CINXE.COM

<!DOCTYPE html> <html lang="en" data-content_root="../"> <head> <meta charset="utf-8" /> <meta name="viewport" content="width=device-width, initial-scale=1.0" /><meta name="viewport" content="width=device-width, initial-scale=1" /> <meta property="og:title" content="8. Compound statements" /> <meta property="og:type" content="website" /> <meta property="og:url" content="https://docs.python.org/3/reference/compound_stmts.html" /> <meta property="og:site_name" content="Python documentation" /> <meta property="og:description" content="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 i..." /> <meta property="og:image" content="https://docs.python.org/3/_static/og-image.png" /> <meta property="og:image:alt" content="Python documentation" /> <meta name="description" content="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 i..." /> <meta property="og:image:width" content="200" /> <meta property="og:image:height" content="200" /> <meta name="theme-color" content="#3776ab" /> <title>8. Compound statements — Python 3.13.0 documentation</title><meta name="viewport" content="width=device-width, initial-scale=1.0"> <link rel="stylesheet" type="text/css" href="../_static/pygments.css?v=80d5e7a1" /> <link rel="stylesheet" type="text/css" href="../_static/pydoctheme.css?v=41b4f12d" /> <link rel="stylesheet" type="text/css" href="../_static/sidebar-wrap.css?v=65806c5a" /> <link id="pygments_dark_css" media="(prefers-color-scheme: dark)" rel="stylesheet" type="text/css" href="../_static/pygments_dark.css?v=b20cc3f5" /> <script src="../_static/documentation_options.js?v=6aaf4047"></script> <script src="../_static/doctools.js?v=9bcbadda"></script> <script src="../_static/sphinx_highlight.js?v=dc90522c"></script> <script src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 3.13.0 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" /> <script defer data-domain="docs.python.org" src="https://plausible.io/js/script.js"></script> <link rel="canonical" href="https://docs.python.org/3/reference/compound_stmts.html" /> <style> @media only screen { table.full-width-table { width: 100%; } } </style> <link rel="stylesheet" href="../_static/pydoctheme_dark.css" media="(prefers-color-scheme: dark)" id="pydoctheme_dark_css"> <link rel="shortcut icon" type="image/png" href="../_static/py.svg" /> <script type="text/javascript" src="../_static/copybutton.js"></script> <script type="text/javascript" src="../_static/menu.js"></script> <script type="text/javascript" src="../_static/search-focus.js"></script> <script type="text/javascript" src="../_static/themetoggle.js"></script> <script type="text/javascript" src="../_static/rtd_switcher.js"></script> <meta name="readthedocs-addons-api-version" content="1"> </head> <body> <div class="mobile-nav"> <input type="checkbox" id="menuToggler" class="toggler__input" aria-controls="navigation" aria-pressed="false" aria-expanded="false" role="button" aria-label="Menu" /> <nav class="nav-content" role="navigation"> <label for="menuToggler" class="toggler__label"> </label> <a href="https://www.python.org/" class="nav-logo"> <img src="../_static/py.svg" alt="Python logo"/> </a> <form role="search" class="search" action="../search.html" method="get"> <svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 24 24" class="search-icon"> <path fill-rule="nonzero" fill="currentColor" d="M15.5 14h-.79l-.28-.27a6.5 6.5 0 001.48-5.34c-.47-2.78-2.79-5-5.59-5.34a6.505 6.505 0 00-7.27 7.27c.34 2.8 2.56 5.12 5.34 5.59a6.5 6.5 0 005.34-1.48l.27.28v.79l4.25 4.25c.41.41 1.08.41 1.49 0 .41-.41.41-1.08 0-1.49L15.5 14zm-6 0C7.01 14 5 11.99 5 9.5S7.01 5 9.5 5 14 7.01 14 9.5 11.99 14 9.5 14z"></path> </svg> <input placeholder="Quick search" aria-label="Quick search" type="search" name="q" /> <input type="submit" value="Go"/> </form> </nav> <div class="menu-wrapper"> <nav class="menu" role="navigation" aria-label="main navigation"> <div class="language_switcher_placeholder"></div> <label class="theme-selector-label"> Theme <select class="theme-selector" oninput="activateTheme(this.value)"> <option value="auto" selected>Auto</option> <option value="light">Light</option> <option value="dark">Dark</option> </select> </label> <div> <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="xref std std-keyword docutils literal notranslate">if</code> statement</a></li> <li><a class="reference internal" href="#the-while-statement">8.2. The <code class="xref std std-keyword docutils literal notranslate">while</code> statement</a></li> <li><a class="reference internal" href="#the-for-statement">8.3. The <code class="xref std std-keyword docutils literal notranslate">for</code> statement</a></li> <li><a class="reference internal" href="#the-try-statement">8.4. The <code class="xref std std-keyword docutils literal notranslate">try</code> statement</a><ul> <li><a class="reference internal" href="#except-clause">8.4.1. <code class="xref std std-keyword docutils literal notranslate">except</code> clause</a></li> <li><a class="reference internal" href="#except-star">8.4.2. <code class="xref std std-keyword docutils literal notranslate">except*</code> clause</a></li> <li><a class="reference internal" href="#else-clause">8.4.3. <code class="xref std std-keyword docutils literal notranslate">else</code> clause</a></li> <li><a class="reference internal" href="#finally-clause">8.4.4. <code class="xref std std-keyword docutils literal notranslate">finally</code> clause</a></li> </ul> </li> <li><a class="reference internal" href="#the-with-statement">8.5. The <code class="xref std std-keyword docutils literal notranslate">with</code> statement</a></li> <li><a class="reference internal" href="#the-match-statement">8.6. The <code class="xref std std-keyword docutils literal notranslate">match</code> statement</a><ul> <li><a class="reference internal" href="#overview">8.6.1. Overview</a></li> <li><a class="reference internal" href="#guards">8.6.2. Guards</a></li> <li><a class="reference internal" href="#irrefutable-case-blocks">8.6.3. Irrefutable Case Blocks</a></li> <li><a class="reference internal" href="#patterns">8.6.4. Patterns</a><ul> <li><a class="reference internal" href="#or-patterns">8.6.4.1. OR Patterns</a></li> <li><a class="reference internal" href="#as-patterns">8.6.4.2. AS Patterns</a></li> <li><a class="reference internal" href="#literal-patterns">8.6.4.3. Literal Patterns</a></li> <li><a class="reference internal" href="#capture-patterns">8.6.4.4. Capture Patterns</a></li> <li><a class="reference internal" href="#wildcard-patterns">8.6.4.5. Wildcard Patterns</a></li> <li><a class="reference internal" href="#value-patterns">8.6.4.6. Value Patterns</a></li> <li><a class="reference internal" href="#group-patterns">8.6.4.7. Group Patterns</a></li> <li><a class="reference internal" href="#sequence-patterns">8.6.4.8. Sequence Patterns</a></li> <li><a class="reference internal" href="#mapping-patterns">8.6.4.9. Mapping Patterns</a></li> <li><a class="reference internal" href="#class-patterns">8.6.4.10. Class Patterns</a></li> </ul> </li> </ul> </li> <li><a class="reference internal" href="#function-definitions">8.7. Function definitions</a></li> <li><a class="reference internal" href="#class-definitions">8.8. Class definitions</a></li> <li><a class="reference internal" href="#coroutines">8.9. Coroutines</a><ul> <li><a class="reference internal" href="#coroutine-function-definition">8.9.1. Coroutine function definition</a></li> <li><a class="reference internal" href="#the-async-for-statement">8.9.2. The <code class="xref std std-keyword docutils literal notranslate">async for</code> statement</a></li> <li><a class="reference internal" href="#the-async-with-statement">8.9.3. The <code class="xref std std-keyword docutils literal notranslate">async with</code> statement</a></li> </ul> </li> <li><a class="reference internal" href="#type-parameter-lists">8.10. Type parameter lists</a><ul> <li><a class="reference internal" href="#generic-functions">8.10.1. Generic functions</a></li> <li><a class="reference internal" href="#generic-classes">8.10.2. Generic classes</a></li> <li><a class="reference internal" href="#generic-type-aliases">8.10.3. Generic type aliases</a></li> </ul> </li> </ul> </li> </ul> </div> <div> <h4>Previous topic</h4> <a href="simple_stmts.html" title="previous chapter">7. Simple statements</a> </div> <div> <h4>Next topic</h4> <a href="toplevel_components.html" title="next chapter">9. Top-level components</a> </div> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li> <a href="https://github.com/python/cpython/blob/main/Doc/reference/compound_stmts.rst" rel="nofollow">Show Source </a> </li> </ul> </div> </nav> </div> </div> <div class="related" role="navigation" aria-label="Related"> <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.svg" alt="Python logo" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li class="switchers"> <div class="language_switcher_placeholder"></div> <div class="version_switcher_placeholder"></div> </li> <li> </li> <li id="cpython-language-and-version"> <a href="../index.html">3.13.0 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">The Python Language Reference</a> »</li> <li class="nav-item nav-item-this"><a href="">8. Compound statements</a></li> <li class="right"> <div class="inline-search" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" aria-label="Quick search" type="search" name="q" id="search-box" /> <input type="submit" value="Go" /> </form> </div> | </li> <li class="right"> <label class="theme-selector-label"> Theme <select class="theme-selector" oninput="activateTheme(this.value)"> <option value="auto" selected>Auto</option> <option value="light">Light</option> <option value="dark">Dark</option> </select> </label> |</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <section id="compound-statements"> <h1>8. Compound statements<a class="headerlink" href="#compound-statements" title="Link to this heading">¶</a></h1> 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. The <a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate">if</code></a>, <a class="reference internal" href="#while"><code class="xref std std-keyword docutils literal notranslate">while</code></a> and <a class="reference internal" href="#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> statements implement traditional control flow constructs. <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</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">with</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. 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">if</code></a> clause a following <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clause would belong: <div class="highlight-python3 notranslate"><div class="highlight"><pre>if test1: if test2: print(x) </pre></div> </div> 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">print()</code></a> calls are executed: <div class="highlight-python3 notranslate"><div class="highlight"><pre>if x < y < z: print(x); print(y); print(z) </pre></div> </div> Summarizing: <pre> compound_stmt ::= <a class="reference internal" href="#grammar-token-python-grammar-if_stmt"><code class="xref docutils literal notranslate">if_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-while_stmt"><code class="xref docutils literal notranslate">while_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-for_stmt"><code class="xref docutils literal notranslate">for_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-try_stmt"><code class="xref docutils literal notranslate">try_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-with_stmt"><code class="xref docutils literal notranslate">with_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-match_stmt"><code class="xref docutils literal notranslate">match_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-funcdef"><code class="xref docutils literal notranslate">funcdef</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-classdef"><code class="xref docutils literal notranslate">classdef</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-async_with_stmt"><code class="xref docutils literal notranslate">async_with_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-async_for_stmt"><code class="xref docutils literal notranslate">async_for_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-async_funcdef"><code class="xref docutils literal notranslate">async_funcdef</code></a> suite ::= <a class="reference internal" href="#grammar-token-python-grammar-stmt_list"><code class="xref docutils literal notranslate">stmt_list</code></a> NEWLINE | NEWLINE INDENT <a class="reference internal" href="#grammar-token-python-grammar-statement"><code class="xref docutils literal notranslate">statement</code></a>+ DEDENT statement ::= <a class="reference internal" href="#grammar-token-python-grammar-stmt_list"><code class="xref docutils literal notranslate">stmt_list</code></a> NEWLINE | <a class="reference internal" href="#grammar-token-python-grammar-compound_stmt"><code class="xref docutils literal notranslate">compound_stmt</code></a> stmt_list ::= <a class="reference internal" href="simple_stmts.html#grammar-token-python-grammar-simple_stmt"><code class="xref docutils literal notranslate">simple_stmt</code></a> (";" <a class="reference internal" href="simple_stmts.html#grammar-token-python-grammar-simple_stmt"><code class="xref docutils literal notranslate">simple_stmt</code></a>)* [";"] </pre> Note that statements always end in a <code class="docutils literal notranslate">NEWLINE</code> possibly followed by a <code class="docutils literal notranslate">DEDENT</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">else</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">if</code></a> statements to be indented). The formatting of the grammar rules in the following sections places each clause on a separate line for clarity. <section id="the-if-statement"> <h2>8.1. The <code class="xref std std-keyword docutils literal notranslate">if</code> statement<a class="headerlink" href="#the-if-statement" title="Link to this heading">¶</a></h2> The <a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate">if</code></a> statement is used for conditional execution: <pre> if_stmt ::= "if" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-assignment_expression"><code class="xref docutils literal notranslate">assignment_expression</code></a> ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a> ("elif" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-assignment_expression"><code class="xref docutils literal notranslate">assignment_expression</code></a> ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a>)* ["else" ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a>] </pre> 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">Boolean operations</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">if</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">else</code></a> clause, if present, is executed. </section> <section id="the-while-statement"> <h2>8.2. The <code class="xref std std-keyword docutils literal notranslate">while</code> statement<a class="headerlink" href="#the-while-statement" title="Link to this heading">¶</a></h2> The <a class="reference internal" href="#while"><code class="xref std std-keyword docutils literal notranslate">while</code></a> statement is used for repeated execution as long as an expression is true: <pre> while_stmt ::= "while" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-assignment_expression"><code class="xref docutils literal notranslate">assignment_expression</code></a> ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a> ["else" ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a>] </pre> 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 <code class="xref std std-keyword docutils literal notranslate">else</code> clause, if present, is executed and the loop terminates. A <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a> statement executed in the first suite terminates the loop without executing the <code class="xref std std-keyword docutils literal notranslate">else</code> clause’s suite. A <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> statement executed in the first suite skips the rest of the suite and goes back to testing the expression. </section> <section id="the-for-statement"> <h2>8.3. The <code class="xref std std-keyword docutils literal notranslate">for</code> statement<a class="headerlink" href="#the-for-statement" title="Link to this heading">¶</a></h2> The <a class="reference internal" href="#for"><code class="xref std std-keyword docutils literal notranslate">for</code></a> statement is used to iterate over the elements of a sequence (such as a string, tuple or list) or other iterable object: <pre> for_stmt ::= "for" <a class="reference internal" href="simple_stmts.html#grammar-token-python-grammar-target_list"><code class="xref docutils literal notranslate">target_list</code></a> "in" <code class="xref docutils literal notranslate">starred_list</code> ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a> ["else" ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a>] </pre> The <code class="docutils literal notranslate">starred_list</code> expression is evaluated once; it should yield an <a class="reference internal" href="../glossary.html#term-iterable">iterable</a> object. An <a class="reference internal" href="../glossary.html#term-iterator">iterator</a> is created for that iterable. The first item provided by the iterator is then assigned to the target list using the standard rules for assignments (see <a class="reference internal" href="simple_stmts.html#assignment">Assignment statements</a>), and the suite is executed. This repeats for each item provided by the iterator. When the iterator is exhausted, the suite in the <code class="xref std std-keyword docutils literal notranslate">else</code> clause, if present, is executed, and the loop terminates. A <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a> statement executed in the first suite terminates the loop without executing the <code class="xref std std-keyword docutils literal notranslate">else</code> clause’s suite. A <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> statement executed in the first suite skips the rest of the suite and continues with the next item, or with the <code class="xref std std-keyword docutils literal notranslate">else</code> clause if there is no next item. The for-loop makes assignments to the variables in the target list. This overwrites all previous assignments to those variables including those made in the suite of the for-loop: <div class="highlight-python3 notranslate"><div class="highlight"><pre>for i in range(10): print(i) i = 5 # this will not affect the for-loop # because i will be overwritten with the next # index in the range </pre></div> </div> 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 type <a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-func docutils literal notranslate">range()</code></a> represents immutable arithmetic sequences of integers. For instance, iterating <code class="docutils literal notranslate">range(3)</code> successively yields 0, 1, and then 2. <div class="versionchanged"> Changed in version 3.11: Starred elements are now allowed in the expression list. </div> </section> <section id="the-try-statement"> <h2>8.4. The <code class="xref std std-keyword docutils literal notranslate">try</code> statement<a class="headerlink" href="#the-try-statement" title="Link to this heading">¶</a></h2> The <code class="xref std std-keyword docutils literal notranslate">try</code> statement specifies exception handlers and/or cleanup code for a group of statements: <pre> try_stmt ::= <a class="reference internal" href="#grammar-token-python-grammar-try1_stmt"><code class="xref docutils literal notranslate">try1_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-try2_stmt"><code class="xref docutils literal notranslate">try2_stmt</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-try3_stmt"><code class="xref docutils literal notranslate">try3_stmt</code></a> try1_stmt ::= "try" ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a> ("except" [<a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>]] ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a>)+ ["else" ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a>] ["finally" ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a>] try2_stmt ::= "try" ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a> ("except" "*" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a> ["as" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a>] ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a>)+ ["else" ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a>] ["finally" ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a>] try3_stmt ::= "try" ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a> "finally" ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a> </pre> Additional information on exceptions can be found in section <a class="reference internal" href="executionmodel.html#exceptions">Exceptions</a>, and information on using the <a class="reference internal" href="simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate">raise</code></a> statement to generate exceptions may be found in section <a class="reference internal" href="simple_stmts.html#raise">The raise statement</a>. <section id="except-clause"> <h3>8.4.1. <code class="xref std std-keyword docutils literal notranslate">except</code> clause<a class="headerlink" href="#except-clause" title="Link to this heading">¶</a></h3> The <code class="xref std std-keyword docutils literal notranslate">except</code> 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">try</code></a> clause, no exception handler is executed. When an exception occurs in the <code class="xref std std-keyword docutils literal notranslate">try</code> suite, a search for an exception handler is started. This search inspects the <code class="xref std std-keyword docutils literal notranslate">except</code> clauses in turn until one is found that matches the exception. An expression-less <code class="xref std std-keyword docutils literal notranslate">except</code> clause, if present, must be last; it matches any exception. For an <code class="xref std std-keyword docutils literal notranslate">except</code> clause with an expression, the expression must evaluate to an exception type or a tuple of exception types. The raised exception matches an <code class="xref std std-keyword docutils literal notranslate">except</code> clause whose expression evaluates to the class or a <a class="reference internal" href="../glossary.html#term-abstract-base-class">non-virtual base class</a> of the exception object, or to a tuple that contains such a class. If no <code class="xref std std-keyword docutils literal notranslate">except</code> clause matches the exception, the search for an exception handler continues in the surrounding code and on the invocation stack. <a class="footnote-reference brackets" href="#id20" id="id1" role="doc-noteref">[1]</a> If the evaluation of an expression in the header of an <code class="xref std std-keyword docutils literal notranslate">except</code> 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">try</code></a> statement raised the exception). When a matching <code class="xref std std-keyword docutils literal notranslate">except</code> clause is found, the exception is assigned to the target specified after the <code class="xref std std-keyword docutils literal notranslate">as</code> keyword in that <code class="xref std std-keyword docutils literal notranslate">except</code> clause, if present, and the <code class="xref std std-keyword docutils literal notranslate">except</code> clause’s suite is executed. All <code class="xref std std-keyword docutils literal notranslate">except</code> clauses must have an executable block. When the end of this block is reached, execution continues normally after the entire <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> statement. (This means that if two nested handlers exist for the same exception, and the exception occurs in the <code class="xref std std-keyword docutils literal notranslate">try</code> clause of the inner handler, the outer handler will not handle the exception.) When an exception has been assigned using <code class="docutils literal notranslate">as target</code>, it is cleared at the end of the <code class="xref std std-keyword docutils literal notranslate">except</code> clause. This is as if <div class="highlight-python3 notranslate"><div class="highlight"><pre>except E as N: foo </pre></div> </div> was translated to <div class="highlight-python3 notranslate"><div class="highlight"><pre>except E as N: try: foo finally: del N </pre></div> </div> This means the exception must be assigned to a different name to be able to refer to it after the <code class="xref std std-keyword docutils literal notranslate">except</code> 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. Before an <code class="xref std std-keyword docutils literal notranslate">except</code> clause’s suite is executed, the exception is 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">sys</code></a> module, where it can be accessed from within the body of the <code class="xref std std-keyword docutils literal notranslate">except</code> clause by calling <a class="reference internal" href="../library/sys.html#sys.exception" title="sys.exception"><code class="xref py py-func docutils literal notranslate">sys.exception()</code></a>. When leaving an exception handler, the exception 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">sys</code></a> module is reset to its previous value: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> print(sys.exception()) None >>> try: ... raise TypeError ... except: ... print(repr(sys.exception())) ... try: ... raise ValueError ... except: ... print(repr(sys.exception())) ... print(repr(sys.exception())) ... TypeError() ValueError() TypeError() >>> print(sys.exception()) None </pre></div> </div> </section> <section id="except-star"> <h3>8.4.2. <code class="xref std std-keyword docutils literal notranslate">except*</code> clause<a class="headerlink" href="#except-star" title="Link to this heading">¶</a></h3> The <code class="xref std std-keyword docutils literal notranslate">except*</code> clause(s) are used for handling <a class="reference internal" href="../library/exceptions.html#ExceptionGroup" title="ExceptionGroup"><code class="xref py py-exc docutils literal notranslate">ExceptionGroup</code></a>s. The exception type for matching is interpreted as in the case of <a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a>, but in the case of exception groups we can have partial matches when the type matches some of the exceptions in the group. This means that multiple <code class="xref std std-keyword docutils literal notranslate">except*</code> clauses can execute, each handling part of the exception group. Each clause executes at most once and handles an exception group of all matching exceptions. Each exception in the group is handled by at most one <code class="xref std std-keyword docutils literal notranslate">except*</code> clause, the first that matches it. <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> try: ... raise ExceptionGroup("eg", ... [ValueError(1), TypeError(2), OSError(3), OSError(4)]) ... except* TypeError as e: ... print(f'caught {type(e)} with nested {e.exceptions}') ... except* OSError as e: ... print(f'caught {type(e)} with nested {e.exceptions}') ... caught <class 'ExceptionGroup'> with nested (TypeError(2),) caught <class 'ExceptionGroup'> with nested (OSError(3), OSError(4)) + Exception Group Traceback (most recent call last): | File "<stdin>", line 2, in <module> | ExceptionGroup: eg +-+---------------- 1 ---------------- | ValueError: 1 +------------------------------------ </pre></div> </div> Any remaining exceptions that were not handled by any <code class="xref std std-keyword docutils literal notranslate">except*</code> clause are re-raised at the end, along with all exceptions that were raised from within the <code class="xref std std-keyword docutils literal notranslate">except*</code> clauses. If this list contains more than one exception to reraise, they are combined into an exception group. If the raised exception is not an exception group and its type matches one of the <code class="xref std std-keyword docutils literal notranslate">except*</code> clauses, it is caught and wrapped by an exception group with an empty message string. <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> try: ... raise BlockingIOError ... except* BlockingIOError as e: ... print(repr(e)) ... ExceptionGroup('', (BlockingIOError())) </pre></div> </div> An <code class="xref std std-keyword docutils literal notranslate">except*</code> clause must have a matching expression; it cannot be <code class="docutils literal notranslate">except*:</code>. Furthermore, this expression cannot contain exception group types, because that would have ambiguous semantics. It is not possible to mix <a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> and <code class="xref std std-keyword docutils literal notranslate">except*</code> in the same <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a>. <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a>, <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> and <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a> cannot appear in an <code class="xref std std-keyword docutils literal notranslate">except*</code> clause. </section> <section id="else-clause"> <h3>8.4.3. <code class="xref std std-keyword docutils literal notranslate">else</code> clause<a class="headerlink" href="#else-clause" title="Link to this heading">¶</a></h3> The optional <code class="xref std std-keyword docutils literal notranslate">else</code> clause is executed if the control flow leaves the <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> suite, no exception was raised, and no <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a>, <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a>, or <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a> statement was executed. Exceptions in the <code class="xref std std-keyword docutils literal notranslate">else</code> clause are not handled by the preceding <a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> clauses. </section> <section id="finally-clause"> <h3>8.4.4. <code class="xref std std-keyword docutils literal notranslate">finally</code> clause<a class="headerlink" href="#finally-clause" title="Link to this heading">¶</a></h3> If <code class="xref std std-keyword docutils literal notranslate">finally</code> is present, it specifies a ‘cleanup’ handler. The <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> clause is executed, including any <a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> and <a class="reference internal" href="#else"><code class="xref std std-keyword docutils literal notranslate">else</code></a> clauses. If an exception occurs in any of the clauses and is not handled, the exception is temporarily saved. The <code class="xref std std-keyword docutils literal notranslate">finally</code> clause is executed. If there is a saved exception it is re-raised at the end of the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause. If the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause raises another exception, the saved exception is set as the context of the new exception. If the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause executes a <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a>, <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a> or <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> statement, the saved exception is discarded: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> def f(): ... try: ... 1/0 ... finally: ... return 42 ... >>> f() 42 </pre></div> </div> The exception information is not available to the program during execution of the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause. When a <a class="reference internal" href="simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a>, <a class="reference internal" href="simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a> or <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> statement is executed in the <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a> suite of a <code class="xref std std-keyword docutils literal notranslate">try</code>…<code class="xref std std-keyword docutils literal notranslate">finally</code> statement, the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause is also executed ‘on the way out.’ 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">return</code></a> statement executed. Since the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause always executes, a <code class="xref std std-keyword docutils literal notranslate">return</code> statement executed in the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause will always be the last one executed: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> def foo(): ... try: ... return 'try' ... finally: ... return 'finally' ... >>> foo() 'finally' </pre></div> </div> <div class="versionchanged"> Changed in version 3.8: Prior to Python 3.8, a <a class="reference internal" href="simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> statement was illegal in the <code class="xref std std-keyword docutils literal notranslate">finally</code> clause due to a problem with the implementation. </div> </section> </section> <section id="the-with-statement"> <h2>8.5. The <code class="xref std std-keyword docutils literal notranslate">with</code> statement<a class="headerlink" href="#the-with-statement" title="Link to this heading">¶</a></h2> The <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate">with</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">With Statement Context Managers</a>). This allows common <a class="reference internal" href="#try"><code class="xref std std-keyword docutils literal notranslate">try</code></a>…<a class="reference internal" href="#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a>…<a class="reference internal" href="#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> usage patterns to be encapsulated for convenient reuse. <pre> with_stmt ::= "with" ( "(" <a class="reference internal" href="#grammar-token-python-grammar-with_stmt_contents"><code class="xref docutils literal notranslate">with_stmt_contents</code></a> ","? ")" | <a class="reference internal" href="#grammar-token-python-grammar-with_stmt_contents"><code class="xref docutils literal notranslate">with_stmt_contents</code></a> ) ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a> with_stmt_contents ::= <a class="reference internal" href="#grammar-token-python-grammar-with_item"><code class="xref docutils literal notranslate">with_item</code></a> ("," <a class="reference internal" href="#grammar-token-python-grammar-with_item"><code class="xref docutils literal notranslate">with_item</code></a>)* with_item ::= <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a> ["as" <a class="reference internal" href="simple_stmts.html#grammar-token-python-grammar-target"><code class="xref docutils literal notranslate">target</code></a>] </pre> The execution of the <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate">with</code></a> statement with one “item” proceeds as follows: <ol class="arabic"> <li>The context expression (the expression given in the <a class="reference internal" href="#grammar-token-python-grammar-with_item"><code class="xref std std-token docutils literal notranslate">with_item</code></a>) is evaluated to obtain a context manager.</li> <li>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">__enter__()</code></a> is loaded for later use.</li> <li>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">__exit__()</code></a> is loaded for later use.</li> <li>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">__enter__()</code></a> method is invoked.</li> <li>If a target was included in the <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate">with</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">__enter__()</code></a> is assigned to it. <div class="admonition note"> Note The <a class="reference internal" href="#with"><code class="xref std std-keyword docutils literal notranslate">with</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">__enter__()</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">__exit__()</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 7 below. </div> </li> <li>The suite is executed.</li> <li>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">__exit__()</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">__exit__()</code></a>. Otherwise, three <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a> arguments are supplied. 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">__exit__()</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">with</code></a> statement. 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">__exit__()</code></a> is ignored, and execution proceeds at the normal location for the kind of exit that was taken. </li> </ol> The following code: <div class="highlight-python3 notranslate"><div class="highlight"><pre>with EXPRESSION as TARGET: SUITE </pre></div> </div> is semantically equivalent to: <div class="highlight-python3 notranslate"><div class="highlight"><pre>manager = (EXPRESSION) enter = type(manager).__enter__ exit = type(manager).__exit__ value = enter(manager) try: TARGET = value SUITE except: if not exit(manager, *sys.exc_info()): raise else: exit(manager, None, None, None) </pre></div> </div> 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">with</code></a> statements were nested: <div class="highlight-python3 notranslate"><div class="highlight"><pre>with A() as a, B() as b: SUITE </pre></div> </div> is semantically equivalent to: <div class="highlight-python3 notranslate"><div class="highlight"><pre>with A() as a: with B() as b: SUITE </pre></div> </div> You can also write multi-item context managers in multiple lines if the items are surrounded by parentheses. For example: <div class="highlight-python3 notranslate"><div class="highlight"><pre>with ( A() as a, B() as b, ): SUITE </pre></div> </div> <div class="versionchanged"> Changed in version 3.1: Support for multiple context expressions. </div> <div class="versionchanged"> Changed in version 3.10: Support for using grouping parentheses to break the statement in multiple lines. </div> <div class="admonition seealso"> See also <dl class="simple"> <dt><a class="pep reference external" href="https://peps.python.org/pep-0343/">PEP 343</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">with</code></a> statement. </dd> </dl> </div> </section> <section id="the-match-statement"> <h2>8.6. The <code class="xref std std-keyword docutils literal notranslate">match</code> statement<a class="headerlink" href="#the-match-statement" title="Link to this heading">¶</a></h2> <div class="versionadded" id="index-18"> Added in version 3.10. </div> The match statement is used for pattern matching. Syntax: <pre> match_stmt ::= 'match' <a class="reference internal" href="#grammar-token-python-grammar-subject_expr"><code class="xref docutils literal notranslate">subject_expr</code></a> ":" NEWLINE INDENT <a class="reference internal" href="#grammar-token-python-grammar-case_block"><code class="xref docutils literal notranslate">case_block</code></a>+ DEDENT subject_expr ::= <code class="xref docutils literal notranslate">star_named_expression</code> "," <code class="xref docutils literal notranslate">star_named_expressions</code>? | <code class="xref docutils literal notranslate">named_expression</code> case_block ::= 'case' <a class="reference internal" href="#grammar-token-python-grammar-patterns"><code class="xref docutils literal notranslate">patterns</code></a> [<a class="reference internal" href="#grammar-token-python-grammar-guard"><code class="xref docutils literal notranslate">guard</code></a>] ":" <code class="xref docutils literal notranslate">block</code> </pre> <div class="admonition note"> Note This section uses single quotes to denote <a class="reference internal" href="lexical_analysis.html#soft-keywords">soft keywords</a>. </div> Pattern matching takes a pattern as input (following <code class="docutils literal notranslate">case</code>) and a subject value (following <code class="docutils literal notranslate">match</code>). The pattern (which may contain subpatterns) is matched against the subject value. The outcomes are: <ul class="simple"> <li>A match success or failure (also termed a pattern success or failure).</li> <li>Possible binding of matched values to a name. The prerequisites for this are further discussed below.</li> </ul> The <code class="docutils literal notranslate">match</code> and <code class="docutils literal notranslate">case</code> keywords are <a class="reference internal" href="lexical_analysis.html#soft-keywords">soft keywords</a>. <div class="admonition seealso"> See also <ul class="simple"> <li><a class="pep reference external" href="https://peps.python.org/pep-0634/">PEP 634</a> – Structural Pattern Matching: Specification</li> <li><a class="pep reference external" href="https://peps.python.org/pep-0636/">PEP 636</a> – Structural Pattern Matching: Tutorial</li> </ul> </div> <section id="overview"> <h3>8.6.1. Overview<a class="headerlink" href="#overview" title="Link to this heading">¶</a></h3> Here’s an overview of the logical flow of a match statement: <ol class="arabic"> <li>The subject expression <code class="docutils literal notranslate">subject_expr</code> is evaluated and a resulting subject value obtained. If the subject expression contains a comma, a tuple is constructed using <a class="reference internal" href="../library/stdtypes.html#typesseq-tuple">the standard rules</a>.</li> <li>Each pattern in a <code class="docutils literal notranslate">case_block</code> is attempted to match with the subject value. The specific rules for success or failure are described below. The match attempt can also bind some or all of the standalone names within the pattern. The precise pattern binding rules vary per pattern type and are specified below. Name bindings made during a successful pattern match outlive the executed block and can be used after the match statement. <div class="admonition note"> Note During failed pattern matches, some subpatterns may succeed. Do not rely on bindings being made for a failed match. Conversely, do not rely on variables remaining unchanged after a failed match. The exact behavior is dependent on implementation and may vary. This is an intentional decision made to allow different implementations to add optimizations. </div> </li> <li>If the pattern succeeds, the corresponding guard (if present) is evaluated. In this case all name bindings are guaranteed to have happened. <ul class="simple"> <li>If the guard evaluates as true or is missing, the <code class="docutils literal notranslate">block</code> inside <code class="docutils literal notranslate">case_block</code> is executed.</li> <li>Otherwise, the next <code class="docutils literal notranslate">case_block</code> is attempted as described above.</li> <li>If there are no further case blocks, the match statement is completed.</li> </ul> </li> </ol> <div class="admonition note"> Note Users should generally never rely on a pattern being evaluated. Depending on implementation, the interpreter may cache values or use other optimizations which skip repeated evaluations. </div> A sample match statement: <div class="highlight-python3 notranslate"><div class="highlight"><pre>>>> flag = False >>> match (100, 200): ... case (100, 300): # Mismatch: 200 != 300 ... print('Case 1') ... case (100, 200) if flag: # Successful match, but guard fails ... print('Case 2') ... case (100, y): # Matches and binds y to 200 ... print(f'Case 3, y: {y}') ... case _: # Pattern not attempted ... print('Case 4, I match anything!') ... Case 3, y: 200 </pre></div> </div> In this case, <code class="docutils literal notranslate">if flag</code> is a guard. Read more about that in the next section. </section> <section id="guards"> <h3>8.6.2. Guards<a class="headerlink" href="#guards" title="Link to this heading">¶</a></h3> <pre id="index-21"> guard ::= "if" <code class="xref docutils literal notranslate">named_expression</code> </pre> A <code class="docutils literal notranslate">guard</code> (which is part of the <code class="docutils literal notranslate">case</code>) must succeed for code inside the <code class="docutils literal notranslate">case</code> block to execute. It takes the form: <a class="reference internal" href="#if"><code class="xref std std-keyword docutils literal notranslate">if</code></a> followed by an expression. The logical flow of a <code class="docutils literal notranslate">case</code> block with a <code class="docutils literal notranslate">guard</code> follows: <ol class="arabic simple"> <li>Check that the pattern in the <code class="docutils literal notranslate">case</code> block succeeded. If the pattern failed, the <code class="docutils literal notranslate">guard</code> is not evaluated and the next <code class="docutils literal notranslate">case</code> block is checked.</li> <li>If the pattern succeeded, evaluate the <code class="docutils literal notranslate">guard</code>. <ul class="simple"> <li>If the <code class="docutils literal notranslate">guard</code> condition evaluates as true, the case block is selected.</li> <li>If the <code class="docutils literal notranslate">guard</code> condition evaluates as false, the case block is not selected.</li> <li>If the <code class="docutils literal notranslate">guard</code> raises an exception during evaluation, the exception bubbles up.</li> </ul> </li> </ol> Guards are allowed to have side effects as they are expressions. Guard evaluation must proceed from the first to the last case block, one at a time, skipping case blocks whose pattern(s) don’t all succeed. (I.e., guard evaluation must happen in order.) Guard evaluation must stop once a case block is selected. </section> <section id="irrefutable-case-blocks"> <h3>8.6.3. Irrefutable Case Blocks<a class="headerlink" href="#irrefutable-case-blocks" title="Link to this heading">¶</a></h3> An irrefutable case block is a match-all case block. A match statement may have at most one irrefutable case block, and it must be last. A case block is considered irrefutable if it has no guard and its pattern is irrefutable. A pattern is considered irrefutable if we can prove from its syntax alone that it will always succeed. Only the following patterns are irrefutable: <ul class="simple"> <li><a class="reference internal" href="#as-patterns">AS Patterns</a> whose left-hand side is irrefutable</li> <li><a class="reference internal" href="#or-patterns">OR Patterns</a> containing at least one irrefutable pattern</li> <li><a class="reference internal" href="#capture-patterns">Capture Patterns</a></li> <li><a class="reference internal" href="#wildcard-patterns">Wildcard Patterns</a></li> <li>parenthesized irrefutable patterns</li> </ul> </section> <section id="patterns"> <h3>8.6.4. Patterns<a class="headerlink" href="#patterns" title="Link to this heading">¶</a></h3> <div class="admonition note" id="index-23"> Note This section uses grammar notations beyond standard EBNF: <ul class="simple"> <li>the notation <code class="docutils literal notranslate">SEP.RULE+</code> is shorthand for <code class="docutils literal notranslate">RULE (SEP RULE)*</code></li> <li>the notation <code class="docutils literal notranslate">!RULE</code> is shorthand for a negative lookahead assertion</li> </ul> </div> The top-level syntax for <code class="docutils literal notranslate">patterns</code> is: <pre> patterns ::= <a class="reference internal" href="#grammar-token-python-grammar-open_sequence_pattern"><code class="xref docutils literal notranslate">open_sequence_pattern</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-pattern"><code class="xref docutils literal notranslate">pattern</code></a> pattern ::= <a class="reference internal" href="#grammar-token-python-grammar-as_pattern"><code class="xref docutils literal notranslate">as_pattern</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-or_pattern"><code class="xref docutils literal notranslate">or_pattern</code></a> closed_pattern ::= | <a class="reference internal" href="#grammar-token-python-grammar-literal_pattern"><code class="xref docutils literal notranslate">literal_pattern</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-capture_pattern"><code class="xref docutils literal notranslate">capture_pattern</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-wildcard_pattern"><code class="xref docutils literal notranslate">wildcard_pattern</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-value_pattern"><code class="xref docutils literal notranslate">value_pattern</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-group_pattern"><code class="xref docutils literal notranslate">group_pattern</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-sequence_pattern"><code class="xref docutils literal notranslate">sequence_pattern</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-mapping_pattern"><code class="xref docutils literal notranslate">mapping_pattern</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-class_pattern"><code class="xref docutils literal notranslate">class_pattern</code></a> </pre> The descriptions below will include a description “in simple terms” of what a pattern does for illustration purposes (credits to Raymond Hettinger for a document that inspired most of the descriptions). Note that these descriptions are purely for illustration purposes and may not reflect the underlying implementation. Furthermore, they do not cover all valid forms. <section id="or-patterns"> <h4>8.6.4.1. OR Patterns<a class="headerlink" href="#or-patterns" title="Link to this heading">¶</a></h4> An OR pattern is two or more patterns separated by vertical bars <code class="docutils literal notranslate">|</code>. Syntax: <pre> or_pattern ::= "|".<a class="reference internal" href="#grammar-token-python-grammar-closed_pattern"><code class="xref docutils literal notranslate">closed_pattern</code></a>+ </pre> Only the final subpattern may be <a class="reference internal" href="#irrefutable-case">irrefutable</a>, and each subpattern must bind the same set of names to avoid ambiguity. An OR pattern matches each of its subpatterns in turn to the subject value, until one succeeds. The OR pattern is then considered successful. Otherwise, if none of the subpatterns succeed, the OR pattern fails. In simple terms, <code class="docutils literal notranslate">P1 | P2 | ...</code> will try to match <code class="docutils literal notranslate">P1</code>, if it fails it will try to match <code class="docutils literal notranslate">P2</code>, succeeding immediately if any succeeds, failing otherwise. </section> <section id="as-patterns"> <h4>8.6.4.2. AS Patterns<a class="headerlink" href="#as-patterns" title="Link to this heading">¶</a></h4> An AS pattern matches an OR pattern on the left of the <a class="reference internal" href="#as"><code class="xref std std-keyword docutils literal notranslate">as</code></a> keyword against a subject. Syntax: <pre> as_pattern ::= <a class="reference internal" href="#grammar-token-python-grammar-or_pattern"><code class="xref docutils literal notranslate">or_pattern</code></a> "as" <a class="reference internal" href="#grammar-token-python-grammar-capture_pattern"><code class="xref docutils literal notranslate">capture_pattern</code></a> </pre> If the OR pattern fails, the AS pattern fails. Otherwise, the AS pattern binds the subject to the name on the right of the as keyword and succeeds. <code class="docutils literal notranslate">capture_pattern</code> cannot be a <code class="docutils literal notranslate">_</code>. In simple terms <code class="docutils literal notranslate">P as NAME</code> will match with <code class="docutils literal notranslate">P</code>, and on success it will set <code class="docutils literal notranslate">NAME = <subject></code>. </section> <section id="literal-patterns"> <h4>8.6.4.3. Literal Patterns<a class="headerlink" href="#literal-patterns" title="Link to this heading">¶</a></h4> A literal pattern corresponds to most <a class="reference internal" href="lexical_analysis.html#literals">literals</a> in Python. Syntax: <pre> literal_pattern ::= <a class="reference internal" href="#grammar-token-python-grammar-signed_number"><code class="xref docutils literal notranslate">signed_number</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-signed_number"><code class="xref docutils literal notranslate">signed_number</code></a> "+" NUMBER | <a class="reference internal" href="#grammar-token-python-grammar-signed_number"><code class="xref docutils literal notranslate">signed_number</code></a> "-" NUMBER | <code class="xref docutils literal notranslate">strings</code> | "None" | "True" | "False" signed_number ::= ["-"] NUMBER </pre> The rule <code class="docutils literal notranslate">strings</code> and the token <code class="docutils literal notranslate">NUMBER</code> are defined in the <a class="reference internal" href="grammar.html">standard Python grammar</a>. Triple-quoted strings are supported. Raw strings and byte strings are supported. <a class="reference internal" href="lexical_analysis.html#f-strings">f-strings</a> are not supported. The forms <code class="docutils literal notranslate">signed_number '+' NUMBER</code> and <code class="docutils literal notranslate">signed_number '-' NUMBER</code> are for expressing <a class="reference internal" href="lexical_analysis.html#imaginary">complex numbers</a>; they require a real number on the left and an imaginary number on the right. E.g. <code class="docutils literal notranslate">3 + 4j</code>. In simple terms, <code class="docutils literal notranslate">LITERAL</code> will succeed only if <code class="docutils literal notranslate"><subject> == LITERAL</code>. For the singletons <code class="docutils literal notranslate">None</code>, <code class="docutils literal notranslate">True</code> and <code class="docutils literal notranslate">False</code>, the <a class="reference internal" href="expressions.html#is"><code class="xref std std-keyword docutils literal notranslate">is</code></a> operator is used. </section> <section id="capture-patterns"> <h4>8.6.4.4. Capture Patterns<a class="headerlink" href="#capture-patterns" title="Link to this heading">¶</a></h4> A capture pattern binds the subject value to a name. Syntax: <pre> capture_pattern ::= !'_' NAME </pre> A single underscore <code class="docutils literal notranslate">_</code> is not a capture pattern (this is what <code class="docutils literal notranslate">!'_'</code> expresses). It is instead treated as a <a class="reference internal" href="#grammar-token-python-grammar-wildcard_pattern"><code class="xref std std-token docutils literal notranslate">wildcard_pattern</code></a>. In a given pattern, a given name can only be bound once. E.g. <code class="docutils literal notranslate">case x, x: ...</code> is invalid while <code class="docutils literal notranslate">case [x] | x: ...</code> is allowed. Capture patterns always succeed. The binding follows scoping rules established by the assignment expression operator in <a class="pep reference external" href="https://peps.python.org/pep-0572/">PEP 572</a>; the name becomes a local variable in the closest containing function scope unless there’s an applicable <a class="reference internal" href="simple_stmts.html#global"><code class="xref std std-keyword docutils literal notranslate">global</code></a> or <a class="reference internal" href="simple_stmts.html#nonlocal"><code class="xref std std-keyword docutils literal notranslate">nonlocal</code></a> statement. In simple terms <code class="docutils literal notranslate">NAME</code> will always succeed and it will set <code class="docutils literal notranslate">NAME = <subject></code>. </section> <section id="wildcard-patterns"> <h4>8.6.4.5. Wildcard Patterns<a class="headerlink" href="#wildcard-patterns" title="Link to this heading">¶</a></h4> A wildcard pattern always succeeds (matches anything) and binds no name. Syntax: <pre> wildcard_pattern ::= '_' </pre> <code class="docutils literal notranslate">_</code> is a <a class="reference internal" href="lexical_analysis.html#soft-keywords">soft keyword</a> within any pattern, but only within patterns. It is an identifier, as usual, even within <code class="docutils literal notranslate">match</code> subject expressions, <code class="docutils literal notranslate">guard</code>s, and <code class="docutils literal notranslate">case</code> blocks. In simple terms, <code class="docutils literal notranslate">_</code> will always succeed. </section> <section id="value-patterns"> <h4>8.6.4.6. Value Patterns<a class="headerlink" href="#value-patterns" title="Link to this heading">¶</a></h4> A value pattern represents a named value in Python. Syntax: <pre> value_pattern ::= <a class="reference internal" href="#grammar-token-python-grammar-attr"><code class="xref docutils literal notranslate">attr</code></a> attr ::= <a class="reference internal" href="#grammar-token-python-grammar-name_or_attr"><code class="xref docutils literal notranslate">name_or_attr</code></a> "." NAME name_or_attr ::= <a class="reference internal" href="#grammar-token-python-grammar-attr"><code class="xref docutils literal notranslate">attr</code></a> | NAME </pre> The dotted name in the pattern is looked up using standard Python <a class="reference internal" href="executionmodel.html#resolve-names">name resolution rules</a>. The pattern succeeds if the value found compares equal to the subject value (using the <code class="docutils literal notranslate">==</code> equality operator). In simple terms <code class="docutils literal notranslate">NAME1.NAME2</code> will succeed only if <code class="docutils literal notranslate"><subject> == NAME1.NAME2</code> <div class="admonition note"> Note If the same value occurs multiple times in the same match statement, the interpreter may cache the first value found and reuse it rather than repeat the same lookup. This cache is strictly tied to a given execution of a given match statement. </div> </section> <section id="group-patterns"> <h4>8.6.4.7. Group Patterns<a class="headerlink" href="#group-patterns" title="Link to this heading">¶</a></h4> A group pattern allows users to add parentheses around patterns to emphasize the intended grouping. Otherwise, it has no additional syntax. Syntax: <pre> group_pattern ::= "(" <a class="reference internal" href="#grammar-token-python-grammar-pattern"><code class="xref docutils literal notranslate">pattern</code></a> ")" </pre> In simple terms <code class="docutils literal notranslate">(P)</code> has the same effect as <code class="docutils literal notranslate">P</code>. </section> <section id="sequence-patterns"> <h4>8.6.4.8. Sequence Patterns<a class="headerlink" href="#sequence-patterns" title="Link to this heading">¶</a></h4> A sequence pattern contains several subpatterns to be matched against sequence elements. The syntax is similar to the unpacking of a list or tuple. <pre> sequence_pattern ::= "[" [<a class="reference internal" href="#grammar-token-python-grammar-maybe_sequence_pattern"><code class="xref docutils literal notranslate">maybe_sequence_pattern</code></a>] "]" | "(" [<a class="reference internal" href="#grammar-token-python-grammar-open_sequence_pattern"><code class="xref docutils literal notranslate">open_sequence_pattern</code></a>] ")" open_sequence_pattern ::= <a class="reference internal" href="#grammar-token-python-grammar-maybe_star_pattern"><code class="xref docutils literal notranslate">maybe_star_pattern</code></a> "," [<a class="reference internal" href="#grammar-token-python-grammar-maybe_sequence_pattern"><code class="xref docutils literal notranslate">maybe_sequence_pattern</code></a>] maybe_sequence_pattern ::= ",".<a class="reference internal" href="#grammar-token-python-grammar-maybe_star_pattern"><code class="xref docutils literal notranslate">maybe_star_pattern</code></a>+ ","? maybe_star_pattern ::= <a class="reference internal" href="#grammar-token-python-grammar-star_pattern"><code class="xref docutils literal notranslate">star_pattern</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-pattern"><code class="xref docutils literal notranslate">pattern</code></a> star_pattern ::= "*" (<a class="reference internal" href="#grammar-token-python-grammar-capture_pattern"><code class="xref docutils literal notranslate">capture_pattern</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-wildcard_pattern"><code class="xref docutils literal notranslate">wildcard_pattern</code></a>) </pre> There is no difference if parentheses or square brackets are used for sequence patterns (i.e. <code class="docutils literal notranslate">(...)</code> vs <code class="docutils literal notranslate">[...]</code> ). <div class="admonition note"> Note A single pattern enclosed in parentheses without a trailing comma (e.g. <code class="docutils literal notranslate">(3 | 4)</code>) is a <a class="reference internal" href="#group-patterns">group pattern</a>. While a single pattern enclosed in square brackets (e.g. <code class="docutils literal notranslate">[3 | 4]</code>) is still a sequence pattern. </div> At most one star subpattern may be in a sequence pattern. The star subpattern may occur in any position. If no star subpattern is present, the sequence pattern is a fixed-length sequence pattern; otherwise it is a variable-length sequence pattern. The following is the logical flow for matching a sequence pattern against a subject value: <ol class="arabic"> <li>If the subject value is not a sequence <a class="footnote-reference brackets" href="#id21" id="id11" role="doc-noteref">[2]</a>, the sequence pattern fails.</li> <li>If the subject value is an instance of <code class="docutils literal notranslate">str</code>, <code class="docutils literal notranslate">bytes</code> or <code class="docutils literal notranslate">bytearray</code> the sequence pattern fails.</li> <li>The subsequent steps depend on whether the sequence pattern is fixed or variable-length. If the sequence pattern is fixed-length: <ol class="arabic simple"> <li>If the length of the subject sequence is not equal to the number of subpatterns, the sequence pattern fails</li> <li>Subpatterns in the sequence pattern are matched to their corresponding items in the subject sequence from left to right. Matching stops as soon as a subpattern fails. If all subpatterns succeed in matching their corresponding item, the sequence pattern succeeds.</li> </ol> Otherwise, if the sequence pattern is variable-length: <ol class="arabic simple"> <li>If the length of the subject sequence is less than the number of non-star subpatterns, the sequence pattern fails.</li> <li>The leading non-star subpatterns are matched to their corresponding items as for fixed-length sequences.</li> <li>If the previous step succeeds, the star subpattern matches a list formed of the remaining subject items, excluding the remaining items corresponding to non-star subpatterns following the star subpattern.</li> <li>Remaining non-star subpatterns are matched to their corresponding subject items, as for a fixed-length sequence.</li> </ol> <div class="admonition note"> Note The length of the subject sequence is obtained via <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate">len()</code></a> (i.e. via the <code class="xref py py-meth docutils literal notranslate">__len__()</code> protocol). This length may be cached by the interpreter in a similar manner as <a class="reference internal" href="#value-patterns">value patterns</a>. </div> </li> </ol> In simple terms <code class="docutils literal notranslate">[P1, P2, P3,</code> … <code class="docutils literal notranslate">, P<N>]</code> matches only if all the following happens: <ul class="simple"> <li>check <code class="docutils literal notranslate"><subject></code> is a sequence</li> <li><code class="docutils literal notranslate">len(subject) == <N></code></li> <li><code class="docutils literal notranslate">P1</code> matches <code class="docutils literal notranslate"><subject>[0]</code> (note that this match can also bind names)</li> <li><code class="docutils literal notranslate">P2</code> matches <code class="docutils literal notranslate"><subject>[1]</code> (note that this match can also bind names)</li> <li>… and so on for the corresponding pattern/element.</li> </ul> </section> <section id="mapping-patterns"> <h4>8.6.4.9. Mapping Patterns<a class="headerlink" href="#mapping-patterns" title="Link to this heading">¶</a></h4> A mapping pattern contains one or more key-value patterns. The syntax is similar to the construction of a dictionary. Syntax: <pre> mapping_pattern ::= "{" [<a class="reference internal" href="#grammar-token-python-grammar-items_pattern"><code class="xref docutils literal notranslate">items_pattern</code></a>] "}" items_pattern ::= ",".<a class="reference internal" href="#grammar-token-python-grammar-key_value_pattern"><code class="xref docutils literal notranslate">key_value_pattern</code></a>+ ","? key_value_pattern ::= (<a class="reference internal" href="#grammar-token-python-grammar-literal_pattern"><code class="xref docutils literal notranslate">literal_pattern</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-value_pattern"><code class="xref docutils literal notranslate">value_pattern</code></a>) ":" <a class="reference internal" href="#grammar-token-python-grammar-pattern"><code class="xref docutils literal notranslate">pattern</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-double_star_pattern"><code class="xref docutils literal notranslate">double_star_pattern</code></a> double_star_pattern ::= "**" <a class="reference internal" href="#grammar-token-python-grammar-capture_pattern"><code class="xref docutils literal notranslate">capture_pattern</code></a> </pre> At most one double star pattern may be in a mapping pattern. The double star pattern must be the last subpattern in the mapping pattern. Duplicate keys in mapping patterns are disallowed. Duplicate literal keys will raise a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate">SyntaxError</code></a>. Two keys that otherwise have the same value will raise a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> at runtime. The following is the logical flow for matching a mapping pattern against a subject value: <ol class="arabic simple"> <li>If the subject value is not a mapping <a class="footnote-reference brackets" href="#id22" id="id13" role="doc-noteref">[3]</a>,the mapping pattern fails.</li> <li>If every key given in the mapping pattern is present in the subject mapping, and the pattern for each key matches the corresponding item of the subject mapping, the mapping pattern succeeds.</li> <li>If duplicate keys are detected in the mapping pattern, the pattern is considered invalid. A <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate">SyntaxError</code></a> is raised for duplicate literal values; or a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> for named keys of the same value.</li> </ol> <div class="admonition note"> Note Key-value pairs are matched using the two-argument form of the mapping subject’s <code class="docutils literal notranslate">get()</code> method. Matched key-value pairs must already be present in the mapping, and not created on-the-fly via <code class="xref py py-meth docutils literal notranslate">__missing__()</code> or <a class="reference internal" href="datamodel.html#object.__getitem__" title="object.__getitem__"><code class="xref py py-meth docutils literal notranslate">__getitem__()</code></a>. </div> In simple terms <code class="docutils literal notranslate">{KEY1: P1, KEY2: P2, ... }</code> matches only if all the following happens: <ul class="simple"> <li>check <code class="docutils literal notranslate"><subject></code> is a mapping</li> <li><code class="docutils literal notranslate">KEY1 in <subject></code></li> <li><code class="docutils literal notranslate">P1</code> matches <code class="docutils literal notranslate"><subject>[KEY1]</code></li> <li>… and so on for the corresponding KEY/pattern pair.</li> </ul> </section> <section id="class-patterns"> <h4>8.6.4.10. Class Patterns<a class="headerlink" href="#class-patterns" title="Link to this heading">¶</a></h4> A class pattern represents a class and its positional and keyword arguments (if any). Syntax: <pre> class_pattern ::= <a class="reference internal" href="#grammar-token-python-grammar-name_or_attr"><code class="xref docutils literal notranslate">name_or_attr</code></a> "(" [<a class="reference internal" href="#grammar-token-python-grammar-pattern_arguments"><code class="xref docutils literal notranslate">pattern_arguments</code></a> ","?] ")" pattern_arguments ::= <a class="reference internal" href="#grammar-token-python-grammar-positional_patterns"><code class="xref docutils literal notranslate">positional_patterns</code></a> ["," <a class="reference internal" href="#grammar-token-python-grammar-keyword_patterns"><code class="xref docutils literal notranslate">keyword_patterns</code></a>] | <a class="reference internal" href="#grammar-token-python-grammar-keyword_patterns"><code class="xref docutils literal notranslate">keyword_patterns</code></a> positional_patterns ::= ",".<a class="reference internal" href="#grammar-token-python-grammar-pattern"><code class="xref docutils literal notranslate">pattern</code></a>+ keyword_patterns ::= ",".<a class="reference internal" href="#grammar-token-python-grammar-keyword_pattern"><code class="xref docutils literal notranslate">keyword_pattern</code></a>+ keyword_pattern ::= NAME "=" <a class="reference internal" href="#grammar-token-python-grammar-pattern"><code class="xref docutils literal notranslate">pattern</code></a> </pre> The same keyword should not be repeated in class patterns. The following is the logical flow for matching a class pattern against a subject value: <ol class="arabic"> <li>If <code class="docutils literal notranslate">name_or_attr</code> is not an instance of the builtin <a class="reference internal" href="../library/functions.html#type" title="type"><code class="xref py py-class docutils literal notranslate">type</code></a> , raise <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>.</li> <li>If the subject value is not an instance of <code class="docutils literal notranslate">name_or_attr</code> (tested via <a class="reference internal" href="../library/functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal notranslate">isinstance()</code></a>), the class pattern fails.</li> <li>If no pattern arguments are present, the pattern succeeds. Otherwise, the subsequent steps depend on whether keyword or positional argument patterns are present. For a number of built-in types (specified below), a single positional subpattern is accepted which will match the entire subject; for these types keyword patterns also work as for other types. If only keyword patterns are present, they are processed as follows, one by one: I. The keyword is looked up as an attribute on the subject. <blockquote> <div><ul class="simple"> <li>If this raises an exception other than <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate">AttributeError</code></a>, the exception bubbles up.</li> <li>If this raises <a class="reference internal" href="../library/exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate">AttributeError</code></a>, the class pattern has failed.</li> <li>Else, the subpattern associated with the keyword pattern is matched against the subject’s attribute value. If this fails, the class pattern fails; if this succeeds, the match proceeds to the next keyword.</li> </ul> </div></blockquote> II. If all keyword patterns succeed, the class pattern succeeds. If any positional patterns are present, they are converted to keyword patterns using the <a class="reference internal" href="datamodel.html#object.__match_args__" title="object.__match_args__"><code class="xref py py-data docutils literal notranslate">__match_args__</code></a> attribute on the class <code class="docutils literal notranslate">name_or_attr</code> before matching: I. The equivalent of <code class="docutils literal notranslate">getattr(cls, "__match_args__", ())</code> is called. <blockquote> <div><ul class="simple"> <li>If this raises an exception, the exception bubbles up.</li> <li>If the returned value is not a tuple, the conversion fails and <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> is raised.</li> <li>If there are more positional patterns than <code class="docutils literal notranslate">len(cls.__match_args__)</code>, <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> is raised.</li> <li>Otherwise, positional pattern <code class="docutils literal notranslate">i</code> is converted to a keyword pattern using <code class="docutils literal notranslate">__match_args__[i]</code> as the keyword. <code class="docutils literal notranslate">__match_args__[i]</code> must be a string; if not <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> is raised.</li> <li>If there are duplicate keywords, <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> is raised.</li> </ul> <div class="admonition seealso"> See also <a class="reference internal" href="datamodel.html#class-pattern-matching">Customizing positional arguments in class pattern matching</a> </div> </div></blockquote> <dl class="simple"> <dt>II. Once all positional patterns have been converted to keyword patterns,</dt><dd>the match proceeds as if there were only keyword patterns. </dd> </dl> For the following built-in types the handling of positional subpatterns is different: <ul class="simple"> <li><a class="reference internal" href="../library/functions.html#bool" title="bool"><code class="xref py py-class docutils literal notranslate">bool</code></a></li> <li><a class="reference internal" href="../library/stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal notranslate">bytearray</code></a></li> <li><a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate">bytes</code></a></li> <li><a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a></li> <li><a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate">float</code></a></li> <li><a class="reference internal" href="../library/stdtypes.html#frozenset" title="frozenset"><code class="xref py py-class docutils literal notranslate">frozenset</code></a></li> <li><a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a></li> <li><a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate">list</code></a></li> <li><a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate">set</code></a></li> <li><a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate">str</code></a></li> <li><a class="reference internal" href="../library/stdtypes.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate">tuple</code></a></li> </ul> These classes accept a single positional argument, and the pattern there is matched against the whole object rather than an attribute. For example <code class="docutils literal notranslate">int(0|1)</code> matches the value <code class="docutils literal notranslate">0</code>, but not the value <code class="docutils literal notranslate">0.0</code>. </li> </ol> In simple terms <code class="docutils literal notranslate">CLS(P1, attr=P2)</code> matches only if the following happens: <ul class="simple"> <li><code class="docutils literal notranslate">isinstance(<subject>, CLS)</code></li> <li>convert <code class="docutils literal notranslate">P1</code> to a keyword pattern using <code class="docutils literal notranslate">CLS.__match_args__</code></li> <li>For each keyword argument <code class="docutils literal notranslate">attr=P2</code>: <ul> <li><code class="docutils literal notranslate">hasattr(<subject>, "attr")</code></li> <li><code class="docutils literal notranslate">P2</code> matches <code class="docutils literal notranslate"><subject>.attr</code></li> </ul> </li> <li>… and so on for the corresponding keyword argument/pattern pair.</li> </ul> <div class="admonition seealso"> See also <ul class="simple"> <li><a class="pep reference external" href="https://peps.python.org/pep-0634/">PEP 634</a> – Structural Pattern Matching: Specification</li> <li><a class="pep reference external" href="https://peps.python.org/pep-0636/">PEP 636</a> – Structural Pattern Matching: Tutorial</li> </ul> </div> </section> </section> </section> <section id="function-definitions"> <h2>8.7. Function definitions<a class="headerlink" href="#function-definitions" title="Link to this heading">¶</a></h2> A function definition defines a user-defined function object (see section <a class="reference internal" href="datamodel.html#types">The standard type hierarchy</a>): <pre> funcdef ::= [<a class="reference internal" href="#grammar-token-python-grammar-decorators"><code class="xref docutils literal notranslate">decorators</code></a>] "def" <a class="reference internal" href="#grammar-token-python-grammar-funcname"><code class="xref docutils literal notranslate">funcname</code></a> [<a class="reference internal" href="#grammar-token-python-grammar-type_params"><code class="xref docutils literal notranslate">type_params</code></a>] "(" [<a class="reference internal" href="#grammar-token-python-grammar-parameter_list"><code class="xref docutils literal notranslate">parameter_list</code></a>] ")" ["->" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a>] ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a> decorators ::= <a class="reference internal" href="#grammar-token-python-grammar-decorator"><code class="xref docutils literal notranslate">decorator</code></a>+ decorator ::= "@" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-assignment_expression"><code class="xref docutils literal notranslate">assignment_expression</code></a> NEWLINE parameter_list ::= <a class="reference internal" href="#grammar-token-python-grammar-defparameter"><code class="xref docutils literal notranslate">defparameter</code></a> ("," <a class="reference internal" href="#grammar-token-python-grammar-defparameter"><code class="xref docutils literal notranslate">defparameter</code></a>)* "," "/" ["," [<a class="reference internal" href="#grammar-token-python-grammar-parameter_list_no_posonly"><code class="xref docutils literal notranslate">parameter_list_no_posonly</code></a>]] | <a class="reference internal" href="#grammar-token-python-grammar-parameter_list_no_posonly"><code class="xref docutils literal notranslate">parameter_list_no_posonly</code></a> parameter_list_no_posonly ::= <a class="reference internal" href="#grammar-token-python-grammar-defparameter"><code class="xref docutils literal notranslate">defparameter</code></a> ("," <a class="reference internal" href="#grammar-token-python-grammar-defparameter"><code class="xref docutils literal notranslate">defparameter</code></a>)* ["," [<a class="reference internal" href="#grammar-token-python-grammar-parameter_list_starargs"><code class="xref docutils literal notranslate">parameter_list_starargs</code></a>]] | <a class="reference internal" href="#grammar-token-python-grammar-parameter_list_starargs"><code class="xref docutils literal notranslate">parameter_list_starargs</code></a> parameter_list_starargs ::= "*" [<a class="reference internal" href="#grammar-token-python-grammar-star_parameter"><code class="xref docutils literal notranslate">star_parameter</code></a>] ("," <a class="reference internal" href="#grammar-token-python-grammar-defparameter"><code class="xref docutils literal notranslate">defparameter</code></a>)* ["," ["**" <a class="reference internal" href="#grammar-token-python-grammar-parameter"><code class="xref docutils literal notranslate">parameter</code></a> [","]]] | "**" <a class="reference internal" href="#grammar-token-python-grammar-parameter"><code class="xref docutils literal notranslate">parameter</code></a> [","] parameter ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> [":" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a>] star_parameter ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> [":" ["*"] <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a>] defparameter ::= <a class="reference internal" href="#grammar-token-python-grammar-parameter"><code class="xref docutils literal notranslate">parameter</code></a> ["=" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a>] funcname ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> </pre> 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. The function definition does not execute the function body; this gets executed only when the function is called. <a class="footnote-reference brackets" href="#id23" id="id15" role="doc-noteref">[4]</a> A function definition may be wrapped by one or more <a class="reference internal" href="../glossary.html#term-decorator">decorator</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 <div class="highlight-python3 notranslate"><div class="highlight"><pre>@f1(arg) @f2 def func(): pass </pre></div> </div> is roughly equivalent to <div class="highlight-python3 notranslate"><div class="highlight"><pre>def func(): pass func = f1(arg)(f2(func)) </pre></div> </div> except that the original function is not temporarily bound to the name <code class="docutils literal notranslate">func</code>. <div class="versionchanged"> Changed in version 3.9: Functions may be decorated with any valid <a class="reference internal" href="expressions.html#grammar-token-python-grammar-assignment_expression"><code class="xref std std-token docutils literal notranslate">assignment_expression</code></a>. Previously, the grammar was much more restrictive; see <a class="pep reference external" href="https://peps.python.org/pep-0614/">PEP 614</a> for details. </div> A list of <a class="reference internal" href="#type-params">type parameters</a> may be given in square brackets between the function’s name and the opening parenthesis for its parameter list. This indicates to static type checkers that the function is generic. At runtime, the type parameters can be retrieved from the function’s <a class="reference internal" href="datamodel.html#function.__type_params__" title="function.__type_params__"><code class="xref py py-attr docutils literal notranslate">__type_params__</code></a> attribute. See <a class="reference internal" href="#generic-functions">Generic functions</a> for more. <div class="versionchanged"> Changed in version 3.12: Type parameter lists are new in Python 3.12. </div> When one or more <a class="reference internal" href="../glossary.html#term-parameter">parameters</a> have the form parameter <code class="docutils literal notranslate">=</code> expression, 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">argument</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">*</code>” must also have a default value — this is a syntactic restriction that is not expressed by the grammar. Default parameter values are evaluated from left to right when the function definition is executed. 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 value 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 parameter value is in effect modified. This is generally not what was intended. A way around this is to use <code class="docutils literal notranslate">None</code> as the default, and explicitly test for it in the body of the function, e.g.: <div class="highlight-python3 notranslate"><div class="highlight"><pre>def whats_on_the_telly(penguin=None): if penguin is None: penguin = [] penguin.append("property of the zoo") return penguin </pre></div> </div> Function call semantics are described in more detail in section <a class="reference internal" href="expressions.html#calls">Calls</a>. A function call always assigns values to all parameters mentioned in the parameter list, either from positional arguments, from keyword arguments, or from default values. If the form “<code class="docutils literal notranslate">*identifier</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">**identifier</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">*</code>” or “<code class="docutils literal notranslate">*identifier</code>” are keyword-only parameters and may only be passed by keyword arguments. Parameters before “<code class="docutils literal notranslate">/</code>” are positional-only parameters and may only be passed by positional arguments. <div class="versionchanged"> Changed in version 3.8: The <code class="docutils literal notranslate">/</code> function parameter syntax may be used to indicate positional-only parameters. See <a class="pep reference external" href="https://peps.python.org/pep-0570/">PEP 570</a> for details. </div> Parameters may have an <a class="reference internal" href="../glossary.html#term-function-annotation">annotation</a> of the form “<code class="docutils literal notranslate">: expression</code>” following the parameter name. Any parameter may have an annotation, even those of the form <code class="docutils literal notranslate">*identifier</code> or <code class="docutils literal notranslate">**identifier</code>. (As a special case, parameters of the form <code class="docutils literal notranslate">*identifier</code> may have an annotation “<code class="docutils literal notranslate">: *expression</code>”.) Functions may have “return” annotation of the form “<code class="docutils literal notranslate">-> expression</code>” after the parameter list. These annotations can be any valid Python expression. 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">__annotations__</code> attribute of the function object. If the <code class="docutils literal notranslate">annotations</code> import from <a class="reference internal" href="../library/__future__.html#module-__future__" title="__future__: Future statement definitions"><code class="xref py py-mod docutils literal notranslate">__future__</code></a> is used, annotations are preserved as strings at runtime which enables postponed evaluation. Otherwise, they are evaluated when the function definition is executed. In this case annotations may be evaluated in a different order than they appear in the source code. <div class="versionchanged"> Changed in version 3.11: Parameters of the form “<code class="docutils literal notranslate">*identifier</code>” may have an annotation “<code class="docutils literal notranslate">: *expression</code>”. See <a class="pep reference external" href="https://peps.python.org/pep-0646/">PEP 646</a>. </div> 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">Lambdas</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">def</code></a>” statement can be passed around or assigned to another name just like a function defined by a lambda expression. The “<code class="xref std std-keyword docutils literal notranslate">def</code>” form is actually more powerful since it allows the execution of multiple statements and annotations. Programmer’s note: Functions are first-class objects. A “<code class="docutils literal notranslate">def</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">Naming and binding</a> for details. <div class="admonition seealso"> See also <dl class="simple"> <dt><a class="pep reference external" href="https://peps.python.org/pep-3107/">PEP 3107</a> - Function Annotations</dt><dd>The original specification for function annotations. </dd> <dt><a class="pep reference external" href="https://peps.python.org/pep-0484/">PEP 484</a> - Type Hints</dt><dd>Definition of a standard meaning for annotations: type hints. </dd> <dt><a class="pep reference external" href="https://peps.python.org/pep-0526/">PEP 526</a> - Syntax for Variable Annotations</dt><dd>Ability to type hint variable declarations, including class variables and instance variables. </dd> <dt><a class="pep reference external" href="https://peps.python.org/pep-0563/">PEP 563</a> - Postponed Evaluation of Annotations</dt><dd>Support for forward references within annotations by preserving annotations in a string form at runtime instead of eager evaluation. </dd> <dt><a class="pep reference external" href="https://peps.python.org/pep-0318/">PEP 318</a> - Decorators for Functions and Methods</dt><dd>Function and method decorators were introduced. Class decorators were introduced in <a class="pep reference external" href="https://peps.python.org/pep-3129/">PEP 3129</a>. </dd> </dl> </div> </section> <section id="class-definitions"> <h2>8.8. Class definitions<a class="headerlink" href="#class-definitions" title="Link to this heading">¶</a></h2> A class definition defines a class object (see section <a class="reference internal" href="datamodel.html#types">The standard type hierarchy</a>): <pre> classdef ::= [<a class="reference internal" href="#grammar-token-python-grammar-decorators"><code class="xref docutils literal notranslate">decorators</code></a>] "class" <a class="reference internal" href="#grammar-token-python-grammar-classname"><code class="xref docutils literal notranslate">classname</code></a> [<a class="reference internal" href="#grammar-token-python-grammar-type_params"><code class="xref docutils literal notranslate">type_params</code></a>] [<a class="reference internal" href="#grammar-token-python-grammar-inheritance"><code class="xref docutils literal notranslate">inheritance</code></a>] ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a> inheritance ::= "(" [<a class="reference internal" href="expressions.html#grammar-token-python-grammar-argument_list"><code class="xref docutils literal notranslate">argument_list</code></a>] ")" classname ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> </pre> 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">Metaclasses</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">object</code></a>; hence, <div class="highlight-python3 notranslate"><div class="highlight"><pre>class Foo: pass </pre></div> </div> is equivalent to <div class="highlight-python3 notranslate"><div class="highlight"><pre>class Foo(object): pass </pre></div> </div> The class’s suite is then executed in a new execution frame (see <a class="reference internal" href="executionmodel.html#naming">Naming and binding</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 brackets" href="#id24" id="id16" role="doc-noteref">[5]</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. The order in which attributes are defined in the class body is preserved in the new class’s <a class="reference internal" href="datamodel.html#type.__dict__" title="type.__dict__"><code class="xref py py-attr docutils literal notranslate">__dict__</code></a>. Note that this is reliable only right after the class is created and only for classes that were defined using the definition syntax. Class creation can be customized heavily using <a class="reference internal" href="datamodel.html#metaclasses">metaclasses</a>. Classes can also be decorated: just like when decorating functions, <div class="highlight-python3 notranslate"><div class="highlight"><pre>@f1(arg) @f2 class Foo: pass </pre></div> </div> is roughly equivalent to <div class="highlight-python3 notranslate"><div class="highlight"><pre>class Foo: pass Foo = f1(arg)(f2(Foo)) </pre></div> </div> The evaluation rules for the decorator expressions are the same as for function decorators. The result is then bound to the class name. <div class="versionchanged"> Changed in version 3.9: Classes may be decorated with any valid <a class="reference internal" href="expressions.html#grammar-token-python-grammar-assignment_expression"><code class="xref std std-token docutils literal notranslate">assignment_expression</code></a>. Previously, the grammar was much more restrictive; see <a class="pep reference external" href="https://peps.python.org/pep-0614/">PEP 614</a> for details. </div> A list of <a class="reference internal" href="#type-params">type parameters</a> may be given in square brackets immediately after the class’s name. This indicates to static type checkers that the class is generic. At runtime, the type parameters can be retrieved from the class’s <a class="reference internal" href="datamodel.html#type.__type_params__" title="type.__type_params__"><code class="xref py py-attr docutils literal notranslate">__type_params__</code></a> attribute. See <a class="reference internal" href="#generic-classes">Generic classes</a> for more. <div class="versionchanged"> Changed in version 3.12: Type parameter lists are new in Python 3.12. </div> Programmer’s note: 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">self.name = value</code>. Both class and instance attributes are accessible through the notation “<code class="docutils literal notranslate">self.name</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">Descriptors</a> can be used to create instance variables with different implementation details. <div class="admonition seealso"> See also <dl class="simple"> <dt><a class="pep reference external" href="https://peps.python.org/pep-3115/">PEP 3115</a> - Metaclasses in Python 3000</dt><dd>The proposal that changed the declaration of metaclasses to the current syntax, and the semantics for how classes with metaclasses are constructed. </dd> <dt><a class="pep reference external" href="https://peps.python.org/pep-3129/">PEP 3129</a> - Class Decorators</dt><dd>The proposal that added class decorators. Function and method decorators were introduced in <a class="pep reference external" href="https://peps.python.org/pep-0318/">PEP 318</a>. </dd> </dl> </div> </section> <section id="coroutines"> <h2>8.9. Coroutines<a class="headerlink" href="#coroutines" title="Link to this heading">¶</a></h2> <div class="versionadded"> Added in version 3.5. </div> <section id="coroutine-function-definition"> <h3>8.9.1. Coroutine function definition<a class="headerlink" href="#coroutine-function-definition" title="Link to this heading">¶</a></h3> <pre> async_funcdef ::= [<a class="reference internal" href="#grammar-token-python-grammar-decorators"><code class="xref docutils literal notranslate">decorators</code></a>] "async" "def" <a class="reference internal" href="#grammar-token-python-grammar-funcname"><code class="xref docutils literal notranslate">funcname</code></a> "(" [<a class="reference internal" href="#grammar-token-python-grammar-parameter_list"><code class="xref docutils literal notranslate">parameter_list</code></a>] ")" ["->" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a>] ":" <a class="reference internal" href="#grammar-token-python-grammar-suite"><code class="xref docutils literal notranslate">suite</code></a> </pre> Execution of Python coroutines can be suspended and resumed at many points (see <a class="reference internal" href="../glossary.html#term-coroutine">coroutine</a>). <a class="reference internal" href="expressions.html#await"><code class="xref std std-keyword docutils literal notranslate">await</code></a> expressions, <a class="reference internal" href="#async-for"><code class="xref std std-keyword docutils literal notranslate">async for</code></a> and <a class="reference internal" href="#async-with"><code class="xref std std-keyword docutils literal notranslate">async with</code></a> can only be used in the body of a coroutine function. Functions defined with <code class="docutils literal notranslate">async def</code> syntax are always coroutine functions, even if they do not contain <code class="docutils literal notranslate">await</code> or <code class="docutils literal notranslate">async</code> keywords. It is a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate">SyntaxError</code></a> to use a <code class="docutils literal notranslate">yield from</code> expression inside the body of a coroutine function. An example of a coroutine function: <div class="highlight-python3 notranslate"><div class="highlight"><pre>async def func(param1, param2): do_stuff() await some_coroutine() </pre></div> </div> <div class="versionchanged"> Changed in version 3.7: <code class="docutils literal notranslate">await</code> and <code class="docutils literal notranslate">async</code> are now keywords; previously they were only treated as such inside the body of a coroutine function. </div> </section> <section id="the-async-for-statement"> <h3>8.9.2. The <code class="xref std std-keyword docutils literal notranslate">async for</code> statement<a class="headerlink" href="#the-async-for-statement" title="Link to this heading">¶</a></h3> <pre> async_for_stmt ::= "async" <a class="reference internal" href="#grammar-token-python-grammar-for_stmt"><code class="xref docutils literal notranslate">for_stmt</code></a> </pre> An <a class="reference internal" href="../glossary.html#term-asynchronous-iterable">asynchronous iterable</a> provides an <code class="docutils literal notranslate">__aiter__</code> method that directly returns an <a class="reference internal" href="../glossary.html#term-asynchronous-iterator">asynchronous iterator</a>, which can call asynchronous code in its <code class="docutils literal notranslate">__anext__</code> method. The <code class="docutils literal notranslate">async for</code> statement allows convenient iteration over asynchronous iterables. The following code: <div class="highlight-python3 notranslate"><div class="highlight"><pre>async for TARGET in ITER: SUITE else: SUITE2 </pre></div> </div> Is semantically equivalent to: <div class="highlight-python3 notranslate"><div class="highlight"><pre>iter = (ITER) iter = type(iter).__aiter__(iter) running = True while running: try: TARGET = await type(iter).__anext__(iter) except StopAsyncIteration: running = False else: SUITE else: SUITE2 </pre></div> </div> See also <a class="reference internal" href="datamodel.html#object.__aiter__" title="object.__aiter__"><code class="xref py py-meth docutils literal notranslate">__aiter__()</code></a> and <a class="reference internal" href="datamodel.html#object.__anext__" title="object.__anext__"><code class="xref py py-meth docutils literal notranslate">__anext__()</code></a> for details. It is a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate">SyntaxError</code></a> to use an <code class="docutils literal notranslate">async for</code> statement outside the body of a coroutine function. </section> <section id="the-async-with-statement"> <h3>8.9.3. The <code class="xref std std-keyword docutils literal notranslate">async with</code> statement<a class="headerlink" href="#the-async-with-statement" title="Link to this heading">¶</a></h3> <pre> async_with_stmt ::= "async" <a class="reference internal" href="#grammar-token-python-grammar-with_stmt"><code class="xref docutils literal notranslate">with_stmt</code></a> </pre> An <a class="reference internal" href="../glossary.html#term-asynchronous-context-manager">asynchronous context manager</a> is a <a class="reference internal" href="../glossary.html#term-context-manager">context manager</a> that is able to suspend execution in its enter and exit methods. The following code: <div class="highlight-python3 notranslate"><div class="highlight"><pre>async with EXPRESSION as TARGET: SUITE </pre></div> </div> is semantically equivalent to: <div class="highlight-python3 notranslate"><div class="highlight"><pre>manager = (EXPRESSION) aenter = type(manager).__aenter__ aexit = type(manager).__aexit__ value = await aenter(manager) hit_except = False try: TARGET = value SUITE except: hit_except = True if not await aexit(manager, *sys.exc_info()): raise finally: if not hit_except: await aexit(manager, None, None, None) </pre></div> </div> See also <a class="reference internal" href="datamodel.html#object.__aenter__" title="object.__aenter__"><code class="xref py py-meth docutils literal notranslate">__aenter__()</code></a> and <a class="reference internal" href="datamodel.html#object.__aexit__" title="object.__aexit__"><code class="xref py py-meth docutils literal notranslate">__aexit__()</code></a> for details. It is a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate">SyntaxError</code></a> to use an <code class="docutils literal notranslate">async with</code> statement outside the body of a coroutine function. <div class="admonition seealso"> See also <dl class="simple"> <dt><a class="pep reference external" href="https://peps.python.org/pep-0492/">PEP 492</a> - Coroutines with async and await syntax</dt><dd>The proposal that made coroutines a proper standalone concept in Python, and added supporting syntax. </dd> </dl> </div> </section> </section> <section id="type-parameter-lists"> <h2>8.10. Type parameter lists<a class="headerlink" href="#type-parameter-lists" title="Link to this heading">¶</a></h2> <div class="versionadded"> Added in version 3.12. </div> <div class="versionchanged"> Changed in version 3.13: Support for default values was added (see <a class="pep reference external" href="https://peps.python.org/pep-0696/">PEP 696</a>). </div> <pre id="index-55"> type_params ::= "[" <a class="reference internal" href="#grammar-token-python-grammar-type_param"><code class="xref docutils literal notranslate">type_param</code></a> ("," <a class="reference internal" href="#grammar-token-python-grammar-type_param"><code class="xref docutils literal notranslate">type_param</code></a>)* "]" type_param ::= <a class="reference internal" href="#grammar-token-python-grammar-typevar"><code class="xref docutils literal notranslate">typevar</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-typevartuple"><code class="xref docutils literal notranslate">typevartuple</code></a> | <a class="reference internal" href="#grammar-token-python-grammar-paramspec"><code class="xref docutils literal notranslate">paramspec</code></a> typevar ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> (":" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a>)? ("=" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a>)? typevartuple ::= "*" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> ("=" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a>)? paramspec ::= "**" <a class="reference internal" href="lexical_analysis.html#grammar-token-python-grammar-identifier"><code class="xref docutils literal notranslate">identifier</code></a> ("=" <a class="reference internal" href="expressions.html#grammar-token-python-grammar-expression"><code class="xref docutils literal notranslate">expression</code></a>)? </pre> <a class="reference internal" href="#def">Functions</a> (including <a class="reference internal" href="#async-def">coroutines</a>), <a class="reference internal" href="#class">classes</a> and <a class="reference internal" href="simple_stmts.html#type">type aliases</a> may contain a type parameter list: <div class="highlight-python3 notranslate"><div class="highlight"><pre>def max[T](args: list[T]) -> T: ... async def amax[T](args: list[T]) -> T: ... class Bag[T]: def __iter__(self) -> Iterator[T]: ... def add(self, arg: T) -> None: ... type ListOrSet[T] = list[T] | set[T] </pre></div> </div> Semantically, this indicates that the function, class, or type alias is generic over a type variable. This information is primarily used by static type checkers, and at runtime, generic objects behave much like their non-generic counterparts. Type parameters are declared in square brackets (<code class="docutils literal notranslate">[]</code>) immediately after the name of the function, class, or type alias. The type parameters are accessible within the scope of the generic object, but not elsewhere. Thus, after a declaration <code class="docutils literal notranslate">def func[T](): pass</code>, the name <code class="docutils literal notranslate">T</code> is not available in the module scope. Below, the semantics of generic objects are described with more precision. The scope of type parameters is modeled with a special function (technically, an <a class="reference internal" href="executionmodel.html#annotation-scopes">annotation scope</a>) that wraps the creation of the generic object. Generic functions, classes, and type aliases have a <a class="reference internal" href="../library/stdtypes.html#definition.__type_params__" title="definition.__type_params__"><code class="xref py py-attr docutils literal notranslate">__type_params__</code></a> attribute listing their type parameters. Type parameters come in three kinds: <ul class="simple"> <li><a class="reference internal" href="../library/typing.html#typing.TypeVar" title="typing.TypeVar"><code class="xref py py-data docutils literal notranslate">typing.TypeVar</code></a>, introduced by a plain name (e.g., <code class="docutils literal notranslate">T</code>). Semantically, this represents a single type to a type checker.</li> <li><a class="reference internal" href="../library/typing.html#typing.TypeVarTuple" title="typing.TypeVarTuple"><code class="xref py py-data docutils literal notranslate">typing.TypeVarTuple</code></a>, introduced by a name prefixed with a single asterisk (e.g., <code class="docutils literal notranslate">*Ts</code>). Semantically, this stands for a tuple of any number of types.</li> <li><a class="reference internal" href="../library/typing.html#typing.ParamSpec" title="typing.ParamSpec"><code class="xref py py-data docutils literal notranslate">typing.ParamSpec</code></a>, introduced by a name prefixed with two asterisks (e.g., <code class="docutils literal notranslate">**P</code>). Semantically, this stands for the parameters of a callable.</li> </ul> <a class="reference internal" href="../library/typing.html#typing.TypeVar" title="typing.TypeVar"><code class="xref py py-data docutils literal notranslate">typing.TypeVar</code></a> declarations can define bounds and constraints with a colon (<code class="docutils literal notranslate">:</code>) followed by an expression. A single expression after the colon indicates a bound (e.g. <code class="docutils literal notranslate">T: int</code>). Semantically, this means that the <code class="xref py py-data docutils literal notranslate">typing.TypeVar</code> can only represent types that are a subtype of this bound. A parenthesized tuple of expressions after the colon indicates a set of constraints (e.g. <code class="docutils literal notranslate">T: (str, bytes)</code>). Each member of the tuple should be a type (again, this is not enforced at runtime). Constrained type variables can only take on one of the types in the list of constraints. For <code class="xref py py-data docutils literal notranslate">typing.TypeVar</code>s declared using the type parameter list syntax, the bound and constraints are not evaluated when the generic object is created, but only when the value is explicitly accessed through the attributes <code class="docutils literal notranslate">__bound__</code> and <code class="docutils literal notranslate">__constraints__</code>. To accomplish this, the bounds or constraints are evaluated in a separate <a class="reference internal" href="executionmodel.html#annotation-scopes">annotation scope</a>. <a class="reference internal" href="../library/typing.html#typing.TypeVarTuple" title="typing.TypeVarTuple"><code class="xref py py-data docutils literal notranslate">typing.TypeVarTuple</code></a>s and <a class="reference internal" href="../library/typing.html#typing.ParamSpec" title="typing.ParamSpec"><code class="xref py py-data docutils literal notranslate">typing.ParamSpec</code></a>s cannot have bounds or constraints. All three flavors of type parameters can also have a default value, which is used when the type parameter is not explicitly provided. This is added by appending a single equals sign (<code class="docutils literal notranslate">=</code>) followed by an expression. Like the bounds and constraints of type variables, the default value is not evaluated when the object is created, but only when the type parameter’s <code class="docutils literal notranslate">__default__</code> attribute is accessed. To this end, the default value is evaluated in a separate <a class="reference internal" href="executionmodel.html#annotation-scopes">annotation scope</a>. If no default value is specified for a type parameter, the <code class="docutils literal notranslate">__default__</code> attribute is set to the special sentinel object <a class="reference internal" href="../library/typing.html#typing.NoDefault" title="typing.NoDefault"><code class="xref py py-data docutils literal notranslate">typing.NoDefault</code></a>. The following example indicates the full set of allowed type parameter declarations: <div class="highlight-python3 notranslate"><div class="highlight"><pre>def overly_generic[ SimpleTypeVar, TypeVarWithDefault = int, TypeVarWithBound: int, TypeVarWithConstraints: (str, bytes), *SimpleTypeVarTuple = (int, float), **SimpleParamSpec = (str, bytearray), ]( a: SimpleTypeVar, b: TypeVarWithDefault, c: TypeVarWithBound, d: Callable[SimpleParamSpec, TypeVarWithConstraints], *e: SimpleTypeVarTuple, ): ... </pre></div> </div> <section id="generic-functions"> <h3>8.10.1. Generic functions<a class="headerlink" href="#generic-functions" title="Link to this heading">¶</a></h3> Generic functions are declared as follows: <div class="highlight-python3 notranslate"><div class="highlight"><pre>def func[T](arg: T): ... </pre></div> </div> This syntax is equivalent to: <div class="highlight-python3 notranslate"><div class="highlight"><pre>annotation-def TYPE_PARAMS_OF_func(): T = typing.TypeVar("T") def func(arg: T): ... func.__type_params__ = (T,) return func func = TYPE_PARAMS_OF_func() </pre></div> </div> Here <code class="docutils literal notranslate">annotation-def</code> indicates an <a class="reference internal" href="executionmodel.html#annotation-scopes">annotation scope</a>, which is not actually bound to any name at runtime. (One other liberty is taken in the translation: the syntax does not go through attribute access on the <a class="reference internal" href="../library/typing.html#module-typing" title="typing: Support for type hints (see :pep:`484`)."><code class="xref py py-mod docutils literal notranslate">typing</code></a> module, but creates an instance of <a class="reference internal" href="../library/typing.html#typing.TypeVar" title="typing.TypeVar"><code class="xref py py-data docutils literal notranslate">typing.TypeVar</code></a> directly.) The annotations of generic functions are evaluated within the annotation scope used for declaring the type parameters, but the function’s defaults and decorators are not. The following example illustrates the scoping rules for these cases, as well as for additional flavors of type parameters: <div class="highlight-python3 notranslate"><div class="highlight"><pre>@decorator def func[T: int, *Ts, **P](*args: *Ts, arg: Callable[P, T] = some_default): ... </pre></div> </div> Except for the <a class="reference internal" href="executionmodel.html#lazy-evaluation">lazy evaluation</a> of the <a class="reference internal" href="../library/typing.html#typing.TypeVar" title="typing.TypeVar"><code class="xref py py-class docutils literal notranslate">TypeVar</code></a> bound, this is equivalent to: <div class="highlight-python3 notranslate"><div class="highlight"><pre>DEFAULT_OF_arg = some_default annotation-def TYPE_PARAMS_OF_func(): annotation-def BOUND_OF_T(): return int # In reality, BOUND_OF_T() is evaluated only on demand. T = typing.TypeVar("T", bound=BOUND_OF_T()) Ts = typing.TypeVarTuple("Ts") P = typing.ParamSpec("P") def func(*args: *Ts, arg: Callable[P, T] = DEFAULT_OF_arg): ... func.__type_params__ = (T, Ts, P) return func func = decorator(TYPE_PARAMS_OF_func()) </pre></div> </div> The capitalized names like <code class="docutils literal notranslate">DEFAULT_OF_arg</code> are not actually bound at runtime. </section> <section id="generic-classes"> <h3>8.10.2. Generic classes<a class="headerlink" href="#generic-classes" title="Link to this heading">¶</a></h3> Generic classes are declared as follows: <div class="highlight-python3 notranslate"><div class="highlight"><pre>class Bag[T]: ... </pre></div> </div> This syntax is equivalent to: <div class="highlight-python3 notranslate"><div class="highlight"><pre>annotation-def TYPE_PARAMS_OF_Bag(): T = typing.TypeVar("T") class Bag(typing.Generic[T]): __type_params__ = (T,) ... return Bag Bag = TYPE_PARAMS_OF_Bag() </pre></div> </div> Here again <code class="docutils literal notranslate">annotation-def</code> (not a real keyword) indicates an <a class="reference internal" href="executionmodel.html#annotation-scopes">annotation scope</a>, and the name <code class="docutils literal notranslate">TYPE_PARAMS_OF_Bag</code> is not actually bound at runtime. Generic classes implicitly inherit from <a class="reference internal" href="../library/typing.html#typing.Generic" title="typing.Generic"><code class="xref py py-data docutils literal notranslate">typing.Generic</code></a>. The base classes and keyword arguments of generic classes are evaluated within the type scope for the type parameters, and decorators are evaluated outside that scope. This is illustrated by this example: <div class="highlight-python3 notranslate"><div class="highlight"><pre>@decorator class Bag(Base[T], arg=T): ... </pre></div> </div> This is equivalent to: <div class="highlight-python3 notranslate"><div class="highlight"><pre>annotation-def TYPE_PARAMS_OF_Bag(): T = typing.TypeVar("T") class Bag(Base[T], typing.Generic[T], arg=T): __type_params__ = (T,) ... return Bag Bag = decorator(TYPE_PARAMS_OF_Bag()) </pre></div> </div> </section> <section id="generic-type-aliases"> <h3>8.10.3. Generic type aliases<a class="headerlink" href="#generic-type-aliases" title="Link to this heading">¶</a></h3> The <a class="reference internal" href="simple_stmts.html#type"><code class="xref std std-keyword docutils literal notranslate">type</code></a> statement can also be used to create a generic type alias: <div class="highlight-python3 notranslate"><div class="highlight"><pre>type ListOrSet[T] = list[T] | set[T] </pre></div> </div> Except for the <a class="reference internal" href="executionmodel.html#lazy-evaluation">lazy evaluation</a> of the value, this is equivalent to: <div class="highlight-python3 notranslate"><div class="highlight"><pre>annotation-def TYPE_PARAMS_OF_ListOrSet(): T = typing.TypeVar("T") annotation-def VALUE_OF_ListOrSet(): return list[T] | set[T] # In reality, the value is lazily evaluated return typing.TypeAliasType("ListOrSet", VALUE_OF_ListOrSet(), type_params=(T,)) ListOrSet = TYPE_PARAMS_OF_ListOrSet() </pre></div> </div> Here, <code class="docutils literal notranslate">annotation-def</code> (not a real keyword) indicates an <a class="reference internal" href="executionmodel.html#annotation-scopes">annotation scope</a>. The capitalized names like <code class="docutils literal notranslate">TYPE_PARAMS_OF_ListOrSet</code> are not actually bound at runtime. Footnotes <aside class="footnote-list brackets"> <aside class="footnote brackets" id="id20" role="doc-footnote"> [<a role="doc-backlink" href="#id1">1</a>] 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">finally</code></a> clause which happens to raise another exception. That new exception causes the old one to be lost. </aside> <aside class="footnote brackets" id="id21" role="doc-footnote"> [<a role="doc-backlink" href="#id11">2</a>] In pattern matching, a sequence is defined as one of the following: <ul class="simple"> <li>a class that inherits from <a class="reference internal" href="../library/collections.abc.html#collections.abc.Sequence" title="collections.abc.Sequence"><code class="xref py py-class docutils literal notranslate">collections.abc.Sequence</code></a></li> <li>a Python class that has been registered as <a class="reference internal" href="../library/collections.abc.html#collections.abc.Sequence" title="collections.abc.Sequence"><code class="xref py py-class docutils literal notranslate">collections.abc.Sequence</code></a></li> <li>a builtin class that has its (CPython) <a class="reference internal" href="../c-api/typeobj.html#c.Py_TPFLAGS_SEQUENCE" title="Py_TPFLAGS_SEQUENCE"><code class="xref c c-macro docutils literal notranslate">Py_TPFLAGS_SEQUENCE</code></a> bit set</li> <li>a class that inherits from any of the above</li> </ul> The following standard library classes are sequences: <ul class="simple"> <li><a class="reference internal" href="../library/array.html#array.array" title="array.array"><code class="xref py py-class docutils literal notranslate">array.array</code></a></li> <li><a class="reference internal" href="../library/collections.html#collections.deque" title="collections.deque"><code class="xref py py-class docutils literal notranslate">collections.deque</code></a></li> <li><a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate">list</code></a></li> <li><a class="reference internal" href="../library/stdtypes.html#memoryview" title="memoryview"><code class="xref py py-class docutils literal notranslate">memoryview</code></a></li> <li><a class="reference internal" href="../library/stdtypes.html#range" title="range"><code class="xref py py-class docutils literal notranslate">range</code></a></li> <li><a class="reference internal" href="../library/stdtypes.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate">tuple</code></a></li> </ul> <div class="admonition note"> Note Subject values of type <code class="docutils literal notranslate">str</code>, <code class="docutils literal notranslate">bytes</code>, and <code class="docutils literal notranslate">bytearray</code> do not match sequence patterns. </div> </aside> <aside class="footnote brackets" id="id22" role="doc-footnote"> [<a role="doc-backlink" href="#id13">3</a>] In pattern matching, a mapping is defined as one of the following: <ul class="simple"> <li>a class that inherits from <a class="reference internal" href="../library/collections.abc.html#collections.abc.Mapping" title="collections.abc.Mapping"><code class="xref py py-class docutils literal notranslate">collections.abc.Mapping</code></a></li> <li>a Python class that has been registered as <a class="reference internal" href="../library/collections.abc.html#collections.abc.Mapping" title="collections.abc.Mapping"><code class="xref py py-class docutils literal notranslate">collections.abc.Mapping</code></a></li> <li>a builtin class that has its (CPython) <a class="reference internal" href="../c-api/typeobj.html#c.Py_TPFLAGS_MAPPING" title="Py_TPFLAGS_MAPPING"><code class="xref c c-macro docutils literal notranslate">Py_TPFLAGS_MAPPING</code></a> bit set</li> <li>a class that inherits from any of the above</li> </ul> The standard library classes <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a> and <a class="reference internal" href="../library/types.html#types.MappingProxyType" title="types.MappingProxyType"><code class="xref py py-class docutils literal notranslate">types.MappingProxyType</code></a> are mappings. </aside> <aside class="footnote brackets" id="id23" role="doc-footnote"> [<a role="doc-backlink" href="#id15">4</a>] A string literal appearing as the first statement in the function body is transformed into the function’s <a class="reference internal" href="datamodel.html#function.__doc__" title="function.__doc__"><code class="xref py py-attr docutils literal notranslate">__doc__</code></a> attribute and therefore the function’s <a class="reference internal" href="../glossary.html#term-docstring">docstring</a>. </aside> <aside class="footnote brackets" id="id24" role="doc-footnote"> [<a role="doc-backlink" href="#id16">5</a>] A string literal appearing as the first statement in the class body is transformed into the namespace’s <a class="reference internal" href="datamodel.html#type.__doc__" title="type.__doc__"><code class="xref py py-attr docutils literal notranslate">__doc__</code></a> item and therefore the class’s <a class="reference internal" href="../glossary.html#term-docstring">docstring</a>. </aside> </aside> </section> </section> </section> <div class="clearer"></div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="Main"> <div class="sphinxsidebarwrapper"> <div> <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="xref std std-keyword docutils literal notranslate">if</code> statement</a></li> <li><a class="reference internal" href="#the-while-statement">8.2. The <code class="xref std std-keyword docutils literal notranslate">while</code> statement</a></li> <li><a class="reference internal" href="#the-for-statement">8.3. The <code class="xref std std-keyword docutils literal notranslate">for</code> statement</a></li> <li><a class="reference internal" href="#the-try-statement">8.4. The <code class="xref std std-keyword docutils literal notranslate">try</code> statement</a><ul> <li><a class="reference internal" href="#except-clause">8.4.1. <code class="xref std std-keyword docutils literal notranslate">except</code> clause</a></li> <li><a class="reference internal" href="#except-star">8.4.2. <code class="xref std std-keyword docutils literal notranslate">except*</code> clause</a></li> <li><a class="reference internal" href="#else-clause">8.4.3. <code class="xref std std-keyword docutils literal notranslate">else</code> clause</a></li> <li><a class="reference internal" href="#finally-clause">8.4.4. <code class="xref std std-keyword docutils literal notranslate">finally</code> clause</a></li> </ul> </li> <li><a class="reference internal" href="#the-with-statement">8.5. The <code class="xref std std-keyword docutils literal notranslate">with</code> statement</a></li> <li><a class="reference internal" href="#the-match-statement">8.6. The <code class="xref std std-keyword docutils literal notranslate">match</code> statement</a><ul> <li><a class="reference internal" href="#overview">8.6.1. Overview</a></li> <li><a class="reference internal" href="#guards">8.6.2. Guards</a></li> <li><a class="reference internal" href="#irrefutable-case-blocks">8.6.3. Irrefutable Case Blocks</a></li> <li><a class="reference internal" href="#patterns">8.6.4. Patterns</a><ul> <li><a class="reference internal" href="#or-patterns">8.6.4.1. OR Patterns</a></li> <li><a class="reference internal" href="#as-patterns">8.6.4.2. AS Patterns</a></li> <li><a class="reference internal" href="#literal-patterns">8.6.4.3. Literal Patterns</a></li> <li><a class="reference internal" href="#capture-patterns">8.6.4.4. Capture Patterns</a></li> <li><a class="reference internal" href="#wildcard-patterns">8.6.4.5. Wildcard Patterns</a></li> <li><a class="reference internal" href="#value-patterns">8.6.4.6. Value Patterns</a></li> <li><a class="reference internal" href="#group-patterns">8.6.4.7. Group Patterns</a></li> <li><a class="reference internal" href="#sequence-patterns">8.6.4.8. Sequence Patterns</a></li> <li><a class="reference internal" href="#mapping-patterns">8.6.4.9. Mapping Patterns</a></li> <li><a class="reference internal" href="#class-patterns">8.6.4.10. Class Patterns</a></li> </ul> </li> </ul> </li> <li><a class="reference internal" href="#function-definitions">8.7. Function definitions</a></li> <li><a class="reference internal" href="#class-definitions">8.8. Class definitions</a></li> <li><a class="reference internal" href="#coroutines">8.9. Coroutines</a><ul> <li><a class="reference internal" href="#coroutine-function-definition">8.9.1. Coroutine function definition</a></li> <li><a class="reference internal" href="#the-async-for-statement">8.9.2. The <code class="xref std std-keyword docutils literal notranslate">async for</code> statement</a></li> <li><a class="reference internal" href="#the-async-with-statement">8.9.3. The <code class="xref std std-keyword docutils literal notranslate">async with</code> statement</a></li> </ul> </li> <li><a class="reference internal" href="#type-parameter-lists">8.10. Type parameter lists</a><ul> <li><a class="reference internal" href="#generic-functions">8.10.1. Generic functions</a></li> <li><a class="reference internal" href="#generic-classes">8.10.2. Generic classes</a></li> <li><a class="reference internal" href="#generic-type-aliases">8.10.3. Generic type aliases</a></li> </ul> </li> </ul> </li> </ul> </div> <div> <h4>Previous topic</h4> <a href="simple_stmts.html" title="previous chapter">7. Simple statements</a> </div> <div> <h4>Next topic</h4> <a href="toplevel_components.html" title="next chapter">9. Top-level components</a> </div> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li> <a href="https://github.com/python/cpython/blob/main/Doc/reference/compound_stmts.rst" rel="nofollow">Show Source </a> </li> </ul> </div> </div> <div id="sidebarbutton" title="Collapse sidebar"> « </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="Related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="toplevel_components.html" title="9. Top-level components" >next</a> |</li> <li class="right" > <a href="simple_stmts.html" title="7. Simple statements" >previous</a> |</li> <li><img src="../_static/py.svg" alt="Python logo" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li class="switchers"> <div class="language_switcher_placeholder"></div> <div class="version_switcher_placeholder"></div> </li> <li> </li> <li id="cpython-language-and-version"> <a href="../index.html">3.13.0 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Language Reference</a> »</li> <li class="nav-item nav-item-this"><a href="">8. Compound statements</a></li> <li class="right"> <div class="inline-search" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" aria-label="Quick search" type="search" name="q" id="search-box" /> <input type="submit" value="Go" /> </form> </div> | </li> <li class="right"> <label class="theme-selector-label"> Theme <select class="theme-selector" oninput="activateTheme(this.value)"> <option value="auto" selected>Auto</option> <option value="light">Light</option> <option value="dark">Dark</option> </select> </label> |</li> </ul> </div> <div class="footer"> © <a href="../copyright.html"> Copyright </a> 2001-2024, Python Software Foundation. This page is licensed under the Python Software Foundation License Version 2. Examples, recipes, and other code in the documentation are additionally licensed under the Zero Clause BSD License. See <a href="/license.html">History and License</a> for more information. The Python Software Foundation is a non-profit corporation. <a href="https://www.python.org/psf/donations/">Please donate.</a> Last updated on Nov 24, 2024 (06:13 UTC). <a href="/bugs.html">Found a bug</a>? Created using <a href="https://www.sphinx-doc.org/">Sphinx</a> 8.1.3. </div> <script type="text/javascript" src="../_static/switchers.js"></script> </body> </html>