed once for each item provided by the iterator, in the order returned by the iterator. Each item in turn is assigned to the target list using the standard rules for assignments (see <a class="reference internal" href="simple_stmts.html#assignment"><span class="std std-ref">Assignment statements</span></a>), and then the suite is executed. When the items are exhausted (which is immediately when the sequence is empty or an iterator raises a <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a> exception), the suite in the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> clause, if present, is executed, and the loop terminates.</p> <p id="index-7">A <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> statement executed in the first suite terminates the loop without executing the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> clause’s suite. A <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> statement executed in the first suite skips the rest of the suite and continues with the next item, or with the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> clause if there is no next item.</p> <p>The for-loop makes assignments to the variables(s) in the target list. This overwrites all previous assignments to those variables including those made in the suite of the for-loop:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span> <span class="nb">print</span><span class="p">(</span><span class="n">i</span><span class="p">)</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">5</span> <span class="c1"># this will not affect the for-loop</span> <span class="c1"># because i will be overwritten with the next</span> <span class="c1"># index in the range</span> </pre></div> </div> <p id="index-8">Names in the target list are not deleted when the loop is finished, but if the sequence is empty, they will not have been assigned to at all by the loop. Hint: the built-in function <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-func docutils literal notranslate"><span class="pre">range()</span></code></a> returns an iterator of integers suitable to emulate the effect of Pascal’s <code class="docutils literal notranslate"><span class="pre">for</span> <span class="pre">i</span> <span class="pre">:=</span> <span class="pre">a</span> <span class="pre">to</span> <span class="pre">b</span> <span class="pre">do</span></code>; e.g., <code class="docutils literal notranslate"><span class="pre">list(range(3))</span></code> returns the list <code class="docutils literal notranslate"><span class="pre">[0,</span> <span class="pre">1,</span> <span class="pre">2]</span></code>.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p id="index-9">There is a subtlety when the sequence is being modified by the loop (this can only occur for mutable sequences, e.g. lists). An internal counter is used to keep track of which item is used next, and this is incremented on each iteration. When this counter has reached the length of the sequence the loop terminates. This means that if the suite deletes the current (or a previous) item from the sequence, the next item will be skipped (since it gets the index of the current item which has already been treated). Likewise, if the suite inserts an item in the sequence before the current item, the current item will be treated again the next time through the loop. This can lead to nasty bugs that can be avoided by making a temporary copy using a slice of the whole sequence, e.g.,</p> <div class="last highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">a</span><span class="p">[:]:</span> <span class="k">if</span> <span class="n">x</span> <span class="o"><</span> <span class="mi">0</span><span class="p">:</span> <span class="n">a</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> </pre></div> </div> </div> </div> <div class="section" id="the-try-statement"> <span id="finally"></span><span id="except"></span><span id="try"></span><h2>8.4. The <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement<a class="headerlink" href="#the-try-statement" title="Permalink to this headline">¶</a></h2> <span class="target" id="index-10"></span><p id="index-11">The <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement specifies exception handlers and/or cleanup code for a group of statements:</p> <pre> <strong id="grammar-token-try_stmt">try_stmt </strong> ::= <a class="reference internal" href="#grammar-token-try1_stmt"><code class="xref docutils literal notranslate"><span class="pre">try1_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-try2_stmt"><code class="xref docutils literal notranslate"><span class="pre">try2_stmt</span></code></a> <strong id="grammar-token-try1_stmt">try1_stmt</strong> ::= "try" ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a> ("except" [<a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a>]] ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a>)+ ["else" ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a>] ["finally" ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a>] <strong id="grammar-token-try2_stmt">try2_stmt</strong> ::= "try" ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a> "finally" ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a> </pre> <p>The <a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> clause(s) specify one or more exception handlers. When no exception occurs in the <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> clause, no exception handler is executed. When an exception occurs in the <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> suite, a search for an exception handler is started. This search inspects the except clauses in turn until one is found that matches the exception. An expression-less except clause, if present, must be last; it matches any exception. For an except clause with an expression, that expression is evaluated, and the clause matches the exception if the resulting object is “compatible” with the exception. An object is compatible with an exception if it is the class or a base class of the exception object or a tuple containing an item compatible with the exception.</p> <p>If no except clause matches the exception, the search for an exception handler continues in the surrounding code and on the invocation stack. <a class="footnote-reference" href="#id5" id="id1">[1]</a></p> <p>If the evaluation of an expression in the header of an except clause raises an exception, the original search for a handler is canceled and a search starts for the new exception in the surrounding code and on the call stack (it is treated as if the entire <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement raised the exception).</p> <p>When a matching except clause is found, the exception is assigned to the target specified after the <a class="reference internal" href="#as"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code></a> keyword in that except clause, if present, and the except clause’s suite is executed. All except clauses must have an executable block. When the end of this block is reached, execution continues normally after the entire try statement. (This means that if two nested handlers exist for the same exception, and the exception occurs in the try clause of the inner handler, the outer handler will not handle the exception.)</p> <p>When an exception has been assigned using <code class="docutils literal notranslate"><span class="pre">as</span> <span class="pre">target</span></code>, it is cleared at the end of the except clause. This is as if</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">except</span> <span class="n">E</span> <span class="k">as</span> <span class="n">N</span><span class="p">:</span> <span class="n">foo</span> </pre></div> </div> <p>was translated to</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">except</span> <span class="n">E</span> <span class="k">as</span> <span class="n">N</span><span class="p">:</span> <span class="k">try</span><span class="p">:</span> <span class="n">foo</span> <span class="k">finally</span><span class="p">:</span> <span class="k">del</span> <span class="n">N</span> </pre></div> </div> <p>This means the exception must be assigned to a different name to be able to refer to it after the except clause. Exceptions are cleared because with the traceback attached to them, they form a reference cycle with the stack frame, keeping all locals in that frame alive until the next garbage collection occurs.</p> <p id="index-12">Before an except clause’s suite is executed, details about the exception are stored in the <a class="reference internal" href="../library/sys.html#module-sys" title="sys: Access system-specific parameters and functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">sys</span></code></a> module and can be accessed via <a class="reference internal" href="../library/sys.html#sys.exc_info" title="sys.exc_info"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.exc_info()</span></code></a>. <a class="reference internal" href="../library/sys.html#sys.exc_info" title="sys.exc_info"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.exc_info()</span></code></a> returns a 3-tuple consisting of the exception class, the exception instance and a traceback object (see section <a class="reference internal" href="datamodel.html#types"><span class="std std-ref">The standard type hierarchy</span></a>) identifying the point in the program where the exception occurred. <a class="reference internal" href="../library/sys.html#sys.exc_info" title="sys.exc_info"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.exc_info()</span></code></a> values are restored to their previous values (before the call) when returning from a function that handled an exception.</p> <p id="index-13">The optional <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> clause is executed if and when control flows off the end of the <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> clause. <a class="footnote-reference" href="#id6" id="id2">[2]</a> Exceptions in the <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> clause are not handled by the preceding <a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> clauses.</p> <p id="index-14">If <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> is present, it specifies a ‘cleanup’ handler. The <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> clause is executed, including any <a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> and <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> clauses. If an exception occurs in any of the clauses and is not handled, the exception is temporarily saved. The <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause is executed. If there is a saved exception it is re-raised at the end of the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause. If the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause raises another exception, the saved exception is set as the context of the new exception. If the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause executes a <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> or <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> statement, the saved exception is discarded:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">f</span><span class="p">():</span> <span class="gp">... </span> <span class="k">try</span><span class="p">:</span> <span class="gp">... </span> <span class="mi">1</span><span class="o">/</span><span class="mi">0</span> <span class="gp">... </span> <span class="k">finally</span><span class="p">:</span> <span class="gp">... </span> <span class="k">return</span> <span class="mi">42</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">f</span><span class="p">()</span> <span class="go">42</span> </pre></div> </div> <p>The exception information is not available to the program during execution of the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause.</p> <p id="index-15">When a <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a>, <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> or <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> statement is executed in the <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> suite of a <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a>…<a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> statement, the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause is also executed ‘on the way out.’ A <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> statement is illegal in the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause. (The reason is a problem with the current implementation — this restriction may be lifted in the future).</p> <p>The return value of a function is determined by the last <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement executed. Since the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause always executes, a <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement executed in the <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause will always be the last one executed:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">foo</span><span class="p">():</span> <span class="gp">... </span> <span class="k">try</span><span class="p">:</span> <span class="gp">... </span> <span class="k">return</span> <span class="s1">'try'</span> <span class="gp">... </span> <span class="k">finally</span><span class="p">:</span> <span class="gp">... </span> <span class="k">return</span> <span class="s1">'finally'</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">foo</span><span class="p">()</span> <span class="go">'finally'</span> </pre></div> </div> <p>Additional information on exceptions can be found in section <a class="reference internal" href="executionmodel.html#exceptions"><span class="std std-ref">Exceptions</span></a>, and information on using the <a class="reference internal" href="simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> statement to generate exceptions may be found in section <a class="reference internal" href="simple_stmts.html#raise"><span class="std std-ref">The raise statement</span></a>.</p> </div> <div class="section" id="the-with-statement"> <span id="as"></span><span id="with"></span><h2>8.5. The <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement<a class="headerlink" href="#the-with-statement" title="Permalink to this headline">¶</a></h2> <p id="index-16">The <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement is used to wrap the execution of a block with methods defined by a context manager (see section <a class="reference internal" href="datamodel.html#context-managers"><span class="std std-ref">With Statement Context Managers</span></a>). This allows common <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a>…<a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a>…<a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> usage patterns to be encapsulated for convenient reuse.</p> <pre> <strong id="grammar-token-with_stmt">with_stmt</strong> ::= "with" <a class="reference internal" href="#grammar-token-with_item"><code class="xref docutils literal notranslate"><span class="pre">with_item</span></code></a> ("," <a class="reference internal" href="#grammar-token-with_item"><code class="xref docutils literal notranslate"><span class="pre">with_item</span></code></a>)* ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a> <strong id="grammar-token-with_item">with_item</strong> ::= <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ["as" <a class="reference internal" href="simple_stmts.html#grammar-token-target"><code class="xref docutils literal notranslate"><span class="pre">target</span></code></a>] </pre> <p>The execution of the <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement with one “item” proceeds as follows:</p> <ol class="arabic"> <li><p class="first">The context expression (the expression given in the <a class="reference internal" href="#grammar-token-with_item"><code class="xref std std-token docutils literal notranslate"><span class="pre">with_item</span></code></a>) is evaluated to obtain a context manager.</p> </li> <li><p class="first">The context manager’s <a class="reference internal" href="datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> is loaded for later use.</p> </li> <li><p class="first">The context manager’s <a class="reference internal" href="datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> method is invoked.</p> </li> <li><p class="first">If a target was included in the <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement, the return value from <a class="reference internal" href="datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> is assigned to it.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">The <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement guarantees that if the <a class="reference internal" href="datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> method returns without an error, then <a class="reference internal" href="datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> will always be called. Thus, if an error occurs during the assignment to the target list, it will be treated the same as an error occurring within the suite would be. See step 6 below.</p> </div> </li> <li><p class="first">The suite is executed.</p> </li> <li><p class="first">The context manager’s <a class="reference internal" href="datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> method is invoked. If an exception caused the suite to be exited, its type, value, and traceback are passed as arguments to <a class="reference internal" href="datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a>. Otherwise, three <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a> arguments are supplied.</p> <p>If the suite was exited due to an exception, and the return value from the <a class="reference internal" href="datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> method was false, the exception is reraised. If the return value was true, the exception is suppressed, and execution continues with the statement following the <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement.</p> <p>If the suite was exited for any reason other than an exception, the return value from <a class="reference internal" href="datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> is ignored, and execution proceeds at the normal location for the kind of exit that was taken.</p> </li> </ol> <p>With more than one item, the context managers are processed as if multiple <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statements were nested:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">A</span><span class="p">()</span> <span class="k">as</span> <span class="n">a</span><span class="p">,</span> <span class="n">B</span><span class="p">()</span> <span class="k">as</span> <span class="n">b</span><span class="p">:</span> <span class="n">suite</span> </pre></div> </div> <p>is equivalent to</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">A</span><span class="p">()</span> <span class="k">as</span> <span class="n">a</span><span class="p">:</span> <span class="k">with</span> <span class="n">B</span><span class="p">()</span> <span class="k">as</span> <span class="n">b</span><span class="p">:</span> <span class="n">suite</span> </pre></div> </div> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.1: </span>Support for multiple context expressions.</p> </div> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><span class="target" id="index-17"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0343"><strong>PEP 343</strong></a> - The “with” statement</dt> <dd>The specification, background, and examples for the Python <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement.</dd> </dl> </div> </div> <div class="section" id="function-definitions"> <span id="def"></span><span id="function"></span><span id="index-18"></span><h2>8.6. Function definitions<a class="headerlink" href="#function-definitions" title="Permalink to this headline">¶</a></h2> <p id="index-19">A function definition defines a user-defined function object (see section <a class="reference internal" href="datamodel.html#types"><span class="std std-ref">The standard type hierarchy</span></a>):</p> <pre> <strong id="grammar-token-funcdef">funcdef </strong> ::= [<a class="reference internal" href="#grammar-token-decorators"><code class="xref docutils literal notranslate"><span class="pre">decorators</span></code></a>] "def" <a class="reference internal" href="#grammar-token-funcname"><code class="xref docutils literal notranslate"><span class="pre">funcname</span></code></a> "(" [<a class="reference internal" href="#grammar-token-parameter_list"><code class="xref docutils literal notranslate"><span class="pre">parameter_list</span></code></a>] ")" ["->" <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>] ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a> <strong id="grammar-token-decorators">decorators </strong> ::= <a class="reference internal" href="#grammar-token-decorator"><code class="xref docutils literal notranslate"><span class="pre">decorator</span></code></a>+ <strong id="grammar-token-decorator">decorator </strong> ::= "@" <a class="reference internal" href="#grammar-token-dotted_name"><code class="xref docutils literal notranslate"><span class="pre">dotted_name</span></code></a> ["(" [<a class="reference internal" href="expressions.html#grammar-token-argument_list"><code class="xref docutils literal notranslate"><span class="pre">argument_list</span></code></a> [","]] ")"] NEWLINE <strong id="grammar-token-dotted_name">dotted_name </strong> ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> ("." <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a>)* <strong id="grammar-token-parameter_list">parameter_list </strong> ::= <a class="reference internal" href="#grammar-token-defparameter"><code class="xref docutils literal notranslate"><span class="pre">defparameter</span></code></a> ("," <a class="reference internal" href="#grammar-token-defparameter"><code class="xref docutils literal notranslate"><span class="pre">defparameter</span></code></a>)* ["," [<a class="reference internal" href="#grammar-token-parameter_list_starargs"><code class="xref docutils literal notranslate"><span class="pre">parameter_list_starargs</span></code></a>]] | <a class="reference internal" href="#grammar-token-parameter_list_starargs"><code class="xref docutils literal notranslate"><span class="pre">parameter_list_starargs</span></code></a> <strong id="grammar-token-parameter_list_starargs">parameter_list_starargs</strong> ::= "*" [<a class="reference internal" href="#grammar-token-parameter"><code class="xref docutils literal notranslate"><span class="pre">parameter</span></code></a>] ("," <a class="reference internal" href="#grammar-token-defparameter"><code class="xref docutils literal notranslate"><span class="pre">defparameter</span></code></a>)* ["," ["**" <a class="reference internal" href="#grammar-token-parameter"><code class="xref docutils literal notranslate"><span class="pre">parameter</span></code></a> [","]]] | "**" <a class="reference internal" href="#grammar-token-parameter"><code class="xref docutils literal notranslate"><span class="pre">parameter</span></code></a> [","] <strong id="grammar-token-parameter">parameter </strong> ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> [":" <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>] <strong id="grammar-token-defparameter">defparameter </strong> ::= <a class="reference internal" href="#grammar-token-parameter"><code class="xref docutils literal notranslate"><span class="pre">parameter</span></code></a> ["=" <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>] <strong id="grammar-token-funcname">funcname </strong> ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> </pre> <p>A function definition is an executable statement. Its execution binds the function name in the current local namespace to a function object (a wrapper around the executable code for the function). This function object contains a reference to the current global namespace as the global namespace to be used when the function is called.</p> <p>The function definition does not execute the function body; this gets executed only when the function is called. <a class="footnote-reference" href="#id7" id="id3">[3]</a></p> <p id="index-20">A function definition may be wrapped by one or more <a class="reference internal" href="../glossary.html#term-decorator"><span class="xref std std-term">decorator</span></a> expressions. Decorator expressions are evaluated when the function is defined, in the scope that contains the function definition. The result must be a callable, which is invoked with the function object as the only argument. The returned value is bound to the function name instead of the function object. Multiple decorators are applied in nested fashion. For example, the following code</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@f1</span><span class="p">(</span><span class="n">arg</span><span class="p">)</span> <span class="nd">@f2</span> <span class="k">def</span> <span class="nf">func</span><span class="p">():</span> <span class="k">pass</span> </pre></div> </div> <p>is roughly equivalent to</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">func</span><span class="p">():</span> <span class="k">pass</span> <span class="n">func</span> <span class="o">=</span> <span class="n">f1</span><span class="p">(</span><span class="n">arg</span><span class="p">)(</span><span class="n">f2</span><span class="p">(</span><span class="n">func</span><span class="p">))</span> </pre></div> </div> <p>except that the original function is not temporarily bound to the name <code class="docutils literal notranslate"><span class="pre">func</span></code>.</p> <p id="index-21">When one or more <a class="reference internal" href="../glossary.html#term-parameter"><span class="xref std std-term">parameters</span></a> have the form <em>parameter</em> <code class="docutils literal notranslate"><span class="pre">=</span></code> <em>expression</em>, the function is said to have “default parameter values.” For a parameter with a default value, the corresponding <a class="reference internal" href="../glossary.html#term-argument"><span class="xref std std-term">argument</span></a> may be omitted from a call, in which case the parameter’s default value is substituted. If a parameter has a default value, all following parameters up until the “<code class="docutils literal notranslate"><span class="pre">*</span></code>” must also have a default value — this is a syntactic restriction that is not expressed by the grammar.</p> <p><strong>Default parameter values are evaluated from left to right when the function definition is executed.</strong> This means that the expression is evaluated once, when the function is defined, and that the same “pre-computed” value is used for each call. This is especially important to understand when a default parameter is a mutable object, such as a list or a dictionary: if the function modifies the object (e.g. by appending an item to a list), the default value is in effect modified. This is generally not what was intended. A way around this is to use <code class="docutils literal notranslate"><span class="pre">None</span></code> as the default, and explicitly test for it in the body of the function, e.g.:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">whats_on_the_telly</span><span class="p">(</span><span class="n">penguin</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span> <span class="k">if</span> <span class="n">penguin</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span> <span class="n">penguin</span> <span class="o">=</span> <span class="p">[]</span> <span class="n">penguin</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="s2">"property of the zoo"</span><span class="p">)</span> <span class="k">return</span> <span class="n">penguin</span> </pre></div> </div> <p id="index-22">Function call semantics are described in more detail in section <a class="reference internal" href="expressions.html#calls"><span class="std std-ref">Calls</span></a>. A function call always assigns values to all parameters mentioned in the parameter list, either from position arguments, from keyword arguments, or from default values. If the form “<code class="docutils literal notranslate"><span class="pre">*identifier</span></code>” is present, it is initialized to a tuple receiving any excess positional parameters, defaulting to the empty tuple. If the form “<code class="docutils literal notranslate"><span class="pre">**identifier</span></code>” is present, it is initialized to a new ordered mapping receiving any excess keyword arguments, defaulting to a new empty mapping of the same type. Parameters after “<code class="docutils literal notranslate"><span class="pre">*</span></code>” or “<code class="docutils literal notranslate"><span class="pre">*identifier</span></code>” are keyword-only parameters and may only be passed used keyword arguments.</p> <p id="index-23">Parameters may have annotations of the form “<code class="docutils literal notranslate"><span class="pre">:</span> <span class="pre">expression</span></code>” following the parameter name. Any parameter may have an annotation even those of the form <code class="docutils literal notranslate"><span class="pre">*identifier</span></code> or <code class="docutils literal notranslate"><span class="pre">**identifier</span></code>. Functions may have “return” annotation of the form “<code class="docutils literal notranslate"><span class="pre">-></span> <span class="pre">expression</span></code>” after the parameter list. These annotations can be any valid Python expression and are evaluated when the function definition is executed. Annotations may be evaluated in a different order than they appear in the source code. The presence of annotations does not change the semantics of a function. The annotation values are available as values of a dictionary keyed by the parameters’ names in the <code class="xref py py-attr docutils literal notranslate"><span class="pre">__annotations__</span></code> attribute of the function object.</p> <p id="index-24">It is also possible to create anonymous functions (functions not bound to a name), for immediate use in expressions. This uses lambda expressions, described in section <a class="reference internal" href="expressions.html#lambda"><span class="std std-ref">Lambdas</span></a>. Note that the lambda expression is merely a shorthand for a simplified function definition; a function defined in a “<a class="reference internal" href="#def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">def</span></code></a>” statement can be passed around or assigned to another name just like a function defined by a lambda expression. The “<a class="reference internal" href="#def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">def</span></code></a>” form is actually more powerful since it allows the execution of multiple statements and annotations.</p> <p><strong>Programmer’s note:</strong> Functions are first-class objects. A “<code class="docutils literal notranslate"><span class="pre">def</span></code>” statement executed inside a function definition defines a local function that can be returned or passed around. Free variables used in the nested function can access the local variables of the function containing the def. See section <a class="reference internal" href="executionmodel.html#naming"><span class="std std-ref">Naming and binding</span></a> for details.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><span class="target" id="index-25"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3107"><strong>PEP 3107</strong></a> - Function Annotations</dt> <dd>The original specification for function annotations.</dd> </dl> </div> </div> <div class="section" id="class-definitions"> <span id="class"></span><h2>8.7. Class definitions<a class="headerlink" href="#class-definitions" title="Permalink to this headline">¶</a></h2> <p id="index-26">A class definition defines a class object (see section <a class="reference internal" href="datamodel.html#types"><span class="std std-ref">The standard type hierarchy</span></a>):</p> <pre> <strong id="grammar-token-classdef">classdef </strong> ::= [<a class="reference internal" href="#grammar-token-decorators"><code class="xref docutils literal notranslate"><span class="pre">decorators</span></code></a>] "class" <a class="reference internal" href="#grammar-token-classname"><code class="xref docutils literal notranslate"><span class="pre">classname</span></code></a> [<a class="reference internal" href="#grammar-token-inheritance"><code class="xref docutils literal notranslate"><span class="pre">inheritance</span></code></a>] ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a> <strong id="grammar-token-inheritance">inheritance</strong> ::= "(" [<a class="reference internal" href="expressions.html#grammar-token-argument_list"><code class="xref docutils literal notranslate"><span class="pre">argument_list</span></code></a>] ")" <strong id="grammar-token-classname">classname </strong> ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> </pre> <p>A class definition is an executable statement. The inheritance list usually gives a list of base classes (see <a class="reference internal" href="datamodel.html#metaclasses"><span class="std std-ref">Metaclasses</span></a> for more advanced uses), so each item in the list should evaluate to a class object which allows subclassing. Classes without an inheritance list inherit, by default, from the base class <a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate"><span class="pre">object</span></code></a>; hence,</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Foo</span><span class="p">:</span> <span class="k">pass</span> </pre></div> </div> <p>is equivalent to</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Foo</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span> <span class="k">pass</span> </pre></div> </div> <p>The class’s suite is then executed in a new execution frame (see <a class="reference internal" href="executionmodel.html#naming"><span class="std std-ref">Naming and binding</span></a>), using a newly created local namespace and the original global namespace. (Usually, the suite contains mostly function definitions.) When the class’s suite finishes execution, its execution frame is discarded but its local namespace is saved. <a class="footnote-reference" href="#id8" id="id4">[4]</a> A class object is then created using the inheritance list for the base classes and the saved local namespace for the attribute dictionary. The class name is bound to this class object in the original local namespace.</p> <p>The order in which attributes are defined in the class body is preserved in the new class’s <code class="docutils literal notranslate"><span class="pre">__dict__</span></code>. Note that this is reliable only right after the class is created and only for classes that were defined using the definition syntax.</p> <p>Class creation can be customized heavily using <a class="reference internal" href="datamodel.html#metaclasses"><span class="std std-ref">metaclasses</span></a>.</p> <p>Classes can also be decorated: just like when decorating functions,</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@f1</span><span class="p">(</span><span class="n">arg</span><span class="p">)</span> <span class="nd">@f2</span> <span class="k">class</span> <span class="nc">Foo</span><span class="p">:</span> <span class="k">pass</span> </pre></div> </div> <p>is roughly equivalent to</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Foo</span><span class="p">:</span> <span class="k">pass</span> <span class="n">Foo</span> <span class="o">=</span> <span class="n">f1</span><span class="p">(</span><span class="n">arg</span><span class="p">)(</span><span class="n">f2</span><span class="p">(</span><span class="n">Foo</span><span class="p">))</span> </pre></div> </div> <p>The evaluation rules for the decorator expressions are the same as for function decorators. The result is then bound to the class name.</p> <p><strong>Programmer’s note:</strong> Variables defined in the class definition are class attributes; they are shared by instances. Instance attributes can be set in a method with <code class="docutils literal notranslate"><span class="pre">self.name</span> <span class="pre">=</span> <span class="pre">value</span></code>. Both class and instance attributes are accessible through the notation “<code class="docutils literal notranslate"><span class="pre">self.name</span></code>”, and an instance attribute hides a class attribute with the same name when accessed in this way. Class attributes can be used as defaults for instance attributes, but using mutable values there can lead to unexpected results. <a class="reference internal" href="datamodel.html#descriptors"><span class="std std-ref">Descriptors</span></a> can be used to create instance variables with different implementation details.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <p class="last"><span class="target" id="index-27"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3115"><strong>PEP 3115</strong></a> - Metaclasses in Python 3 <span class="target" id="index-28"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3129"><strong>PEP 3129</strong></a> - Class Decorators</p> </div> </div> <div class="section" id="coroutines"> <h2>8.8. Coroutines<a class="headerlink" href="#coroutines" title="Permalink to this headline">¶</a></h2> <div class="versionadded"> <p><span class="versionmodified">New in version 3.5.</span></p> </div> <div class="section" id="coroutine-function-definition"> <span id="async-def"></span><span id="index-29"></span><h3>8.8.1. Coroutine function definition<a class="headerlink" href="#coroutine-function-definition" title="Permalink to this headline">¶</a></h3> <pre> <strong id="grammar-token-async_funcdef">async_funcdef</strong> ::= [<a class="reference internal" href="#grammar-token-decorators"><code class="xref docutils literal notranslate"><span class="pre">decorators</span></code></a>] "async" "def" <a class="reference internal" href="#grammar-token-funcname"><code class="xref docutils literal notranslate"><span class="pre">funcname</span></code></a> "(" [<a class="reference internal" href="#grammar-token-parameter_list"><code class="xref docutils literal notranslate"><span class="pre">parameter_list</span></code></a>] ")" ["->" <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>] ":" <a class="reference internal" href="#grammar-token-suite"><code class="xref docutils literal notranslate"><span class="pre">suite</span></code></a> </pre> <p id="index-30">Execution of Python coroutines can be suspended and resumed at many points (see <a class="reference internal" href="../glossary.html#term-coroutine"><span class="xref std std-term">coroutine</span></a>). In the body of a coroutine, any <code class="docutils literal notranslate"><span class="pre">await</span></code> and <code class="docutils literal notranslate"><span class="pre">async</span></code> identifiers become reserved keywords; <a class="reference internal" href="expressions.html#await"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">await</span></code></a> expressions, <a class="reference internal" href="#async-for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code></a> and <a class="reference internal" href="#async-with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span></code></a> can only be used in coroutine bodies.</p> <p>Functions defined with <code class="docutils literal notranslate"><span class="pre">async</span> <span class="pre">def</span></code> syntax are always coroutine functions, even if they do not contain <code class="docutils literal notranslate"><span class="pre">await</span></code> or <code class="docutils literal notranslate"><span class="pre">async</span></code> keywords.</p> <p>It is a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a> to use <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span></code> expressions in <code class="docutils literal notranslate"><span class="pre">async</span> <span class="pre">def</span></code> coroutines.</p> <p>An example of a coroutine function:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">def</span> <span class="nf">func</span><span class="p">(</span><span class="n">param1</span><span class="p">,</span> <span class="n">param2</span><span class="p">):</span> <span class="n">do_stuff</span><span class="p">()</span> <span class="k">await</span> <span class="n">some_coroutine</span><span class="p">()</span> </pre></div> </div> </div> <div class="section" id="the-async-for-statement"> <span id="async-for"></span><span id="index-31"></span><h3>8.8.2. The <a class="reference internal" href="#async-for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code></a> statement<a class="headerlink" href="#the-async-for-statement" title="Permalink to this headline">¶</a></h3> <pre> <strong id="grammar-token-async_for_stmt">async_for_stmt</strong> ::= "async" <a class="reference internal" href="#grammar-token-for_stmt"><code class="xref docutils literal notranslate"><span class="pre">for_stmt</span></code></a> </pre> <p>An <a class="reference internal" href="../glossary.html#term-asynchronous-iterable"><span class="xref std std-term">asynchronous iterable</span></a> is able to call asynchronous code in its <em>iter</em> implementation, and <a class="reference internal" href="../glossary.html#term-asynchronous-iterator"><span class="xref std std-term">asynchronous iterator</span></a> can call asynchronous code in its <em>next</em> method.</p> <p>The <code class="docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code> statement allows convenient iteration over asynchronous iterators.</p> <p>The following code:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">for</span> <span class="n">TARGET</span> <span class="ow">in</span> <span class="n">ITER</span><span class="p">:</span> <span class="n">BLOCK</span> <span class="k">else</span><span class="p">:</span> <span class="n">BLOCK2</span> </pre></div> </div> <p>Is semantically equivalent to:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nb">iter</span> <span class="o">=</span> <span class="p">(</span><span class="n">ITER</span><span class="p">)</span> <span class="nb">iter</span> <span class="o">=</span> <span class="nb">type</span><span class="p">(</span><span class="nb">iter</span><span class="p">)</span><span class="o">.</span><span class="fm">__aiter__</span><span class="p">(</span><span class="nb">iter</span><span class="p">)</span> <span class="n">running</span> <span class="o">=</span> <span class="kc">True</span> <span class="k">while</span> <span class="n">running</span><span class="p">:</span> <span class="k">try</span><span class="p">:</span> <span class="n">TARGET</span> <span class="o">=</span> <span class="k">await</span> <span class="nb">type</span><span class="p">(</span><span class="nb">iter</span><span class="p">)</span><span class="o">.</span><span class="fm">__anext__</span><span class="p">(</span><span class="nb">iter</span><span class="p">)</span> <span class="k">except</span> <span class="n">StopAsyncIteration</span><span class="p">:</span> <span class="n">running</span> <span class="o">=</span> <span class="kc">False</span> <span class="k">else</span><span class="p">:</span> <span class="n">BLOCK</span> <span class="k">else</span><span class="p">:</span> <span class="n">BLOCK2</span> </pre></div> </div> <p>See also <a class="reference internal" href="datamodel.html#object.__aiter__" title="object.__aiter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__aiter__()</span></code></a> and <a class="reference internal" href="datamodel.html#object.__anext__" title="object.__anext__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__anext__()</span></code></a> for details.</p> <p>It is a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a> to use <code class="docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code> statement outside of an <a class="reference internal" href="#async-def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">def</span></code></a> function.</p> </div> <div class="section" id="the-async-with-statement"> <span id="async-with"></span><span id="index-32"></span><h3>8.8.3. The <a class="reference internal" href="#async-with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span></code></a> statement<a class="headerlink" href="#the-async-with-statement" title="Permalink to this headline">¶</a></h3> <pre> <strong id="grammar-token-async_with_stmt">async_with_stmt</strong> ::= "async" <a class="reference internal" href="#grammar-token-with_stmt"><code class="xref docutils literal notranslate"><span class="pre">with_stmt</span></code></a> </pre> <p>An <a class="reference internal" href="../glossary.html#term-asynchronous-context-manager"><span class="xref std std-term">asynchronous context manager</span></a> is a <a class="reference internal" href="../glossary.html#term-context-manager"><span class="xref std std-term">context manager</span></a> that is able to suspend execution in its <em>enter</em> and <em>exit</em> methods.</p> <p>The following code:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">with</span> <span class="n">EXPR</span> <span class="k">as</span> <span class="n">VAR</span><span class="p">:</span> <span class="n">BLOCK</span> </pre></div> </div> <p>Is semantically equivalent to:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">mgr</span> <span class="o">=</span> <span class="p">(</span><span class="n">EXPR</span><span class="p">)</span> <span class="n">aexit</span> <span class="o">=</span> <span class="nb">type</span><span class="p">(</span><span class="n">mgr</span><span class="p">)</span><span class="o">.</span><span class="fm">__aexit__</span> <span class="n">aenter</span> <span class="o">=</span> <span class="nb">type</span><span class="p">(</span><span class="n">mgr</span><span class="p">)</span><span class="o">.</span><span class="fm">__aenter__</span><span class="p">(</span><span class="n">mgr</span><span class="p">)</span> <span class="n">VAR</span> <span class="o">=</span> <span class="k">await</span> <span class="n">aenter</span> <span class="k">try</span><span class="p">:</span> <span class="n">BLOCK</span> <span class="k">except</span><span class="p">:</span> <span class="k">if</span> <span class="ow">not</span> <span class="k">await</span> <span class="n">aexit</span><span class="p">(</span><span class="n">mgr</span><span class="p">,</span> <span class="o">*</span><span class="n">sys</span><span class="o">.</span><span class="n">exc_info</span><span class="p">()):</span> <span class="k">raise</span> <span class="k">else</span><span class="p">:</span> <span class="k">await</span> <span class="n">aexit</span><span class="p">(</span><span class="n">mgr</span><span class="p">,</span> <span class="kc">None</span><span class="p">,</span> <span class="kc">None</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span> </pre></div> </div> <p>See also <a class="reference internal" href="datamodel.html#object.__aenter__" title="object.__aenter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__aenter__()</span></code></a> and <a class="reference internal" href="datamodel.html#object.__aexit__" title="object.__aexit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__aexit__()</span></code></a> for details.</p> <p>It is a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a> to use <code class="docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span></code> statement outside of an <a class="reference internal" href="#async-def"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">def</span></code></a> function.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <p class="last"><span class="target" id="index-33"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0492"><strong>PEP 492</strong></a> - Coroutines with async and await syntax</p> </div> <p class="rubric">Footnotes</p> <table class="docutils footnote" frame="void" id="id5" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>The exception is propagated to the invocation stack unless there is a <a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause which happens to raise another exception. That new exception causes the old one to be lost.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id6" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id2">[2]</a></td><td>Currently, control “flows off the end” except in the case of an exception or the execution of a <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a>, <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a>, or <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> statement.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id7" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id3">[3]</a></td><td>A string literal appearing as the first statement in the function body is transformed into the function’s <code class="docutils literal notranslate"><span class="pre">__doc__</span></code> attribute and therefore the function’s <a class="reference internal" href="../glossary.html#term-docstring"><span class="xref std std-term">docstring</span></a>.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id8" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id4">[4]</a></td><td>A string literal appearing as the first statement in the class body is transformed into the namespace’s <code class="docutils literal notranslate"><span class="pre">__doc__</span></code> item and therefore the class’s <a class="reference internal" href="../glossary.html#term-docstring"><span class="xref std std-term">docstring</span></a>.</td></tr> </tbody> </table> </div> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">8. Compound statements</a><ul> <li><a class="reference internal" href="#the-if-statement">8.1. The <code class="docutils literal notranslate"><span class="pre">if</span></code> statement</a></li> <li><a class="reference internal" href="#the-while-statement">8.2. The <code class="docutils literal notranslate"><span class="pre">while</span></code> statement</a></li> <li><a class="reference internal" href="#the-for-statement">8.3. The <code class="docutils literal notranslate"><span class="pre">for</span></code> statement</a></li> <li><a class="reference internal" href="#the-try-statement">8.4. The <code class="docutils literal notranslate"><span class="pre">try</span></code> statement</a></li> <li><a class="reference internal" href="#the-with-statement">8.5. The <code class="docutils literal notranslate"><span class="pre">with</span></code> statement</a></li> <li><a class="reference internal" href="#function-definitions">8.6. Function definitions</a></li> <li><a class="reference internal" href="#class-definitions">8.7. Class definitions</a></li> <li><a class="reference internal" href="#coroutines">8.8. Coroutines</a><ul> <li><a class="reference internal" href="#coroutine-function-definition">8.8.1. Coroutine function definition</a></li> <li><a class="reference internal" href="#the-async-for-statement">8.8.2. The <code class="docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code> statement</a></li> <li><a class="reference internal" href="#the-async-with-statement">8.8.3. The <code class="docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span></code> statement</a></li> </ul> </li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="simple_stmts.html" title="previous chapter">7. Simple statements</a></p> <h4>Next topic</h4> <p class="topless"><a href="toplevel_components.html" title="next chapter">9. 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