661 lines
45 KiB
HTML
661 lines
45 KiB
HTML
<!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="contextvars — Context Variables" />
|
||
<meta property="og:type" content="website" />
|
||
<meta property="og:url" content="https://docs.python.org/3/library/contextvars.html" />
|
||
<meta property="og:site_name" content="Python documentation" />
|
||
<meta property="og:description" content="This module provides APIs to manage, store, and access context-local state. The ContextVar class is used to declare and work with Context Variables. The copy_context() function and the Context clas..." />
|
||
<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="This module provides APIs to manage, store, and access context-local state. The ContextVar class is used to declare and work with Context Variables. The copy_context() function and the Context clas..." />
|
||
<meta property="og:image:width" content="200">
|
||
<meta property="og:image:height" content="200">
|
||
<meta name="theme-color" content="#3776ab">
|
||
|
||
<title>contextvars — Context Variables — Python 3.13.3 documentation</title><meta name="viewport" content="width=device-width, initial-scale=1.0">
|
||
|
||
<link rel="stylesheet" type="text/css" href="../_static/pygments.css?v=b86133f3" />
|
||
<link rel="stylesheet" type="text/css" href="../_static/pydoctheme.css?v=23252803" />
|
||
<link id="pygments_dark_css" media="(prefers-color-scheme: dark)" rel="stylesheet" type="text/css" href="../_static/pygments_dark.css?v=5349f25f" />
|
||
|
||
<script src="../_static/documentation_options.js?v=5d57ca2d"></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 3.13.3 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="_thread — Low-level threading API" href="_thread.html" />
|
||
<link rel="prev" title="queue — A synchronized queue class" href="queue.html" />
|
||
|
||
<link rel="canonical" href="https://docs.python.org/3/library/contextvars.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">
|
||
<span></span>
|
||
</label>
|
||
<span class="nav-items-wrapper">
|
||
<a href="https://www.python.org/" class="nav-logo">
|
||
<img src="../_static/py.svg" alt="Python logo"/>
|
||
</a>
|
||
<span class="version_switcher_placeholder"></span>
|
||
<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>
|
||
</span>
|
||
</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="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextvars</span></code> — Context Variables</a><ul>
|
||
<li><a class="reference internal" href="#context-variables">Context Variables</a></li>
|
||
<li><a class="reference internal" href="#manual-context-management">Manual Context Management</a></li>
|
||
<li><a class="reference internal" href="#asyncio-support">asyncio support</a></li>
|
||
</ul>
|
||
</li>
|
||
</ul>
|
||
|
||
</div>
|
||
<div>
|
||
<h4>Previous topic</h4>
|
||
<p class="topless"><a href="queue.html"
|
||
title="previous chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">queue</span></code> — A synchronized queue class</a></p>
|
||
</div>
|
||
<div>
|
||
<h4>Next topic</h4>
|
||
<p class="topless"><a href="_thread.html"
|
||
title="next chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">_thread</span></code> — Low-level threading API</a></p>
|
||
</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/library/contextvars.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="_thread.html" title="_thread — Low-level threading API"
|
||
accesskey="N">next</a> |</li>
|
||
<li class="right" >
|
||
<a href="queue.html" title="queue — A synchronized queue class"
|
||
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">3.13.3 Documentation</a> »
|
||
</li>
|
||
|
||
<li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li>
|
||
<li class="nav-item nav-item-2"><a href="concurrency.html" accesskey="U">Concurrent Execution</a> »</li>
|
||
<li class="nav-item nav-item-this"><a href=""><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextvars</span></code> — Context Variables</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="module-contextvars">
|
||
<span id="contextvars-context-variables"></span><h1><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextvars</span></code> — Context Variables<a class="headerlink" href="#module-contextvars" title="Link to this heading">¶</a></h1>
|
||
<hr class="docutils" />
|
||
<p>This module provides APIs to manage, store, and access context-local
|
||
state. The <a class="reference internal" href="#contextvars.ContextVar" title="contextvars.ContextVar"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextVar</span></code></a> class is used to declare
|
||
and work with <em>Context Variables</em>. The <a class="reference internal" href="#contextvars.copy_context" title="contextvars.copy_context"><code class="xref py py-func docutils literal notranslate"><span class="pre">copy_context()</span></code></a>
|
||
function and the <a class="reference internal" href="#contextvars.Context" title="contextvars.Context"><code class="xref py py-class docutils literal notranslate"><span class="pre">Context</span></code></a> class should be used to
|
||
manage the current context in asynchronous frameworks.</p>
|
||
<p>Context managers that have state should use Context Variables
|
||
instead of <a class="reference internal" href="threading.html#threading.local" title="threading.local"><code class="xref py py-func docutils literal notranslate"><span class="pre">threading.local()</span></code></a> to prevent their state from
|
||
bleeding to other code unexpectedly, when used in concurrent code.</p>
|
||
<p>See also <span class="target" id="index-0"></span><a class="pep reference external" href="https://peps.python.org/pep-0567/"><strong>PEP 567</strong></a> for additional details.</p>
|
||
<div class="versionadded">
|
||
<p><span class="versionmodified added">Added in version 3.7.</span></p>
|
||
</div>
|
||
<section id="context-variables">
|
||
<h2>Context Variables<a class="headerlink" href="#context-variables" title="Link to this heading">¶</a></h2>
|
||
<dl class="py class">
|
||
<dt class="sig sig-object py" id="contextvars.ContextVar">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">contextvars.</span></span><span class="sig-name descname"><span class="pre">ContextVar</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">name</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">*</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">default</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.ContextVar" title="Link to this definition">¶</a></dt>
|
||
<dd><p>This class is used to declare a new Context Variable, e.g.:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">var</span><span class="p">:</span> <span class="n">ContextVar</span><span class="p">[</span><span class="nb">int</span><span class="p">]</span> <span class="o">=</span> <span class="n">ContextVar</span><span class="p">(</span><span class="s1">'var'</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">42</span><span class="p">)</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>The required <em>name</em> parameter is used for introspection and debug
|
||
purposes.</p>
|
||
<p>The optional keyword-only <em>default</em> parameter is returned by
|
||
<a class="reference internal" href="#contextvars.ContextVar.get" title="contextvars.ContextVar.get"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ContextVar.get()</span></code></a> when no value for the variable is found
|
||
in the current context.</p>
|
||
<p><strong>Important:</strong> Context Variables should be created at the top module
|
||
level and never in closures. <a class="reference internal" href="#contextvars.Context" title="contextvars.Context"><code class="xref py py-class docutils literal notranslate"><span class="pre">Context</span></code></a> objects hold strong
|
||
references to context variables which prevents context variables
|
||
from being properly garbage collected.</p>
|
||
<dl class="py attribute">
|
||
<dt class="sig sig-object py" id="contextvars.ContextVar.name">
|
||
<span class="sig-name descname"><span class="pre">name</span></span><a class="headerlink" href="#contextvars.ContextVar.name" title="Link to this definition">¶</a></dt>
|
||
<dd><p>The name of the variable. This is a read-only property.</p>
|
||
<div class="versionadded">
|
||
<p><span class="versionmodified added">Added in version 3.7.1.</span></p>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="contextvars.ContextVar.get">
|
||
<span class="sig-name descname"><span class="pre">get</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">default</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.ContextVar.get" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Return a value for the context variable for the current context.</p>
|
||
<p>If there is no value for the variable in the current context,
|
||
the method will:</p>
|
||
<ul class="simple">
|
||
<li><p>return the value of the <em>default</em> argument of the method,
|
||
if provided; or</p></li>
|
||
<li><p>return the default value for the context variable,
|
||
if it was created with one; or</p></li>
|
||
<li><p>raise a <a class="reference internal" href="exceptions.html#LookupError" title="LookupError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">LookupError</span></code></a>.</p></li>
|
||
</ul>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="contextvars.ContextVar.set">
|
||
<span class="sig-name descname"><span class="pre">set</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">value</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.ContextVar.set" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Call to set a new value for the context variable in the current
|
||
context.</p>
|
||
<p>The required <em>value</em> argument is the new value for the context
|
||
variable.</p>
|
||
<p>Returns a <a class="reference internal" href="#contextvars.Token" title="contextvars.Token"><code class="xref py py-class docutils literal notranslate"><span class="pre">Token</span></code></a> object that can be used
|
||
to restore the variable to its previous value via the
|
||
<a class="reference internal" href="#contextvars.ContextVar.reset" title="contextvars.ContextVar.reset"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ContextVar.reset()</span></code></a> method.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="contextvars.ContextVar.reset">
|
||
<span class="sig-name descname"><span class="pre">reset</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">token</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.ContextVar.reset" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Reset the context variable to the value it had before the
|
||
<a class="reference internal" href="#contextvars.ContextVar.set" title="contextvars.ContextVar.set"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ContextVar.set()</span></code></a> that created the <em>token</em> was used.</p>
|
||
<p>For example:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">var</span> <span class="o">=</span> <span class="n">ContextVar</span><span class="p">(</span><span class="s1">'var'</span><span class="p">)</span>
|
||
|
||
<span class="n">token</span> <span class="o">=</span> <span class="n">var</span><span class="o">.</span><span class="n">set</span><span class="p">(</span><span class="s1">'new value'</span><span class="p">)</span>
|
||
<span class="c1"># code that uses 'var'; var.get() returns 'new value'.</span>
|
||
<span class="n">var</span><span class="o">.</span><span class="n">reset</span><span class="p">(</span><span class="n">token</span><span class="p">)</span>
|
||
|
||
<span class="c1"># After the reset call the var has no value again, so</span>
|
||
<span class="c1"># var.get() would raise a LookupError.</span>
|
||
</pre></div>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
</dd></dl>
|
||
|
||
<dl class="py class">
|
||
<dt class="sig sig-object py" id="contextvars.Token">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">contextvars.</span></span><span class="sig-name descname"><span class="pre">Token</span></span><a class="headerlink" href="#contextvars.Token" title="Link to this definition">¶</a></dt>
|
||
<dd><p><em>Token</em> objects are returned by the <a class="reference internal" href="#contextvars.ContextVar.set" title="contextvars.ContextVar.set"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ContextVar.set()</span></code></a> method.
|
||
They can be passed to the <a class="reference internal" href="#contextvars.ContextVar.reset" title="contextvars.ContextVar.reset"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ContextVar.reset()</span></code></a> method to revert
|
||
the value of the variable to what it was before the corresponding
|
||
<em>set</em>.</p>
|
||
<dl class="py attribute">
|
||
<dt class="sig sig-object py" id="contextvars.Token.var">
|
||
<span class="sig-name descname"><span class="pre">var</span></span><a class="headerlink" href="#contextvars.Token.var" title="Link to this definition">¶</a></dt>
|
||
<dd><p>A read-only property. Points to the <a class="reference internal" href="#contextvars.ContextVar" title="contextvars.ContextVar"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextVar</span></code></a> object
|
||
that created the token.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py attribute">
|
||
<dt class="sig sig-object py" id="contextvars.Token.old_value">
|
||
<span class="sig-name descname"><span class="pre">old_value</span></span><a class="headerlink" href="#contextvars.Token.old_value" title="Link to this definition">¶</a></dt>
|
||
<dd><p>A read-only property. Set to the value the variable had before
|
||
the <a class="reference internal" href="#contextvars.ContextVar.set" title="contextvars.ContextVar.set"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ContextVar.set()</span></code></a> method call that created the token.
|
||
It points to <a class="reference internal" href="#contextvars.Token.MISSING" title="contextvars.Token.MISSING"><code class="xref py py-attr docutils literal notranslate"><span class="pre">Token.MISSING</span></code></a> if the variable was not set
|
||
before the call.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py attribute">
|
||
<dt class="sig sig-object py" id="contextvars.Token.MISSING">
|
||
<span class="sig-name descname"><span class="pre">MISSING</span></span><a class="headerlink" href="#contextvars.Token.MISSING" title="Link to this definition">¶</a></dt>
|
||
<dd><p>A marker object used by <a class="reference internal" href="#contextvars.Token.old_value" title="contextvars.Token.old_value"><code class="xref py py-attr docutils literal notranslate"><span class="pre">Token.old_value</span></code></a>.</p>
|
||
</dd></dl>
|
||
|
||
</dd></dl>
|
||
|
||
</section>
|
||
<section id="manual-context-management">
|
||
<h2>Manual Context Management<a class="headerlink" href="#manual-context-management" title="Link to this heading">¶</a></h2>
|
||
<dl class="py function">
|
||
<dt class="sig sig-object py" id="contextvars.copy_context">
|
||
<span class="sig-prename descclassname"><span class="pre">contextvars.</span></span><span class="sig-name descname"><span class="pre">copy_context</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.copy_context" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Returns a copy of the current <a class="reference internal" href="#contextvars.Context" title="contextvars.Context"><code class="xref py py-class docutils literal notranslate"><span class="pre">Context</span></code></a> object.</p>
|
||
<p>The following snippet gets a copy of the current context and prints
|
||
all variables and their values that are set in it:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">ctx</span><span class="p">:</span> <span class="n">Context</span> <span class="o">=</span> <span class="n">copy_context</span><span class="p">()</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="nb">list</span><span class="p">(</span><span class="n">ctx</span><span class="o">.</span><span class="n">items</span><span class="p">()))</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>The function has an <em>O</em>(1) complexity, i.e. works equally fast for
|
||
contexts with a few context variables and for contexts that have
|
||
a lot of them.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py class">
|
||
<dt class="sig sig-object py" id="contextvars.Context">
|
||
<em class="property"><span class="k"><span class="pre">class</span></span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">contextvars.</span></span><span class="sig-name descname"><span class="pre">Context</span></span><a class="headerlink" href="#contextvars.Context" title="Link to this definition">¶</a></dt>
|
||
<dd><p>A mapping of <a class="reference internal" href="#contextvars.ContextVar" title="contextvars.ContextVar"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextVars</span></code></a> to their values.</p>
|
||
<p><code class="docutils literal notranslate"><span class="pre">Context()</span></code> creates an empty context with no values in it.
|
||
To get a copy of the current context use the
|
||
<a class="reference internal" href="#contextvars.copy_context" title="contextvars.copy_context"><code class="xref py py-func docutils literal notranslate"><span class="pre">copy_context()</span></code></a> function.</p>
|
||
<p>Each thread has its own effective stack of <code class="xref py py-class docutils literal notranslate"><span class="pre">Context</span></code> objects. The
|
||
<a class="reference internal" href="../glossary.html#term-current-context"><span class="xref std std-term">current context</span></a> is the <code class="xref py py-class docutils literal notranslate"><span class="pre">Context</span></code> object at the top of the
|
||
current thread’s stack. All <code class="xref py py-class docutils literal notranslate"><span class="pre">Context</span></code> objects in the stacks are
|
||
considered to be <em>entered</em>.</p>
|
||
<p><em>Entering</em> a context, which can be done by calling its <a class="reference internal" href="#contextvars.Context.run" title="contextvars.Context.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a>
|
||
method, makes the context the current context by pushing it onto the top of
|
||
the current thread’s context stack.</p>
|
||
<p><em>Exiting</em> from the current context, which can be done by returning from the
|
||
callback passed to the <a class="reference internal" href="#contextvars.Context.run" title="contextvars.Context.run"><code class="xref py py-meth docutils literal notranslate"><span class="pre">run()</span></code></a> method, restores the current
|
||
context to what it was before the context was entered by popping the context
|
||
off the top of the context stack.</p>
|
||
<p>Since each thread has its own context stack, <a class="reference internal" href="#contextvars.ContextVar" title="contextvars.ContextVar"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextVar</span></code></a> objects
|
||
behave in a similar fashion to <a class="reference internal" href="threading.html#threading.local" title="threading.local"><code class="xref py py-func docutils literal notranslate"><span class="pre">threading.local()</span></code></a> when values are
|
||
assigned in different threads.</p>
|
||
<p>Attempting to enter an already entered context, including contexts entered in
|
||
other threads, raises a <a class="reference internal" href="exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a>.</p>
|
||
<p>After exiting a context, it can later be re-entered (from any thread).</p>
|
||
<p>Any changes to <a class="reference internal" href="#contextvars.ContextVar" title="contextvars.ContextVar"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextVar</span></code></a> values via the <a class="reference internal" href="#contextvars.ContextVar.set" title="contextvars.ContextVar.set"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ContextVar.set()</span></code></a>
|
||
method are recorded in the current context. The <a class="reference internal" href="#contextvars.ContextVar.get" title="contextvars.ContextVar.get"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ContextVar.get()</span></code></a>
|
||
method returns the value associated with the current context. Exiting a
|
||
context effectively reverts any changes made to context variables while the
|
||
context was entered (if needed, the values can be restored by re-entering the
|
||
context).</p>
|
||
<p>Context implements the <a class="reference internal" href="collections.abc.html#collections.abc.Mapping" title="collections.abc.Mapping"><code class="xref py py-class docutils literal notranslate"><span class="pre">collections.abc.Mapping</span></code></a> interface.</p>
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="contextvars.Context.run">
|
||
<span class="sig-name descname"><span class="pre">run</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">callable</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">*</span></span><span class="n"><span class="pre">args</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">**</span></span><span class="n"><span class="pre">kwargs</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.Context.run" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Enters the Context, executes <code class="docutils literal notranslate"><span class="pre">callable(*args,</span> <span class="pre">**kwargs)</span></code>, then exits the
|
||
Context. Returns <em>callable</em>’s return value, or propagates an exception if
|
||
one occurred.</p>
|
||
<p>Example:</p>
|
||
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">contextvars</span>
|
||
|
||
<span class="n">var</span> <span class="o">=</span> <span class="n">contextvars</span><span class="o">.</span><span class="n">ContextVar</span><span class="p">(</span><span class="s1">'var'</span><span class="p">)</span>
|
||
<span class="n">var</span><span class="o">.</span><span class="n">set</span><span class="p">(</span><span class="s1">'spam'</span><span class="p">)</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="n">var</span><span class="o">.</span><span class="n">get</span><span class="p">())</span> <span class="c1"># 'spam'</span>
|
||
|
||
<span class="n">ctx</span> <span class="o">=</span> <span class="n">contextvars</span><span class="o">.</span><span class="n">copy_context</span><span class="p">()</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
|
||
<span class="c1"># 'var' was set to 'spam' before</span>
|
||
<span class="c1"># calling 'copy_context()' and 'ctx.run(main)', so:</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="n">var</span><span class="o">.</span><span class="n">get</span><span class="p">())</span> <span class="c1"># 'spam'</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="n">ctx</span><span class="p">[</span><span class="n">var</span><span class="p">])</span> <span class="c1"># 'spam'</span>
|
||
|
||
<span class="n">var</span><span class="o">.</span><span class="n">set</span><span class="p">(</span><span class="s1">'ham'</span><span class="p">)</span>
|
||
|
||
<span class="c1"># Now, after setting 'var' to 'ham':</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="n">var</span><span class="o">.</span><span class="n">get</span><span class="p">())</span> <span class="c1"># 'ham'</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="n">ctx</span><span class="p">[</span><span class="n">var</span><span class="p">])</span> <span class="c1"># 'ham'</span>
|
||
|
||
<span class="c1"># Any changes that the 'main' function makes to 'var'</span>
|
||
<span class="c1"># will be contained in 'ctx'.</span>
|
||
<span class="n">ctx</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">)</span>
|
||
|
||
<span class="c1"># The 'main()' function was run in the 'ctx' context,</span>
|
||
<span class="c1"># so changes to 'var' are contained in it:</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="n">ctx</span><span class="p">[</span><span class="n">var</span><span class="p">])</span> <span class="c1"># 'ham'</span>
|
||
|
||
<span class="c1"># However, outside of 'ctx', 'var' is still set to 'spam':</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="n">var</span><span class="o">.</span><span class="n">get</span><span class="p">())</span> <span class="c1"># 'spam'</span>
|
||
</pre></div>
|
||
</div>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="contextvars.Context.copy">
|
||
<span class="sig-name descname"><span class="pre">copy</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.Context.copy" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Return a shallow copy of the context object.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="describe">
|
||
<dt class="sig sig-object">
|
||
<span class="sig-name descname"><span class="pre">var</span> <span class="pre">in</span> <span class="pre">context</span></span></dt>
|
||
<dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the <em>context</em> has a value for <em>var</em> set;
|
||
return <code class="docutils literal notranslate"><span class="pre">False</span></code> otherwise.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="describe">
|
||
<dt class="sig sig-object">
|
||
<span class="sig-name descname"><span class="pre">context[var]</span></span></dt>
|
||
<dd><p>Return the value of the <em>var</em> <a class="reference internal" href="#contextvars.ContextVar" title="contextvars.ContextVar"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextVar</span></code></a> variable.
|
||
If the variable is not set in the context object, a
|
||
<a class="reference internal" href="exceptions.html#KeyError" title="KeyError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyError</span></code></a> is raised.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="contextvars.Context.get">
|
||
<span class="sig-name descname"><span class="pre">get</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">var</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">default</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.Context.get" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Return the value for <em>var</em> if <em>var</em> has the value in the context
|
||
object. Return <em>default</em> otherwise. If <em>default</em> is not given,
|
||
return <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="describe">
|
||
<dt class="sig sig-object">
|
||
<span class="sig-name descname"><span class="pre">iter(context)</span></span></dt>
|
||
<dd><p>Return an iterator over the variables stored in the context
|
||
object.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="describe">
|
||
<dt class="sig sig-object">
|
||
<span class="sig-name descname"><span class="pre">len(proxy)</span></span></dt>
|
||
<dd><p>Return the number of variables set in the context object.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="contextvars.Context.keys">
|
||
<span class="sig-name descname"><span class="pre">keys</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.Context.keys" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Return a list of all variables in the context object.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="contextvars.Context.values">
|
||
<span class="sig-name descname"><span class="pre">values</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.Context.values" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Return a list of all variables’ values in the context object.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="py method">
|
||
<dt class="sig sig-object py" id="contextvars.Context.items">
|
||
<span class="sig-name descname"><span class="pre">items</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.Context.items" title="Link to this definition">¶</a></dt>
|
||
<dd><p>Return a list of 2-tuples containing all variables and their
|
||
values in the context object.</p>
|
||
</dd></dl>
|
||
|
||
</dd></dl>
|
||
|
||
</section>
|
||
<section id="asyncio-support">
|
||
<h2>asyncio support<a class="headerlink" href="#asyncio-support" title="Link to this heading">¶</a></h2>
|
||
<p>Context variables are natively supported in <a class="reference internal" href="asyncio.html#module-asyncio" title="asyncio: Asynchronous I/O."><code class="xref py py-mod docutils literal notranslate"><span class="pre">asyncio</span></code></a> and are
|
||
ready to be used without any extra configuration. For example, here
|
||
is a simple echo server, that uses a context variable to make the
|
||
address of a remote client available in the Task that handles that
|
||
client:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
|
||
<span class="kn">import</span><span class="w"> </span><span class="nn">contextvars</span>
|
||
|
||
<span class="n">client_addr_var</span> <span class="o">=</span> <span class="n">contextvars</span><span class="o">.</span><span class="n">ContextVar</span><span class="p">(</span><span class="s1">'client_addr'</span><span class="p">)</span>
|
||
|
||
<span class="k">def</span><span class="w"> </span><span class="nf">render_goodbye</span><span class="p">():</span>
|
||
<span class="c1"># The address of the currently handled client can be accessed</span>
|
||
<span class="c1"># without passing it explicitly to this function.</span>
|
||
|
||
<span class="n">client_addr</span> <span class="o">=</span> <span class="n">client_addr_var</span><span class="o">.</span><span class="n">get</span><span class="p">()</span>
|
||
<span class="k">return</span> <span class="sa">f</span><span class="s1">'Good bye, client @ </span><span class="si">{</span><span class="n">client_addr</span><span class="si">}</span><span class="se">\r\n</span><span class="s1">'</span><span class="o">.</span><span class="n">encode</span><span class="p">()</span>
|
||
|
||
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">handle_request</span><span class="p">(</span><span class="n">reader</span><span class="p">,</span> <span class="n">writer</span><span class="p">):</span>
|
||
<span class="n">addr</span> <span class="o">=</span> <span class="n">writer</span><span class="o">.</span><span class="n">transport</span><span class="o">.</span><span class="n">get_extra_info</span><span class="p">(</span><span class="s1">'socket'</span><span class="p">)</span><span class="o">.</span><span class="n">getpeername</span><span class="p">()</span>
|
||
<span class="n">client_addr_var</span><span class="o">.</span><span class="n">set</span><span class="p">(</span><span class="n">addr</span><span class="p">)</span>
|
||
|
||
<span class="c1"># In any code that we call is now possible to get</span>
|
||
<span class="c1"># client's address by calling 'client_addr_var.get()'.</span>
|
||
|
||
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
|
||
<span class="n">line</span> <span class="o">=</span> <span class="k">await</span> <span class="n">reader</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>
|
||
<span class="nb">print</span><span class="p">(</span><span class="n">line</span><span class="p">)</span>
|
||
<span class="k">if</span> <span class="ow">not</span> <span class="n">line</span><span class="o">.</span><span class="n">strip</span><span class="p">():</span>
|
||
<span class="k">break</span>
|
||
|
||
<span class="n">writer</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="sa">b</span><span class="s1">'HTTP/1.1 200 OK</span><span class="se">\r\n</span><span class="s1">'</span><span class="p">)</span> <span class="c1"># status line</span>
|
||
<span class="n">writer</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="sa">b</span><span class="s1">'</span><span class="se">\r\n</span><span class="s1">'</span><span class="p">)</span> <span class="c1"># headers</span>
|
||
<span class="n">writer</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">render_goodbye</span><span class="p">())</span> <span class="c1"># body</span>
|
||
<span class="n">writer</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
|
||
|
||
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
|
||
<span class="n">srv</span> <span class="o">=</span> <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">start_server</span><span class="p">(</span>
|
||
<span class="n">handle_request</span><span class="p">,</span> <span class="s1">'127.0.0.1'</span><span class="p">,</span> <span class="mi">8081</span><span class="p">)</span>
|
||
|
||
<span class="k">async</span> <span class="k">with</span> <span class="n">srv</span><span class="p">:</span>
|
||
<span class="k">await</span> <span class="n">srv</span><span class="o">.</span><span class="n">serve_forever</span><span class="p">()</span>
|
||
|
||
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>
|
||
|
||
<span class="c1"># To test it you can use telnet or curl:</span>
|
||
<span class="c1"># telnet 127.0.0.1 8081</span>
|
||
<span class="c1"># curl 127.0.0.1:8081</span>
|
||
</pre></div>
|
||
</div>
|
||
</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="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextvars</span></code> — Context Variables</a><ul>
|
||
<li><a class="reference internal" href="#context-variables">Context Variables</a></li>
|
||
<li><a class="reference internal" href="#manual-context-management">Manual Context Management</a></li>
|
||
<li><a class="reference internal" href="#asyncio-support">asyncio support</a></li>
|
||
</ul>
|
||
</li>
|
||
</ul>
|
||
|
||
</div>
|
||
<div>
|
||
<h4>Previous topic</h4>
|
||
<p class="topless"><a href="queue.html"
|
||
title="previous chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">queue</span></code> — A synchronized queue class</a></p>
|
||
</div>
|
||
<div>
|
||
<h4>Next topic</h4>
|
||
<p class="topless"><a href="_thread.html"
|
||
title="next chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">_thread</span></code> — Low-level threading API</a></p>
|
||
</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/library/contextvars.rst"
|
||
rel="nofollow">Show Source
|
||
</a>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
<div id="sidebarbutton" title="Collapse sidebar">
|
||
<span>«</span>
|
||
</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="_thread.html" title="_thread — Low-level threading API"
|
||
>next</a> |</li>
|
||
<li class="right" >
|
||
<a href="queue.html" title="queue — A synchronized queue class"
|
||
>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">3.13.3 Documentation</a> »
|
||
</li>
|
||
|
||
<li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li>
|
||
<li class="nav-item nav-item-2"><a href="concurrency.html" >Concurrent Execution</a> »</li>
|
||
<li class="nav-item nav-item-this"><a href=""><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextvars</span></code> — Context Variables</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>
|
||
2001-2025, Python Software Foundation.
|
||
<br />
|
||
This page is licensed under the Python Software Foundation License Version 2.
|
||
<br />
|
||
Examples, recipes, and other code in the documentation are additionally licensed under the Zero Clause BSD License.
|
||
<br />
|
||
|
||
See <a href="/license.html">History and License</a> for more information.<br />
|
||
|
||
|
||
<br />
|
||
|
||
The Python Software Foundation is a non-profit corporation.
|
||
<a href="https://www.python.org/psf/donations/">Please donate.</a>
|
||
<br />
|
||
<br />
|
||
Last updated on Apr 08, 2025 (14:33 UTC).
|
||
|
||
<a href="/bugs.html">Found a bug</a>?
|
||
|
||
<br />
|
||
|
||
Created using <a href="https://www.sphinx-doc.org/">Sphinx</a> 8.2.3.
|
||
</div>
|
||
|
||
</body>
|
||
</html> |