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  194.   <section id="module-contextvars">

  195. <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="Link to this heading">¶</a></h1>

  196. <hr class="docutils" />

  197. <p>This module provides APIs to manage, store, and access context-local

  198. 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

  199. 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>

  200. 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

  201. manage the current context in asynchronous frameworks.</p>

  202. <p>Context managers that have state should use Context Variables

  203. 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

  204. bleeding to other code unexpectedly, when used in concurrent code.</p>

  205. <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>

  206. <div class="versionadded">

  207. <p><span class="versionmodified added">New in version 3.7.</span></p>

  208. </div>

  209. <section id="context-variables">

  210. <h2>Context Variables<a class="headerlink" href="#context-variables" title="Link to this heading">¶</a></h2>

  211. <dl class="py class">

  212. <dt class="sig sig-object py" id="contextvars.ContextVar">

  213. <em class="property"><span class="pre">class</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>

  214. <dd><p>This class is used to declare a new Context Variable, e.g.:</p>

  215. <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">&#39;var&#39;</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="mi">42</span><span class="p">)</span>

  216. </pre></div>

  217. </div>

  218. <p>The required <em>name</em> parameter is used for introspection and debug

  219. purposes.</p>

  220. <p>The optional keyword-only <em>default</em> parameter is returned by

  221. <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

  222. in the current context.</p>

  223. <p><strong>Important:</strong> Context Variables should be created at the top module

  224. 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

  225. references to context variables which prevents context variables

  226. from being properly garbage collected.</p>

  227. <dl class="py attribute">

  228. <dt class="sig sig-object py" id="contextvars.ContextVar.name">

  229. <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>

  230. <dd><p>The name of the variable.  This is a read-only property.</p>

  231. <div class="versionadded">

  232. <p><span class="versionmodified added">New in version 3.7.1.</span></p>

  233. </div>

  234. </dd></dl>

  235. 

  236. <dl class="py method">

  237. <dt class="sig sig-object py" id="contextvars.ContextVar.get">

  238. <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>

  239. <dd><p>Return a value for the context variable for the current context.</p>

  240. <p>If there is no value for the variable in the current context,

  241. the method will:</p>

  242. <ul class="simple">

  243. <li><p>return the value of the <em>default</em> argument of the method,

  244. if provided; or</p></li>

  245. <li><p>return the default value for the context variable,

  246. if it was created with one; or</p></li>

  247. <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>

  248. </ul>

  249. </dd></dl>

  250. 

  251. <dl class="py method">

  252. <dt class="sig sig-object py" id="contextvars.ContextVar.set">

  253. <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>

  254. <dd><p>Call to set a new value for the context variable in the current

  255. context.</p>

  256. <p>The required <em>value</em> argument is the new value for the context

  257. variable.</p>

  258. <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

  259. to restore the variable to its previous value via the

  260. <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>

  261. </dd></dl>

  262. 

  263. <dl class="py method">

  264. <dt class="sig sig-object py" id="contextvars.ContextVar.reset">

  265. <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>

  266. <dd><p>Reset the context variable to the value it had before the

  267. <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>

  268. <p>For example:</p>

  269. <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">&#39;var&#39;</span><span class="p">)</span>

  270. 

  271. <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">&#39;new value&#39;</span><span class="p">)</span>

  272. <span class="c1"># code that uses &#39;var&#39;; var.get() returns &#39;new value&#39;.</span>

  273. <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>

  274. 

  275. <span class="c1"># After the reset call the var has no value again, so</span>

  276. <span class="c1"># var.get() would raise a LookupError.</span>

  277. </pre></div>

  278. </div>

  279. </dd></dl>

  280. 

  281. </dd></dl>

  282. 

  283. <dl class="py class">

  284. <dt class="sig sig-object py" id="contextvars.Token">

  285. <em class="property"><span class="pre">class</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>

  286. <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.

  287. 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

  288. the value of the variable to what it was before the corresponding

  289. <em>set</em>.</p>

  290. <dl class="py attribute">

  291. <dt class="sig sig-object py" id="contextvars.Token.var">

  292. <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>

  293. <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

  294. that created the token.</p>

  295. </dd></dl>

  296. 

  297. <dl class="py attribute">

  298. <dt class="sig sig-object py" id="contextvars.Token.old_value">

  299. <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>

  300. <dd><p>A read-only property.  Set to the value the variable had before

  301. 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.

  302. 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

  303. before the call.</p>

  304. </dd></dl>

  305. 

  306. <dl class="py attribute">

  307. <dt class="sig sig-object py" id="contextvars.Token.MISSING">

  308. <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>

  309. <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>

  310. </dd></dl>

  311. 

  312. </dd></dl>

  313. 

  314. </section>

  315. <section id="manual-context-management">

  316. <h2>Manual Context Management<a class="headerlink" href="#manual-context-management" title="Link to this heading">¶</a></h2>

  317. <dl class="py function">

  318. <dt class="sig sig-object py" id="contextvars.copy_context">

  319. <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>

  320. <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>

  321. <p>The following snippet gets a copy of the current context and prints

  322. all variables and their values that are set in it:</p>

  323. <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>

  324. <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>

  325. </pre></div>

  326. </div>

  327. <p>The function has an <em>O</em>(1) complexity, i.e. works equally fast for

  328. contexts with a few context variables and for contexts that have

  329. a lot of them.</p>

  330. </dd></dl>

  331. 

  332. <dl class="py class">

  333. <dt class="sig sig-object py" id="contextvars.Context">

  334. <em class="property"><span class="pre">class</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>

  335. <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>

  336. <p><code class="docutils literal notranslate"><span class="pre">Context()</span></code> creates an empty context with no values in it.

  337. To get a copy of the current context use the

  338. <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>

  339. <p>Every thread will have a different top-level <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>

  340. object. This means that a <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 behaves in a similar

  341. 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

  342. threads.</p>

  343. <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>

  344. <dl class="py method">

  345. <dt class="sig sig-object py" id="contextvars.Context.run">

  346. <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>

  347. <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

  348. the <em>run</em> method is called on.  Return the result of the execution

  349. or propagate an exception if one occurred.</p>

  350. <p>Any changes to any context variables that <em>callable</em> makes will

  351. be contained in the context object:</p>

  352. <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">&#39;var&#39;</span><span class="p">)</span>

  353. <span class="n">var</span><span class="o">.</span><span class="n">set</span><span class="p">(</span><span class="s1">&#39;spam&#39;</span><span class="p">)</span>

  354. 

  355. <span class="k">def</span> <span class="nf">main</span><span class="p">():</span>

  356.     <span class="c1"># &#39;var&#39; was set to &#39;spam&#39; before</span>

  357.     <span class="c1"># calling &#39;copy_context()&#39; and &#39;ctx.run(main)&#39;, so:</span>

  358.     <span class="c1"># var.get() == ctx[var] == &#39;spam&#39;</span>

  359. 

  360.     <span class="n">var</span><span class="o">.</span><span class="n">set</span><span class="p">(</span><span class="s1">&#39;ham&#39;</span><span class="p">)</span>

  361. 

  362.     <span class="c1"># Now, after setting &#39;var&#39; to &#39;ham&#39;:</span>

  363.     <span class="c1"># var.get() == ctx[var] == &#39;ham&#39;</span>

  364. 

  365. <span class="n">ctx</span> <span class="o">=</span> <span class="n">copy_context</span><span class="p">()</span>

  366. 

  367. <span class="c1"># Any changes that the &#39;main&#39; function makes to &#39;var&#39;</span>

  368. <span class="c1"># will be contained in &#39;ctx&#39;.</span>

  369. <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>

  370. 

  371. <span class="c1"># The &#39;main()&#39; function was run in the &#39;ctx&#39; context,</span>

  372. <span class="c1"># so changes to &#39;var&#39; are contained in it:</span>

  373. <span class="c1"># ctx[var] == &#39;ham&#39;</span>

  374. 

  375. <span class="c1"># However, outside of &#39;ctx&#39;, &#39;var&#39; is still set to &#39;spam&#39;:</span>

  376. <span class="c1"># var.get() == &#39;spam&#39;</span>

  377. </pre></div>

  378. </div>

  379. <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

  380. context object from more than one OS thread, or when called

  381. recursively.</p>

  382. </dd></dl>

  383. 

  384. <dl class="py method">

  385. <dt class="sig sig-object py" id="contextvars.Context.copy">

  386. <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>

  387. <dd><p>Return a shallow copy of the context object.</p>

  388. </dd></dl>

  389. 

  390. <dl class="describe">

  391. <dt class="sig sig-object">

  392. <span class="sig-name descname"><span class="pre">var</span> <span class="pre">in</span> <span class="pre">context</span></span></dt>

  393. <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;

  394. return <code class="docutils literal notranslate"><span class="pre">False</span></code> otherwise.</p>

  395. </dd></dl>

  396. 

  397. <dl class="describe">

  398. <dt class="sig sig-object">

  399. <span class="sig-name descname"><span class="pre">context[var]</span></span></dt>

  400. <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.

  401. If the variable is not set in the context object, a

  402. <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>

  403. </dd></dl>

  404. 

  405. <dl class="py method">

  406. <dt class="sig sig-object py" id="contextvars.Context.get">

  407. <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>

  408. <dd><p>Return the value for <em>var</em> if <em>var</em> has the value in the context

  409. object.  Return <em>default</em> otherwise.  If <em>default</em> is not given,

  410. return <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p>

  411. </dd></dl>

  412. 

  413. <dl class="describe">

  414. <dt class="sig sig-object">

  415. <span class="sig-name descname"><span class="pre">iter(context)</span></span></dt>

  416. <dd><p>Return an iterator over the variables stored in the context

  417. object.</p>

  418. </dd></dl>

  419. 

  420. <dl class="describe">

  421. <dt class="sig sig-object">

  422. <span class="sig-name descname"><span class="pre">len(proxy)</span></span></dt>

  423. <dd><p>Return the number of variables set in the context object.</p>

  424. </dd></dl>

  425. 

  426. <dl class="py method">

  427. <dt class="sig sig-object py" id="contextvars.Context.keys">

  428. <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>

  429. <dd><p>Return a list of all variables in the context object.</p>

  430. </dd></dl>

  431. 

  432. <dl class="py method">

  433. <dt class="sig sig-object py" id="contextvars.Context.values">

  434. <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>

  435. <dd><p>Return a list of all variables’ values in the context object.</p>

  436. </dd></dl>

  437. 

  438. <dl class="py method">

  439. <dt class="sig sig-object py" id="contextvars.Context.items">

  440. <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>

  441. <dd><p>Return a list of 2-tuples containing all variables and their

  442. values in the context object.</p>

  443. </dd></dl>

  444. 

  445. </dd></dl>

  446. 

  447. </section>

  448. <section id="asyncio-support">

  449. <h2>asyncio support<a class="headerlink" href="#asyncio-support" title="Link to this heading">¶</a></h2>

  450. <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

  451. ready to be used without any extra configuration.  For example, here

  452. is a simple echo server, that uses a context variable to make the

  453. address of a remote client available in the Task that handles that

  454. client:</p>

  455. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">asyncio</span>

  456. <span class="kn">import</span> <span class="nn">contextvars</span>

  457. 

  458. <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">&#39;client_addr&#39;</span><span class="p">)</span>

  459. 

  460. <span class="k">def</span> <span class="nf">render_goodbye</span><span class="p">():</span>

  461.     <span class="c1"># The address of the currently handled client can be accessed</span>

  462.     <span class="c1"># without passing it explicitly to this function.</span>

  463. 

  464.     <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>

  465.     <span class="k">return</span> <span class="sa">f</span><span class="s1">&#39;Good bye, client @ </span><span class="si">{</span><span class="n">client_addr</span><span class="si">}</span><span class="se">\n</span><span class="s1">&#39;</span><span class="o">.</span><span class="n">encode</span><span class="p">()</span>

  466. 

  467. <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>

  468.     <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">&#39;socket&#39;</span><span class="p">)</span><span class="o">.</span><span class="n">getpeername</span><span class="p">()</span>

  469.     <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>

  470. 

  471.     <span class="c1"># In any code that we call is now possible to get</span>

  472.     <span class="c1"># client&#39;s address by calling &#39;client_addr_var.get()&#39;.</span>

  473. 

  474.     <span class="k">while</span> <span class="kc">True</span><span class="p">:</span>

  475.         <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>

  476.         <span class="nb">print</span><span class="p">(</span><span class="n">line</span><span class="p">)</span>

  477.         <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>

  478.             <span class="k">break</span>

  479.         <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>

  480. 

  481.     <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>

  482.     <span class="n">writer</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>

  483. 

  484. <span class="k">async</span> <span class="k">def</span> <span class="nf">main</span><span class="p">():</span>

  485.     <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>

  486.         <span class="n">handle_request</span><span class="p">,</span> <span class="s1">&#39;127.0.0.1&#39;</span><span class="p">,</span> <span class="mi">8081</span><span class="p">)</span>

  487. 

  488.     <span class="k">async</span> <span class="k">with</span> <span class="n">srv</span><span class="p">:</span>

  489.         <span class="k">await</span> <span class="n">srv</span><span class="o">.</span><span class="n">serve_forever</span><span class="p">()</span>

  490. 

  491. <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>

  492. 

  493. <span class="c1"># To test it you can use telnet:</span>

  494. <span class="c1">#     telnet 127.0.0.1 8081</span>

  495. </pre></div>

  496. </div>

  497. </section>

  498. </section>

  499. 

  500. 

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  509.     <ul>

  510. <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>

  511. <li><a class="reference internal" href="#context-variables">Context Variables</a></li>

  512. <li><a class="reference internal" href="#manual-context-management">Manual Context Management</a></li>

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