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<div class="section" id="module-contextvars">
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<span id="contextvars-context-variables"></span><h1><a class="reference internal" href="#module-contextvars" title="contextvars: Context Variables"><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextvars</span></code></a> — Context Variables<a class="headerlink" href="#module-contextvars" title="Permalink to this headline">¶</a></h1>
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<hr class="docutils" />
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<p>This module provides APIs to manage, store, and access context-local
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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
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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>
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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
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manage the current context in asynchronous frameworks.</p>
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<p>Context managers that have state should use Context Variables
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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
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bleeding to other code unexpectedly, when used in concurrent code.</p>
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<p>See also <span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0567"><strong>PEP 567</strong></a> for additional details.</p>
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<div class="versionadded">
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<p><span class="versionmodified added">New in version 3.7.</span></p>
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</div>
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<div class="section" id="context-variables">
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<h2>Context Variables<a class="headerlink" href="#context-variables" title="Permalink to this headline">¶</a></h2>
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<dl class="class">
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<dt id="contextvars.ContextVar">
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<em class="property">class </em><code class="descclassname">contextvars.</code><code class="descname">ContextVar</code><span class="sig-paren">(</span><em>name</em><span class="optional">[</span>, <em>*</em>, <em>default</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.ContextVar" title="Permalink to this definition">¶</a></dt>
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<dd><p>This class is used to declare a new Context Variable, e.g.:</p>
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<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>
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</pre></div>
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</div>
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<p>The required <em>name</em> parameter is used for introspection and debug
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purposes.</p>
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<p>The optional keyword-only <em>default</em> parameter is returned by
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<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
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in the current context.</p>
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<p><strong>Important:</strong> Context Variables should be created at the top module
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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
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references to context variables which prevents context variables
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from being properly garbage collected.</p>
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<dl class="attribute">
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<dt id="contextvars.ContextVar.name">
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<code class="descname">name</code><a class="headerlink" href="#contextvars.ContextVar.name" title="Permalink to this definition">¶</a></dt>
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<dd><p>The name of the variable. This is a read-only property.</p>
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<div class="versionadded">
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<p><span class="versionmodified added">New in version 3.7.1.</span></p>
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</div>
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</dd></dl>
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<dl class="method">
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<dt id="contextvars.ContextVar.get">
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<code class="descname">get</code><span class="sig-paren">(</span><span class="optional">[</span><em>default</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.ContextVar.get" title="Permalink to this definition">¶</a></dt>
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<dd><p>Return a value for the context variable for the current context.</p>
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<p>If there is no value for the variable in the current context,
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the method will:</p>
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<ul class="simple">
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<li><p>return the value of the <em>default</em> argument of the method,
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if provided; or</p></li>
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<li><p>return the default value for the context variable,
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if it was created with one; or</p></li>
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<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>
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</ul>
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</dd></dl>
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<dl class="method">
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<dt id="contextvars.ContextVar.set">
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<code class="descname">set</code><span class="sig-paren">(</span><em>value</em><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.ContextVar.set" title="Permalink to this definition">¶</a></dt>
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<dd><p>Call to set a new value for the context variable in the current
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context.</p>
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<p>The required <em>value</em> argument is the new value for the context
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variable.</p>
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<p>Returns a <a class="reference internal" href="#contextvars.contextvars.Token" title="contextvars.contextvars.Token"><code class="xref py py-class docutils literal notranslate"><span class="pre">Token</span></code></a> object that can be used
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to restore the variable to its previous value via the
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<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>
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</dd></dl>
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<dl class="method">
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<dt id="contextvars.ContextVar.reset">
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<code class="descname">reset</code><span class="sig-paren">(</span><em>token</em><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.ContextVar.reset" title="Permalink to this definition">¶</a></dt>
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<dd><p>Reset the context variable to the value it had before the
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<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>
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<p>For example:</p>
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<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>
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<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>
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<span class="c1"># code that uses 'var'; var.get() returns 'new value'.</span>
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<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>
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<span class="c1"># After the reset call the var has no value again, so</span>
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<span class="c1"># var.get() would raise a LookupError.</span>
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</pre></div>
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</div>
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</dd></dl>
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</dd></dl>
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<dl class="class">
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<dt id="contextvars.contextvars.Token">
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<em class="property">class </em><code class="descclassname">contextvars.</code><code class="descname">Token</code><a class="headerlink" href="#contextvars.contextvars.Token" title="Permalink to this definition">¶</a></dt>
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<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.
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||
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
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the value of the variable to what it was before the corresponding
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<em>set</em>.</p>
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<dl class="attribute">
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<dt id="contextvars.contextvars.Token.Token.var">
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<code class="descclassname">Token.</code><code class="descname">var</code><a class="headerlink" href="#contextvars.contextvars.Token.Token.var" title="Permalink to this definition">¶</a></dt>
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<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
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that created the token.</p>
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</dd></dl>
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<dl class="attribute">
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<dt id="contextvars.contextvars.Token.Token.old_value">
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<code class="descclassname">Token.</code><code class="descname">old_value</code><a class="headerlink" href="#contextvars.contextvars.Token.Token.old_value" title="Permalink to this definition">¶</a></dt>
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<dd><p>A read-only property. Set to the value the variable had before
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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.
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It points to <code class="xref py py-attr docutils literal notranslate"><span class="pre">Token.MISSING</span></code> is the variable was not set
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before the call.</p>
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</dd></dl>
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<dl class="attribute">
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<dt id="contextvars.contextvars.Token.Token.MISSING">
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<code class="descclassname">Token.</code><code class="descname">MISSING</code><a class="headerlink" href="#contextvars.contextvars.Token.Token.MISSING" title="Permalink to this definition">¶</a></dt>
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<dd><p>A marker object used by <code class="xref py py-attr docutils literal notranslate"><span class="pre">Token.old_value</span></code>.</p>
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</dd></dl>
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</dd></dl>
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</div>
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<div class="section" id="manual-context-management">
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<h2>Manual Context Management<a class="headerlink" href="#manual-context-management" title="Permalink to this headline">¶</a></h2>
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<dl class="function">
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<dt id="contextvars.copy_context">
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<code class="descclassname">contextvars.</code><code class="descname">copy_context</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.copy_context" title="Permalink to this definition">¶</a></dt>
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<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>
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<p>The following snippet gets a copy of the current context and prints
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all variables and their values that are set in it:</p>
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<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>
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<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>
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</pre></div>
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</div>
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<p>The function has an O(1) complexity, i.e. works equally fast for
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contexts with a few context variables and for contexts that have
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a lot of them.</p>
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</dd></dl>
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<dl class="class">
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<dt id="contextvars.Context">
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<em class="property">class </em><code class="descclassname">contextvars.</code><code class="descname">Context</code><a class="headerlink" href="#contextvars.Context" title="Permalink to this definition">¶</a></dt>
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<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>
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<p><code class="docutils literal notranslate"><span class="pre">Context()</span></code> creates an empty context with no values in it.
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To get a copy of the current context use the
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||
<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>
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||
<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>
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<dl class="method">
|
||
<dt id="contextvars.Context.run">
|
||
<code class="descname">run</code><span class="sig-paren">(</span><em>callable</em>, <em>*args</em>, <em>**kwargs</em><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.Context.run" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Execute <code class="docutils literal notranslate"><span class="pre">callable(*args,</span> <span class="pre">**kwargs)</span></code> code in the context object
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||
the <em>run</em> method is called on. Return the result of the execution
|
||
or propagate an exception if one occurred.</p>
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||
<p>Any changes to any context variables that <em>callable</em> makes will
|
||
be contained in the context object:</p>
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<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>
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<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>
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||
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<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
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<span class="c1"># 'var' was set to 'spam' before</span>
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<span class="c1"># calling 'copy_context()' and 'ctx.run(main)', so:</span>
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||
<span class="c1"># var.get() == ctx[var] == 'spam'</span>
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||
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<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>
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<span class="c1"># Now, after setting 'var' to 'ham':</span>
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<span class="c1"># var.get() == ctx[var] == 'ham'</span>
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<span class="n">ctx</span> <span class="o">=</span> <span class="n">copy_context</span><span class="p">()</span>
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<span class="c1"># Any changes that the 'main' function makes to 'var'</span>
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<span class="c1"># will be contained in 'ctx'.</span>
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<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>
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<span class="c1"># The 'main()' function was run in the 'ctx' context,</span>
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<span class="c1"># so changes to 'var' are contained in it:</span>
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<span class="c1"># ctx[var] == 'ham'</span>
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<span class="c1"># However, outside of 'ctx', 'var' is still set to 'spam':</span>
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<span class="c1"># var.get() == 'spam'</span>
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</pre></div>
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</div>
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<p>The method 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> when called on the same
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context object from more than one OS thread, or when called
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recursively.</p>
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</dd></dl>
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|
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<dl class="method">
|
||
<dt id="contextvars.Context.copy">
|
||
<code class="descname">copy</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.Context.copy" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Return a shallow copy of the context object.</p>
|
||
</dd></dl>
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||
|
||
<dl class="describe">
|
||
<dt>
|
||
<code class="descname">var in context</code></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>
|
||
<code class="descname">context[var]</code></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="method">
|
||
<dt id="contextvars.Context.get">
|
||
<code class="descname">get</code><span class="sig-paren">(</span><em>var</em><span class="optional">[</span>, <em>default</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.Context.get" title="Permalink 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>
|
||
<code class="descname">iter(context)</code></dt>
|
||
<dd><p>Return an iterator over the variables stored in the context
|
||
object.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="describe">
|
||
<dt>
|
||
<code class="descname">len(proxy)</code></dt>
|
||
<dd><p>Return the number of variables set in the context object.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="method">
|
||
<dt id="contextvars.Context.keys">
|
||
<code class="descname">keys</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.Context.keys" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Return a list of all variables in the context object.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="method">
|
||
<dt id="contextvars.Context.values">
|
||
<code class="descname">values</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.Context.values" title="Permalink to this definition">¶</a></dt>
|
||
<dd><p>Return a list of all variables’ values in the context object.</p>
|
||
</dd></dl>
|
||
|
||
<dl class="method">
|
||
<dt id="contextvars.Context.items">
|
||
<code class="descname">items</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextvars.Context.items" title="Permalink 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>
|
||
|
||
</div>
|
||
<div class="section" id="asyncio-support">
|
||
<h2>asyncio support<a class="headerlink" href="#asyncio-support" title="Permalink to this headline">¶</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="nn">asyncio</span>
|
||
<span class="kn">import</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="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="n">f</span><span class="s1">'Good bye, client @ </span><span class="si">{client_addr}</span><span class="se">\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="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="n">line</span><span class="p">)</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="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="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:</span>
|
||
<span class="c1"># telnet 127.0.0.1 8081</span>
|
||
</pre></div>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
|
||
|
||
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|
||
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|
||
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|
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||
<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>
|
||
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|
||
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|
||
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|
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|
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|
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<p class="topless"><a href="dummy_threading.html"
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title="previous chapter"><code class="xref py py-mod docutils literal notranslate"><span class="pre">dummy_threading</span></code> — Drop-in replacement for the <code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code> module</a></p>
|
||
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|
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