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  205.   <section id="module-contextlib">

  206. <span id="contextlib-utilities-for-with-statement-contexts"></span><h1><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextlib</span></code> — Utilities for <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code>-statement contexts<a class="headerlink" href="#module-contextlib" title="Link to this heading">¶</a></h1>

  207. <p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.12/Lib/contextlib.py">Lib/contextlib.py</a></p>

  208. <hr class="docutils" />

  209. <p>This module provides utilities for common tasks involving the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>

  210. statement. For more information see also <a class="reference internal" href="stdtypes.html#typecontextmanager"><span class="std std-ref">Context Manager Types</span></a> and

  211. <a class="reference internal" href="../reference/datamodel.html#context-managers"><span class="std std-ref">With Statement Context Managers</span></a>.</p>

  212. <section id="utilities">

  213. <h2>Utilities<a class="headerlink" href="#utilities" title="Link to this heading">¶</a></h2>

  214. <p>Functions and classes provided:</p>

  215. <dl class="py class">

  216. <dt class="sig sig-object py" id="contextlib.AbstractContextManager">

  217. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">AbstractContextManager</span></span><a class="headerlink" href="#contextlib.AbstractContextManager" title="Link to this definition">¶</a></dt>

  218. <dd><p>An <a class="reference internal" href="../glossary.html#term-abstract-base-class"><span class="xref std std-term">abstract base class</span></a> for classes that implement

  219. <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__enter__()</span></code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__exit__()</span></code></a>. A default

  220. implementation for <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__enter__()</span></code></a> is provided which returns

  221. <code class="docutils literal notranslate"><span class="pre">self</span></code> while <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__exit__()</span></code></a> is an abstract method which by default

  222. returns <code class="docutils literal notranslate"><span class="pre">None</span></code>. See also the definition of <a class="reference internal" href="stdtypes.html#typecontextmanager"><span class="std std-ref">Context Manager Types</span></a>.</p>

  223. <div class="versionadded">

  224. <p><span class="versionmodified added">New in version 3.6.</span></p>

  225. </div>

  226. </dd></dl>

  227. 

  228. <dl class="py class">

  229. <dt class="sig sig-object py" id="contextlib.AbstractAsyncContextManager">

  230. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">AbstractAsyncContextManager</span></span><a class="headerlink" href="#contextlib.AbstractAsyncContextManager" title="Link to this definition">¶</a></dt>

  231. <dd><p>An <a class="reference internal" href="../glossary.html#term-abstract-base-class"><span class="xref std std-term">abstract base class</span></a> for classes that implement

  232. <a class="reference internal" href="../reference/datamodel.html#object.__aenter__" title="object.__aenter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__aenter__()</span></code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__aexit__" title="object.__aexit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__aexit__()</span></code></a>. A default

  233. implementation for <a class="reference internal" href="../reference/datamodel.html#object.__aenter__" title="object.__aenter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__aenter__()</span></code></a> is provided which returns

  234. <code class="docutils literal notranslate"><span class="pre">self</span></code> while <a class="reference internal" href="../reference/datamodel.html#object.__aexit__" title="object.__aexit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">object.__aexit__()</span></code></a> is an abstract method which by default

  235. returns <code class="docutils literal notranslate"><span class="pre">None</span></code>. See also the definition of

  236. <a class="reference internal" href="../reference/datamodel.html#async-context-managers"><span class="std std-ref">Asynchronous Context Managers</span></a>.</p>

  237. <div class="versionadded">

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

  239. </div>

  240. </dd></dl>

  241. 

  242. <dl class="py function">

  243. <dt class="sig sig-object py" id="contextlib.contextmanager">

  244. <span class="sig-prename descclassname"><span class="pre">&#64;</span></span><span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">contextmanager</span></span><a class="headerlink" href="#contextlib.contextmanager" title="Link to this definition">¶</a></dt>

  245. <dd><p>This function is a <a class="reference internal" href="../glossary.html#term-decorator"><span class="xref std std-term">decorator</span></a> that can be used to define a factory

  246. function for <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement context managers, without needing to

  247. create a class or separate <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> methods.</p>

  248. <p>While many objects natively support use in with statements, sometimes a

  249. resource needs to be managed that isn’t a context manager in its own right,

  250. and doesn’t implement a <code class="docutils literal notranslate"><span class="pre">close()</span></code> method for use with <code class="docutils literal notranslate"><span class="pre">contextlib.closing</span></code></p>

  251. <p>An abstract example would be the following to ensure correct resource

  252. management:</p>

  253. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">contextmanager</span>

  254. 

  255. <span class="nd">@contextmanager</span>

  256. <span class="k">def</span> <span class="nf">managed_resource</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">):</span>

  257.     <span class="c1"># Code to acquire resource, e.g.:</span>

  258.     <span class="n">resource</span> <span class="o">=</span> <span class="n">acquire_resource</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">)</span>

  259.     <span class="k">try</span><span class="p">:</span>

  260.         <span class="k">yield</span> <span class="n">resource</span>

  261.     <span class="k">finally</span><span class="p">:</span>

  262.         <span class="c1"># Code to release resource, e.g.:</span>

  263.         <span class="n">release_resource</span><span class="p">(</span><span class="n">resource</span><span class="p">)</span>

  264. </pre></div>

  265. </div>

  266. <p>The function can then be used like this:</p>

  267. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">with</span> <span class="n">managed_resource</span><span class="p">(</span><span class="n">timeout</span><span class="o">=</span><span class="mi">3600</span><span class="p">)</span> <span class="k">as</span> <span class="n">resource</span><span class="p">:</span>

  268. <span class="gp">... </span>    <span class="c1"># Resource is released at the end of this block,</span>

  269. <span class="gp">... </span>    <span class="c1"># even if code in the block raises an exception</span>

  270. </pre></div>

  271. </div>

  272. <p>The function being decorated must return a <a class="reference internal" href="../glossary.html#term-generator"><span class="xref std std-term">generator</span></a>-iterator when

  273. called. This iterator must yield exactly one value, which will be bound to

  274. the targets in the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement’s <code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code> clause, if any.</p>

  275. <p>At the point where the generator yields, the block nested in the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>

  276. statement is executed.  The generator is then resumed after the block is exited.

  277. If an unhandled exception occurs in the block, it is reraised inside the

  278. generator at the point where the yield occurred.  Thus, you can use a

  279. <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a>…<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a>…<a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> statement to trap

  280. the error (if any), or ensure that some cleanup takes place. If an exception is

  281. trapped merely in order to log it or to perform some action (rather than to

  282. suppress it entirely), the generator must reraise that exception. Otherwise the

  283. generator context manager will indicate to the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code> statement that

  284. the exception has been handled, and execution will resume with the statement

  285. immediately following the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code> statement.</p>

  286. <p><a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">contextmanager()</span></code></a> uses <a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextDecorator</span></code></a> so the context managers

  287. it creates can be used as decorators as well as in <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statements.

  288. When used as a decorator, a new generator instance is implicitly created on

  289. each function call (this allows the otherwise “one-shot” context managers

  290. created by <a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">contextmanager()</span></code></a> to meet the requirement that context

  291. managers support multiple invocations in order to be used as decorators).</p>

  292. <div class="versionchanged">

  293. <p><span class="versionmodified changed">Changed in version 3.2: </span>Use of <a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextDecorator</span></code></a>.</p>

  294. </div>

  295. </dd></dl>

  296. 

  297. <dl class="py function">

  298. <dt class="sig sig-object py" id="contextlib.asynccontextmanager">

  299. <span class="sig-prename descclassname"><span class="pre">&#64;</span></span><span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">asynccontextmanager</span></span><a class="headerlink" href="#contextlib.asynccontextmanager" title="Link to this definition">¶</a></dt>

  300. <dd><blockquote>

  301. <div><p>Similar to <a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">contextmanager()</span></code></a>, but creates an

  302. <a class="reference internal" href="../reference/datamodel.html#async-context-managers"><span class="std std-ref">asynchronous context manager</span></a>.</p>

  303. <p>This function is a <a class="reference internal" href="../glossary.html#term-decorator"><span class="xref std std-term">decorator</span></a> that can be used to define a factory

  304. function for <a class="reference internal" href="../reference/compound_stmts.html#async-with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span></code></a> statement asynchronous context managers,

  305. without needing to create a class or separate <a class="reference internal" href="../reference/datamodel.html#object.__aenter__" title="object.__aenter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__aenter__()</span></code></a> and

  306. <a class="reference internal" href="../reference/datamodel.html#object.__aexit__" title="object.__aexit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__aexit__()</span></code></a> methods. It must be applied to an <a class="reference internal" href="../glossary.html#term-asynchronous-generator"><span class="xref std std-term">asynchronous

  307. generator</span></a> function.</p>

  308. <p>A simple example:</p>

  309. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">asynccontextmanager</span>

  310. 

  311. <span class="nd">@asynccontextmanager</span>

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

  313.     <span class="n">conn</span> <span class="o">=</span> <span class="k">await</span> <span class="n">acquire_db_connection</span><span class="p">()</span>

  314.     <span class="k">try</span><span class="p">:</span>

  315.         <span class="k">yield</span> <span class="n">conn</span>

  316.     <span class="k">finally</span><span class="p">:</span>

  317.         <span class="k">await</span> <span class="n">release_db_connection</span><span class="p">(</span><span class="n">conn</span><span class="p">)</span>

  318. 

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

  320.     <span class="k">async</span> <span class="k">with</span> <span class="n">get_connection</span><span class="p">()</span> <span class="k">as</span> <span class="n">conn</span><span class="p">:</span>

  321.         <span class="k">return</span> <span class="n">conn</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="s1">&#39;SELECT ...&#39;</span><span class="p">)</span>

  322. </pre></div>

  323. </div>

  324. <div class="versionadded">

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

  326. </div>

  327. <p>Context managers defined with <a class="reference internal" href="#contextlib.asynccontextmanager" title="contextlib.asynccontextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">asynccontextmanager()</span></code></a> can be used

  328. either as decorators or with <a class="reference internal" href="../reference/compound_stmts.html#async-with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">async</span> <span class="pre">with</span></code></a> statements:</p>

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

  330. <span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">asynccontextmanager</span>

  331. 

  332. <span class="nd">@asynccontextmanager</span>

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

  334.     <span class="n">now</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">monotonic</span><span class="p">()</span>

  335.     <span class="k">try</span><span class="p">:</span>

  336.         <span class="k">yield</span>

  337.     <span class="k">finally</span><span class="p">:</span>

  338.         <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s1">&#39;it took </span><span class="si">{</span><span class="n">time</span><span class="o">.</span><span class="n">monotonic</span><span class="p">()</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">now</span><span class="si">}</span><span class="s1">s to run&#39;</span><span class="p">)</span>

  339. 

  340. <span class="nd">@timeit</span><span class="p">()</span>

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

  342.     <span class="c1"># ... async code ...</span>

  343. </pre></div>

  344. </div>

  345. <p>When used as a decorator, a new generator instance is implicitly created on

  346. each function call. This allows the otherwise “one-shot” context managers

  347. created by <a class="reference internal" href="#contextlib.asynccontextmanager" title="contextlib.asynccontextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">asynccontextmanager()</span></code></a> to meet the requirement that context

  348. managers support multiple invocations in order to be used as decorators.</p>

  349. </div></blockquote>

  350. <div class="versionchanged">

  351. <p><span class="versionmodified changed">Changed in version 3.10: </span>Async context managers created with <a class="reference internal" href="#contextlib.asynccontextmanager" title="contextlib.asynccontextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">asynccontextmanager()</span></code></a> can

  352. be used as decorators.</p>

  353. </div>

  354. </dd></dl>

  355. 

  356. <dl class="py function">

  357. <dt class="sig sig-object py" id="contextlib.closing">

  358. <span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">closing</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">thing</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.closing" title="Link to this definition">¶</a></dt>

  359. <dd><p>Return a context manager that closes <em>thing</em> upon completion of the block.  This

  360. is basically equivalent to:</p>

  361. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">contextmanager</span>

  362. 

  363. <span class="nd">@contextmanager</span>

  364. <span class="k">def</span> <span class="nf">closing</span><span class="p">(</span><span class="n">thing</span><span class="p">):</span>

  365.     <span class="k">try</span><span class="p">:</span>

  366.         <span class="k">yield</span> <span class="n">thing</span>

  367.     <span class="k">finally</span><span class="p">:</span>

  368.         <span class="n">thing</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>

  369. </pre></div>

  370. </div>

  371. <p>And lets you write code like this:</p>

  372. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">closing</span>

  373. <span class="kn">from</span> <span class="nn">urllib.request</span> <span class="kn">import</span> <span class="n">urlopen</span>

  374. 

  375. <span class="k">with</span> <span class="n">closing</span><span class="p">(</span><span class="n">urlopen</span><span class="p">(</span><span class="s1">&#39;https://www.python.org&#39;</span><span class="p">))</span> <span class="k">as</span> <span class="n">page</span><span class="p">:</span>

  376.     <span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">page</span><span class="p">:</span>

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

  378. </pre></div>

  379. </div>

  380. <p>without needing to explicitly close <code class="docutils literal notranslate"><span class="pre">page</span></code>.  Even if an error occurs,

  381. <code class="docutils literal notranslate"><span class="pre">page.close()</span></code> will be called when the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> block is exited.</p>

  382. <div class="admonition note">

  383. <p class="admonition-title">Note</p>

  384. <p>Most types managing resources support the <a class="reference internal" href="../glossary.html#term-context-manager"><span class="xref std std-term">context manager</span></a> protocol,

  385. which closes <em>thing</em> on leaving the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement.

  386. As such, <code class="xref py py-func docutils literal notranslate"><span class="pre">closing()</span></code> is most useful for third party types that don’t

  387. support context managers.

  388. This example is purely for illustration purposes,

  389. as <a class="reference internal" href="urllib.request.html#urllib.request.urlopen" title="urllib.request.urlopen"><code class="xref py py-func docutils literal notranslate"><span class="pre">urlopen()</span></code></a> would normally be used in a context manager.</p>

  390. </div>

  391. </dd></dl>

  392. 

  393. <dl class="py function">

  394. <dt class="sig sig-object py" id="contextlib.aclosing">

  395. <span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">aclosing</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">thing</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.aclosing" title="Link to this definition">¶</a></dt>

  396. <dd><p>Return an async context manager that calls the <code class="docutils literal notranslate"><span class="pre">aclose()</span></code> method of <em>thing</em>

  397. upon completion of the block.  This is basically equivalent to:</p>

  398. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">asynccontextmanager</span>

  399. 

  400. <span class="nd">@asynccontextmanager</span>

  401. <span class="k">async</span> <span class="k">def</span> <span class="nf">aclosing</span><span class="p">(</span><span class="n">thing</span><span class="p">):</span>

  402.     <span class="k">try</span><span class="p">:</span>

  403.         <span class="k">yield</span> <span class="n">thing</span>

  404.     <span class="k">finally</span><span class="p">:</span>

  405.         <span class="k">await</span> <span class="n">thing</span><span class="o">.</span><span class="n">aclose</span><span class="p">()</span>

  406. </pre></div>

  407. </div>

  408. <p>Significantly, <code class="docutils literal notranslate"><span class="pre">aclosing()</span></code> supports deterministic cleanup of async

  409. generators when they happen to exit early by <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> or an

  410. exception.  For example:</p>

  411. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">aclosing</span>

  412. 

  413. <span class="k">async</span> <span class="k">with</span> <span class="n">aclosing</span><span class="p">(</span><span class="n">my_generator</span><span class="p">())</span> <span class="k">as</span> <span class="n">values</span><span class="p">:</span>

  414.     <span class="k">async</span> <span class="k">for</span> <span class="n">value</span> <span class="ow">in</span> <span class="n">values</span><span class="p">:</span>

  415.         <span class="k">if</span> <span class="n">value</span> <span class="o">==</span> <span class="mi">42</span><span class="p">:</span>

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

  417. </pre></div>

  418. </div>

  419. <p>This pattern ensures that the generator’s async exit code is executed in

  420. the same context as its iterations (so that exceptions and context

  421. variables work as expected, and the exit code isn’t run after the

  422. lifetime of some task it depends on).</p>

  423. <div class="versionadded">

  424. <p><span class="versionmodified added">New in version 3.10.</span></p>

  425. </div>

  426. </dd></dl>

  427. 

  428. <dl class="py function" id="simplifying-support-for-single-optional-context-managers">

  429. <dt class="sig sig-object py" id="contextlib.nullcontext">

  430. <span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">nullcontext</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">enter_result</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.nullcontext" title="Link to this definition">¶</a></dt>

  431. <dd><p>Return a context manager that returns <em>enter_result</em> from <code class="docutils literal notranslate"><span class="pre">__enter__</span></code>, but

  432. otherwise does nothing. It is intended to be used as a stand-in for an

  433. optional context manager, for example:</p>

  434. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">myfunction</span><span class="p">(</span><span class="n">arg</span><span class="p">,</span> <span class="n">ignore_exceptions</span><span class="o">=</span><span class="kc">False</span><span class="p">):</span>

  435.     <span class="k">if</span> <span class="n">ignore_exceptions</span><span class="p">:</span>

  436.         <span class="c1"># Use suppress to ignore all exceptions.</span>

  437.         <span class="n">cm</span> <span class="o">=</span> <span class="n">contextlib</span><span class="o">.</span><span class="n">suppress</span><span class="p">(</span><span class="ne">Exception</span><span class="p">)</span>

  438.     <span class="k">else</span><span class="p">:</span>

  439.         <span class="c1"># Do not ignore any exceptions, cm has no effect.</span>

  440.         <span class="n">cm</span> <span class="o">=</span> <span class="n">contextlib</span><span class="o">.</span><span class="n">nullcontext</span><span class="p">()</span>

  441.     <span class="k">with</span> <span class="n">cm</span><span class="p">:</span>

  442.         <span class="c1"># Do something</span>

  443. </pre></div>

  444. </div>

  445. <p>An example using <em>enter_result</em>:</p>

  446. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">process_file</span><span class="p">(</span><span class="n">file_or_path</span><span class="p">):</span>

  447.     <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">file_or_path</span><span class="p">,</span> <span class="nb">str</span><span class="p">):</span>

  448.         <span class="c1"># If string, open file</span>

  449.         <span class="n">cm</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">file_or_path</span><span class="p">)</span>

  450.     <span class="k">else</span><span class="p">:</span>

  451.         <span class="c1"># Caller is responsible for closing file</span>

  452.         <span class="n">cm</span> <span class="o">=</span> <span class="n">nullcontext</span><span class="p">(</span><span class="n">file_or_path</span><span class="p">)</span>

  453. 

  454.     <span class="k">with</span> <span class="n">cm</span> <span class="k">as</span> <span class="n">file</span><span class="p">:</span>

  455.         <span class="c1"># Perform processing on the file</span>

  456. </pre></div>

  457. </div>

  458. <p>It can also be used as a stand-in for

  459. <a class="reference internal" href="../reference/datamodel.html#async-context-managers"><span class="std std-ref">asynchronous context managers</span></a>:</p>

  460. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">def</span> <span class="nf">send_http</span><span class="p">(</span><span class="n">session</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>

  461.     <span class="k">if</span> <span class="ow">not</span> <span class="n">session</span><span class="p">:</span>

  462.         <span class="c1"># If no http session, create it with aiohttp</span>

  463.         <span class="n">cm</span> <span class="o">=</span> <span class="n">aiohttp</span><span class="o">.</span><span class="n">ClientSession</span><span class="p">()</span>

  464.     <span class="k">else</span><span class="p">:</span>

  465.         <span class="c1"># Caller is responsible for closing the session</span>

  466.         <span class="n">cm</span> <span class="o">=</span> <span class="n">nullcontext</span><span class="p">(</span><span class="n">session</span><span class="p">)</span>

  467. 

  468.     <span class="k">async</span> <span class="k">with</span> <span class="n">cm</span> <span class="k">as</span> <span class="n">session</span><span class="p">:</span>

  469.         <span class="c1"># Send http requests with session</span>

  470. </pre></div>

  471. </div>

  472. <div class="versionadded">

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

  474. </div>

  475. <div class="versionchanged">

  476. <p><span class="versionmodified changed">Changed in version 3.10: </span><a class="reference internal" href="../glossary.html#term-asynchronous-context-manager"><span class="xref std std-term">asynchronous context manager</span></a> support was added.</p>

  477. </div>

  478. </dd></dl>

  479. 

  480. <dl class="py function">

  481. <dt class="sig sig-object py" id="contextlib.suppress">

  482. <span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">suppress</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="o"><span class="pre">*</span></span><span class="n"><span class="pre">exceptions</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.suppress" title="Link to this definition">¶</a></dt>

  483. <dd><p>Return a context manager that suppresses any of the specified exceptions

  484. if they occur in the body of a <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code> statement and then

  485. resumes execution with the first statement following the end of the

  486. <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code> statement.</p>

  487. <p>As with any other mechanism that completely suppresses exceptions, this

  488. context manager should be used only to cover very specific errors where

  489. silently continuing with program execution is known to be the right

  490. thing to do.</p>

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

  492. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">suppress</span>

  493. 

  494. <span class="k">with</span> <span class="n">suppress</span><span class="p">(</span><span class="ne">FileNotFoundError</span><span class="p">):</span>

  495.     <span class="n">os</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="s1">&#39;somefile.tmp&#39;</span><span class="p">)</span>

  496. 

  497. <span class="k">with</span> <span class="n">suppress</span><span class="p">(</span><span class="ne">FileNotFoundError</span><span class="p">):</span>

  498.     <span class="n">os</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="s1">&#39;someotherfile.tmp&#39;</span><span class="p">)</span>

  499. </pre></div>

  500. </div>

  501. <p>This code is equivalent to:</p>

  502. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span>

  503.     <span class="n">os</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="s1">&#39;somefile.tmp&#39;</span><span class="p">)</span>

  504. <span class="k">except</span> <span class="ne">FileNotFoundError</span><span class="p">:</span>

  505.     <span class="k">pass</span>

  506. 

  507. <span class="k">try</span><span class="p">:</span>

  508.     <span class="n">os</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="s1">&#39;someotherfile.tmp&#39;</span><span class="p">)</span>

  509. <span class="k">except</span> <span class="ne">FileNotFoundError</span><span class="p">:</span>

  510.     <span class="k">pass</span>

  511. </pre></div>

  512. </div>

  513. <p>This context manager is <a class="reference internal" href="#reentrant-cms"><span class="std std-ref">reentrant</span></a>.</p>

  514. <p>If the code within the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code> block raises a

  515. <a class="reference internal" href="exceptions.html#BaseExceptionGroup" title="BaseExceptionGroup"><code class="xref py py-exc docutils literal notranslate"><span class="pre">BaseExceptionGroup</span></code></a>, suppressed exceptions are removed from the

  516. group.  If any exceptions in the group are not suppressed, a group containing them is re-raised.</p>

  517. <div class="versionadded">

  518. <p><span class="versionmodified added">New in version 3.4.</span></p>

  519. </div>

  520. <div class="versionchanged">

  521. <p><span class="versionmodified changed">Changed in version 3.12: </span><code class="docutils literal notranslate"><span class="pre">suppress</span></code> now supports suppressing exceptions raised as

  522. part of an <a class="reference internal" href="exceptions.html#BaseExceptionGroup" title="BaseExceptionGroup"><code class="xref py py-exc docutils literal notranslate"><span class="pre">BaseExceptionGroup</span></code></a>.</p>

  523. </div>

  524. </dd></dl>

  525. 

  526. <dl class="py function">

  527. <dt class="sig sig-object py" id="contextlib.redirect_stdout">

  528. <span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">redirect_stdout</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">new_target</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.redirect_stdout" title="Link to this definition">¶</a></dt>

  529. <dd><p>Context manager for temporarily redirecting <a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdout</span></code></a> to

  530. another file or file-like object.</p>

  531. <p>This tool adds flexibility to existing functions or classes whose output

  532. is hardwired to stdout.</p>

  533. <p>For example, the output of <a class="reference internal" href="functions.html#help" title="help"><code class="xref py py-func docutils literal notranslate"><span class="pre">help()</span></code></a> normally is sent to <em>sys.stdout</em>.

  534. You can capture that output in a string by redirecting the output to an

  535. <a class="reference internal" href="io.html#io.StringIO" title="io.StringIO"><code class="xref py py-class docutils literal notranslate"><span class="pre">io.StringIO</span></code></a> object. The replacement stream is returned from the

  536. <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> method and so is available as the target of the

  537. <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement:</p>

  538. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">redirect_stdout</span><span class="p">(</span><span class="n">io</span><span class="o">.</span><span class="n">StringIO</span><span class="p">())</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>

  539.     <span class="n">help</span><span class="p">(</span><span class="nb">pow</span><span class="p">)</span>

  540. <span class="n">s</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">getvalue</span><span class="p">()</span>

  541. </pre></div>

  542. </div>

  543. <p>To send the output of <a class="reference internal" href="functions.html#help" title="help"><code class="xref py py-func docutils literal notranslate"><span class="pre">help()</span></code></a> to a file on disk, redirect the output

  544. to a regular file:</p>

  545. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">&#39;help.txt&#39;</span><span class="p">,</span> <span class="s1">&#39;w&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>

  546.     <span class="k">with</span> <span class="n">redirect_stdout</span><span class="p">(</span><span class="n">f</span><span class="p">):</span>

  547.         <span class="n">help</span><span class="p">(</span><span class="nb">pow</span><span class="p">)</span>

  548. </pre></div>

  549. </div>

  550. <p>To send the output of <a class="reference internal" href="functions.html#help" title="help"><code class="xref py py-func docutils literal notranslate"><span class="pre">help()</span></code></a> to <em>sys.stderr</em>:</p>

  551. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">redirect_stdout</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="p">):</span>

  552.     <span class="n">help</span><span class="p">(</span><span class="nb">pow</span><span class="p">)</span>

  553. </pre></div>

  554. </div>

  555. <p>Note that the global side effect on <a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdout</span></code></a> means that this

  556. context manager is not suitable for use in library code and most threaded

  557. applications. It also has no effect on the output of subprocesses.

  558. However, it is still a useful approach for many utility scripts.</p>

  559. <p>This context manager is <a class="reference internal" href="#reentrant-cms"><span class="std std-ref">reentrant</span></a>.</p>

  560. <div class="versionadded">

  561. <p><span class="versionmodified added">New in version 3.4.</span></p>

  562. </div>

  563. </dd></dl>

  564. 

  565. <dl class="py function">

  566. <dt class="sig sig-object py" id="contextlib.redirect_stderr">

  567. <span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">redirect_stderr</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">new_target</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.redirect_stderr" title="Link to this definition">¶</a></dt>

  568. <dd><p>Similar to <a class="reference internal" href="#contextlib.redirect_stdout" title="contextlib.redirect_stdout"><code class="xref py py-func docutils literal notranslate"><span class="pre">redirect_stdout()</span></code></a> but redirecting

  569. <a class="reference internal" href="sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stderr</span></code></a> to another file or file-like object.</p>

  570. <p>This context manager is <a class="reference internal" href="#reentrant-cms"><span class="std std-ref">reentrant</span></a>.</p>

  571. <div class="versionadded">

  572. <p><span class="versionmodified added">New in version 3.5.</span></p>

  573. </div>

  574. </dd></dl>

  575. 

  576. <dl class="py function">

  577. <dt class="sig sig-object py" id="contextlib.chdir">

  578. <span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">chdir</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">path</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.chdir" title="Link to this definition">¶</a></dt>

  579. <dd><p>Non parallel-safe context manager to change the current working directory.

  580. As this changes a global state, the working directory, it is not suitable

  581. for use in most threaded or async contexts. It is also not suitable for most

  582. non-linear code execution, like generators, where the program execution is

  583. temporarily relinquished – unless explicitly desired, you should not yield

  584. when this context manager is active.</p>

  585. <p>This is a simple wrapper around <a class="reference internal" href="os.html#os.chdir" title="os.chdir"><code class="xref py py-func docutils literal notranslate"><span class="pre">chdir()</span></code></a>, it changes the current

  586. working directory upon entering and restores the old one on exit.</p>

  587. <p>This context manager is <a class="reference internal" href="#reentrant-cms"><span class="std std-ref">reentrant</span></a>.</p>

  588. <div class="versionadded">

  589. <p><span class="versionmodified added">New in version 3.11.</span></p>

  590. </div>

  591. </dd></dl>

  592. 

  593. <dl class="py class">

  594. <dt class="sig sig-object py" id="contextlib.ContextDecorator">

  595. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">ContextDecorator</span></span><a class="headerlink" href="#contextlib.ContextDecorator" title="Link to this definition">¶</a></dt>

  596. <dd><p>A base class that enables a context manager to also be used as a decorator.</p>

  597. <p>Context managers inheriting from <code class="docutils literal notranslate"><span class="pre">ContextDecorator</span></code> have to implement

  598. <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> and <code class="docutils literal notranslate"><span class="pre">__exit__</span></code> as normal. <code class="docutils literal notranslate"><span class="pre">__exit__</span></code> retains its optional

  599. exception handling even when used as a decorator.</p>

  600. <p><code class="docutils literal notranslate"><span class="pre">ContextDecorator</span></code> is used by <a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">contextmanager()</span></code></a>, so you get this

  601. functionality automatically.</p>

  602. <p>Example of <code class="docutils literal notranslate"><span class="pre">ContextDecorator</span></code>:</p>

  603. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ContextDecorator</span>

  604. 

  605. <span class="k">class</span> <span class="nc">mycontext</span><span class="p">(</span><span class="n">ContextDecorator</span><span class="p">):</span>

  606.     <span class="k">def</span> <span class="fm">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  607.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Starting&#39;</span><span class="p">)</span>

  608.         <span class="k">return</span> <span class="bp">self</span>

  609. 

  610.     <span class="k">def</span> <span class="fm">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">*</span><span class="n">exc</span><span class="p">):</span>

  611.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Finishing&#39;</span><span class="p">)</span>

  612.         <span class="k">return</span> <span class="kc">False</span>

  613. </pre></div>

  614. </div>

  615. <p>The class can then be used like this:</p>

  616. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="nd">@mycontext</span><span class="p">()</span>

  617. <span class="gp">... </span><span class="k">def</span> <span class="nf">function</span><span class="p">():</span>

  618. <span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;The bit in the middle&#39;</span><span class="p">)</span>

  619. <span class="gp">...</span>

  620. <span class="gp">&gt;&gt;&gt; </span><span class="n">function</span><span class="p">()</span>

  621. <span class="go">Starting</span>

  622. <span class="go">The bit in the middle</span>

  623. <span class="go">Finishing</span>

  624. 

  625. <span class="gp">&gt;&gt;&gt; </span><span class="k">with</span> <span class="n">mycontext</span><span class="p">():</span>

  626. <span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;The bit in the middle&#39;</span><span class="p">)</span>

  627. <span class="gp">...</span>

  628. <span class="go">Starting</span>

  629. <span class="go">The bit in the middle</span>

  630. <span class="go">Finishing</span>

  631. </pre></div>

  632. </div>

  633. <p>This change is just syntactic sugar for any construct of the following form:</p>

  634. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">f</span><span class="p">():</span>

  635.     <span class="k">with</span> <span class="n">cm</span><span class="p">():</span>

  636.         <span class="c1"># Do stuff</span>

  637. </pre></div>

  638. </div>

  639. <p><code class="docutils literal notranslate"><span class="pre">ContextDecorator</span></code> lets you instead write:</p>

  640. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@cm</span><span class="p">()</span>

  641. <span class="k">def</span> <span class="nf">f</span><span class="p">():</span>

  642.     <span class="c1"># Do stuff</span>

  643. </pre></div>

  644. </div>

  645. <p>It makes it clear that the <code class="docutils literal notranslate"><span class="pre">cm</span></code> applies to the whole function, rather than

  646. just a piece of it (and saving an indentation level is nice, too).</p>

  647. <p>Existing context managers that already have a base class can be extended by

  648. using <code class="docutils literal notranslate"><span class="pre">ContextDecorator</span></code> as a mixin class:</p>

  649. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ContextDecorator</span>

  650. 

  651. <span class="k">class</span> <span class="nc">mycontext</span><span class="p">(</span><span class="n">ContextBaseClass</span><span class="p">,</span> <span class="n">ContextDecorator</span><span class="p">):</span>

  652.     <span class="k">def</span> <span class="fm">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  653.         <span class="k">return</span> <span class="bp">self</span>

  654. 

  655.     <span class="k">def</span> <span class="fm">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">*</span><span class="n">exc</span><span class="p">):</span>

  656.         <span class="k">return</span> <span class="kc">False</span>

  657. </pre></div>

  658. </div>

  659. <div class="admonition note">

  660. <p class="admonition-title">Note</p>

  661. <p>As the decorated function must be able to be called multiple times, the

  662. underlying context manager must support use in multiple <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>

  663. statements. If this is not the case, then the original construct with the

  664. explicit <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code> statement inside the function should be used.</p>

  665. </div>

  666. <div class="versionadded">

  667. <p><span class="versionmodified added">New in version 3.2.</span></p>

  668. </div>

  669. </dd></dl>

  670. 

  671. <dl class="py class">

  672. <dt class="sig sig-object py" id="contextlib.AsyncContextDecorator">

  673. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">AsyncContextDecorator</span></span><a class="headerlink" href="#contextlib.AsyncContextDecorator" title="Link to this definition">¶</a></dt>

  674. <dd><p>Similar to <a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextDecorator</span></code></a> but only for asynchronous functions.</p>

  675. <p>Example of <code class="docutils literal notranslate"><span class="pre">AsyncContextDecorator</span></code>:</p>

  676. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">asyncio</span> <span class="kn">import</span> <span class="n">run</span>

  677. <span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">AsyncContextDecorator</span>

  678. 

  679. <span class="k">class</span> <span class="nc">mycontext</span><span class="p">(</span><span class="n">AsyncContextDecorator</span><span class="p">):</span>

  680.     <span class="k">async</span> <span class="k">def</span> <span class="fm">__aenter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  681.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Starting&#39;</span><span class="p">)</span>

  682.         <span class="k">return</span> <span class="bp">self</span>

  683. 

  684.     <span class="k">async</span> <span class="k">def</span> <span class="fm">__aexit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">*</span><span class="n">exc</span><span class="p">):</span>

  685.         <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Finishing&#39;</span><span class="p">)</span>

  686.         <span class="k">return</span> <span class="kc">False</span>

  687. </pre></div>

  688. </div>

  689. <p>The class can then be used like this:</p>

  690. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="nd">@mycontext</span><span class="p">()</span>

  691. <span class="gp">... </span><span class="k">async</span> <span class="k">def</span> <span class="nf">function</span><span class="p">():</span>

  692. <span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;The bit in the middle&#39;</span><span class="p">)</span>

  693. <span class="gp">...</span>

  694. <span class="gp">&gt;&gt;&gt; </span><span class="n">run</span><span class="p">(</span><span class="n">function</span><span class="p">())</span>

  695. <span class="go">Starting</span>

  696. <span class="go">The bit in the middle</span>

  697. <span class="go">Finishing</span>

  698. 

  699. <span class="gp">&gt;&gt;&gt; </span><span class="k">async</span> <span class="k">def</span> <span class="nf">function</span><span class="p">():</span>

  700. <span class="gp">... </span>   <span class="k">async</span> <span class="k">with</span> <span class="n">mycontext</span><span class="p">():</span>

  701. <span class="gp">... </span>        <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;The bit in the middle&#39;</span><span class="p">)</span>

  702. <span class="gp">...</span>

  703. <span class="gp">&gt;&gt;&gt; </span><span class="n">run</span><span class="p">(</span><span class="n">function</span><span class="p">())</span>

  704. <span class="go">Starting</span>

  705. <span class="go">The bit in the middle</span>

  706. <span class="go">Finishing</span>

  707. </pre></div>

  708. </div>

  709. <div class="versionadded">

  710. <p><span class="versionmodified added">New in version 3.10.</span></p>

  711. </div>

  712. </dd></dl>

  713. 

  714. <dl class="py class">

  715. <dt class="sig sig-object py" id="contextlib.ExitStack">

  716. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">ExitStack</span></span><a class="headerlink" href="#contextlib.ExitStack" title="Link to this definition">¶</a></dt>

  717. <dd><p>A context manager that is designed to make it easy to programmatically

  718. combine other context managers and cleanup functions, especially those

  719. that are optional or otherwise driven by input data.</p>

  720. <p>For example, a set of files may easily be handled in a single with

  721. statement as follows:</p>

  722. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>

  723.     <span class="n">files</span> <span class="o">=</span> <span class="p">[</span><span class="n">stack</span><span class="o">.</span><span class="n">enter_context</span><span class="p">(</span><span class="nb">open</span><span class="p">(</span><span class="n">fname</span><span class="p">))</span> <span class="k">for</span> <span class="n">fname</span> <span class="ow">in</span> <span class="n">filenames</span><span class="p">]</span>

  724.     <span class="c1"># All opened files will automatically be closed at the end of</span>

  725.     <span class="c1"># the with statement, even if attempts to open files later</span>

  726.     <span class="c1"># in the list raise an exception</span>

  727. </pre></div>

  728. </div>

  729. <p>The <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> method returns the <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> instance, and

  730. performs no additional operations.</p>

  731. <p>Each instance maintains a stack of registered callbacks that are called in

  732. reverse order when the instance is closed (either explicitly or implicitly

  733. at the end of a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement). Note that callbacks are <em>not</em>

  734. invoked implicitly when the context stack instance is garbage collected.</p>

  735. <p>This stack model is used so that context managers that acquire their

  736. resources in their <code class="docutils literal notranslate"><span class="pre">__init__</span></code> method (such as file objects) can be

  737. handled correctly.</p>

  738. <p>Since registered callbacks are invoked in the reverse order of

  739. registration, this ends up behaving as if multiple nested <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>

  740. statements had been used with the registered set of callbacks. This even

  741. extends to exception handling - if an inner callback suppresses or replaces

  742. an exception, then outer callbacks will be passed arguments based on that

  743. updated state.</p>

  744. <p>This is a relatively low level API that takes care of the details of

  745. correctly unwinding the stack of exit callbacks. It provides a suitable

  746. foundation for higher level context managers that manipulate the exit

  747. stack in application specific ways.</p>

  748. <div class="versionadded">

  749. <p><span class="versionmodified added">New in version 3.3.</span></p>

  750. </div>

  751. <dl class="py method">

  752. <dt class="sig sig-object py" id="contextlib.ExitStack.enter_context">

  753. <span class="sig-name descname"><span class="pre">enter_context</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">cm</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.ExitStack.enter_context" title="Link to this definition">¶</a></dt>

  754. <dd><p>Enters a new context manager and adds its <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> method to

  755. the callback stack. The return value is the result of the context

  756. manager’s own <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> method.</p>

  757. <p>These context managers may suppress exceptions just as they normally

  758. would if used directly as part of a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement.</p>

  759. <div class="versionchanged">

  760. <p><span class="versionmodified changed">Changed in version 3.11: </span>Raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> instead of <a class="reference internal" href="exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> if <em>cm</em>

  761. is not a context manager.</p>

  762. </div>

  763. </dd></dl>

  764. 

  765. <dl class="py method">

  766. <dt class="sig sig-object py" id="contextlib.ExitStack.push">

  767. <span class="sig-name descname"><span class="pre">push</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">exit</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.ExitStack.push" title="Link to this definition">¶</a></dt>

  768. <dd><p>Adds a context manager’s <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> method to the callback stack.</p>

  769. <p>As <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> is <em>not</em> invoked, this method can be used to cover

  770. part of an <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> implementation with a context manager’s own

  771. <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> method.</p>

  772. <p>If passed an object that is not a context manager, this method assumes

  773. it is a callback with the same signature as a context manager’s

  774. <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> method and adds it directly to the callback stack.</p>

  775. <p>By returning true values, these callbacks can suppress exceptions the

  776. same way context manager <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> methods can.</p>

  777. <p>The passed in object is returned from the function, allowing this

  778. method to be used as a function decorator.</p>

  779. </dd></dl>

  780. 

  781. <dl class="py method">

  782. <dt class="sig sig-object py" id="contextlib.ExitStack.callback">

  783. <span class="sig-name descname"><span class="pre">callback</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">callback</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">/</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">kwds</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.ExitStack.callback" title="Link to this definition">¶</a></dt>

  784. <dd><p>Accepts an arbitrary callback function and arguments and adds it to

  785. the callback stack.</p>

  786. <p>Unlike the other methods, callbacks added this way cannot suppress

  787. exceptions (as they are never passed the exception details).</p>

  788. <p>The passed in callback is returned from the function, allowing this

  789. method to be used as a function decorator.</p>

  790. </dd></dl>

  791. 

  792. <dl class="py method">

  793. <dt class="sig sig-object py" id="contextlib.ExitStack.pop_all">

  794. <span class="sig-name descname"><span class="pre">pop_all</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.ExitStack.pop_all" title="Link to this definition">¶</a></dt>

  795. <dd><p>Transfers the callback stack to a fresh <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> instance

  796. and returns it. No callbacks are invoked by this operation - instead,

  797. they will now be invoked when the new stack is closed (either

  798. explicitly or implicitly at the end of a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement).</p>

  799. <p>For example, a group of files can be opened as an “all or nothing”

  800. operation as follows:</p>

  801. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>

  802.     <span class="n">files</span> <span class="o">=</span> <span class="p">[</span><span class="n">stack</span><span class="o">.</span><span class="n">enter_context</span><span class="p">(</span><span class="nb">open</span><span class="p">(</span><span class="n">fname</span><span class="p">))</span> <span class="k">for</span> <span class="n">fname</span> <span class="ow">in</span> <span class="n">filenames</span><span class="p">]</span>

  803.     <span class="c1"># Hold onto the close method, but don&#39;t call it yet.</span>

  804.     <span class="n">close_files</span> <span class="o">=</span> <span class="n">stack</span><span class="o">.</span><span class="n">pop_all</span><span class="p">()</span><span class="o">.</span><span class="n">close</span>

  805.     <span class="c1"># If opening any file fails, all previously opened files will be</span>

  806.     <span class="c1"># closed automatically. If all files are opened successfully,</span>

  807.     <span class="c1"># they will remain open even after the with statement ends.</span>

  808.     <span class="c1"># close_files() can then be invoked explicitly to close them all.</span>

  809. </pre></div>

  810. </div>

  811. </dd></dl>

  812. 

  813. <dl class="py method">

  814. <dt class="sig sig-object py" id="contextlib.ExitStack.close">

  815. <span class="sig-name descname"><span class="pre">close</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.ExitStack.close" title="Link to this definition">¶</a></dt>

  816. <dd><p>Immediately unwinds the callback stack, invoking callbacks in the

  817. reverse order of registration. For any context managers and exit

  818. callbacks registered, the arguments passed in will indicate that no

  819. exception occurred.</p>

  820. </dd></dl>

  821. 

  822. </dd></dl>

  823. 

  824. <dl class="py class">

  825. <dt class="sig sig-object py" id="contextlib.AsyncExitStack">

  826. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">contextlib.</span></span><span class="sig-name descname"><span class="pre">AsyncExitStack</span></span><a class="headerlink" href="#contextlib.AsyncExitStack" title="Link to this definition">¶</a></dt>

  827. <dd><p>An <a class="reference internal" href="../reference/datamodel.html#async-context-managers"><span class="std std-ref">asynchronous context manager</span></a>, similar

  828. to <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a>, that supports combining both synchronous and

  829. asynchronous context managers, as well as having coroutines for

  830. cleanup logic.</p>

  831. <p>The <a class="reference internal" href="#contextlib.ExitStack.close" title="contextlib.ExitStack.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code></a> method is not implemented; <a class="reference internal" href="#contextlib.AsyncExitStack.aclose" title="contextlib.AsyncExitStack.aclose"><code class="xref py py-meth docutils literal notranslate"><span class="pre">aclose()</span></code></a> must be used

  832. instead.</p>

  833. <dl class="py method">

  834. <dt class="sig sig-object py" id="contextlib.AsyncExitStack.enter_async_context">

  835. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-name descname"><span class="pre">enter_async_context</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">cm</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.AsyncExitStack.enter_async_context" title="Link to this definition">¶</a></dt>

  836. <dd><p>Similar to <a class="reference internal" href="#contextlib.ExitStack.enter_context" title="contextlib.ExitStack.enter_context"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ExitStack.enter_context()</span></code></a> but expects an asynchronous context

  837. manager.</p>

  838. <div class="versionchanged">

  839. <p><span class="versionmodified changed">Changed in version 3.11: </span>Raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> instead of <a class="reference internal" href="exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a> if <em>cm</em>

  840. is not an asynchronous context manager.</p>

  841. </div>

  842. </dd></dl>

  843. 

  844. <dl class="py method">

  845. <dt class="sig sig-object py" id="contextlib.AsyncExitStack.push_async_exit">

  846. <span class="sig-name descname"><span class="pre">push_async_exit</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">exit</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.AsyncExitStack.push_async_exit" title="Link to this definition">¶</a></dt>

  847. <dd><p>Similar to <a class="reference internal" href="#contextlib.ExitStack.push" title="contextlib.ExitStack.push"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ExitStack.push()</span></code></a> but expects either an asynchronous context manager

  848. or a coroutine function.</p>

  849. </dd></dl>

  850. 

  851. <dl class="py method">

  852. <dt class="sig sig-object py" id="contextlib.AsyncExitStack.push_async_callback">

  853. <span class="sig-name descname"><span class="pre">push_async_callback</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">callback</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">/</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">kwds</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.AsyncExitStack.push_async_callback" title="Link to this definition">¶</a></dt>

  854. <dd><p>Similar to <a class="reference internal" href="#contextlib.ExitStack.callback" title="contextlib.ExitStack.callback"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ExitStack.callback()</span></code></a> but expects a coroutine function.</p>

  855. </dd></dl>

  856. 

  857. <dl class="py method">

  858. <dt class="sig sig-object py" id="contextlib.AsyncExitStack.aclose">

  859. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-name descname"><span class="pre">aclose</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#contextlib.AsyncExitStack.aclose" title="Link to this definition">¶</a></dt>

  860. <dd><p>Similar to <a class="reference internal" href="#contextlib.ExitStack.close" title="contextlib.ExitStack.close"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ExitStack.close()</span></code></a> but properly handles awaitables.</p>

  861. </dd></dl>

  862. 

  863. <p>Continuing the example for <a class="reference internal" href="#contextlib.asynccontextmanager" title="contextlib.asynccontextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">asynccontextmanager()</span></code></a>:</p>

  864. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">with</span> <span class="n">AsyncExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>

  865.     <span class="n">connections</span> <span class="o">=</span> <span class="p">[</span><span class="k">await</span> <span class="n">stack</span><span class="o">.</span><span class="n">enter_async_context</span><span class="p">(</span><span class="n">get_connection</span><span class="p">())</span>

  866.         <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">)]</span>

  867.     <span class="c1"># All opened connections will automatically be released at the end of</span>

  868.     <span class="c1"># the async with statement, even if attempts to open a connection</span>

  869.     <span class="c1"># later in the list raise an exception.</span>

  870. </pre></div>

  871. </div>

  872. <div class="versionadded">

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

  874. </div>

  875. </dd></dl>

  876. 

  877. </section>

  878. <section id="examples-and-recipes">

  879. <h2>Examples and Recipes<a class="headerlink" href="#examples-and-recipes" title="Link to this heading">¶</a></h2>

  880. <p>This section describes some examples and recipes for making effective use of

  881. the tools provided by <a class="reference internal" href="#module-contextlib" title="contextlib: Utilities for with-statement contexts."><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextlib</span></code></a>.</p>

  882. <section id="supporting-a-variable-number-of-context-managers">

  883. <h3>Supporting a variable number of context managers<a class="headerlink" href="#supporting-a-variable-number-of-context-managers" title="Link to this heading">¶</a></h3>

  884. <p>The primary use case for <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> is the one given in the class

  885. documentation: supporting a variable number of context managers and other

  886. cleanup operations in a single <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement. The variability

  887. may come from the number of context managers needed being driven by user

  888. input (such as opening a user specified collection of files), or from

  889. some of the context managers being optional:</p>

  890. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>

  891.     <span class="k">for</span> <span class="n">resource</span> <span class="ow">in</span> <span class="n">resources</span><span class="p">:</span>

  892.         <span class="n">stack</span><span class="o">.</span><span class="n">enter_context</span><span class="p">(</span><span class="n">resource</span><span class="p">)</span>

  893.     <span class="k">if</span> <span class="n">need_special_resource</span><span class="p">():</span>

  894.         <span class="n">special</span> <span class="o">=</span> <span class="n">acquire_special_resource</span><span class="p">()</span>

  895.         <span class="n">stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="n">release_special_resource</span><span class="p">,</span> <span class="n">special</span><span class="p">)</span>

  896.     <span class="c1"># Perform operations that use the acquired resources</span>

  897. </pre></div>

  898. </div>

  899. <p>As shown, <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> also makes it quite easy to use <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>

  900. statements to manage arbitrary resources that don’t natively support the

  901. context management protocol.</p>

  902. </section>

  903. <section id="catching-exceptions-from-enter-methods">

  904. <h3>Catching exceptions from <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> methods<a class="headerlink" href="#catching-exceptions-from-enter-methods" title="Link to this heading">¶</a></h3>

  905. <p>It is occasionally desirable to catch exceptions from an <code class="docutils literal notranslate"><span class="pre">__enter__</span></code>

  906. method implementation, <em>without</em> inadvertently catching exceptions from

  907. the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement body or the context manager’s <code class="docutils literal notranslate"><span class="pre">__exit__</span></code>

  908. method. By using <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> the steps in the context management

  909. protocol can be separated slightly in order to allow this:</p>

  910. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">stack</span> <span class="o">=</span> <span class="n">ExitStack</span><span class="p">()</span>

  911. <span class="k">try</span><span class="p">:</span>

  912.     <span class="n">x</span> <span class="o">=</span> <span class="n">stack</span><span class="o">.</span><span class="n">enter_context</span><span class="p">(</span><span class="n">cm</span><span class="p">)</span>

  913. <span class="k">except</span> <span class="ne">Exception</span><span class="p">:</span>

  914.     <span class="c1"># handle __enter__ exception</span>

  915. <span class="k">else</span><span class="p">:</span>

  916.     <span class="k">with</span> <span class="n">stack</span><span class="p">:</span>

  917.         <span class="c1"># Handle normal case</span>

  918. </pre></div>

  919. </div>

  920. <p>Actually needing to do this is likely to indicate that the underlying API

  921. should be providing a direct resource management interface for use with

  922. <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a>/<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a>/<a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> statements, but not

  923. all APIs are well designed in that regard. When a context manager is the

  924. only resource management API provided, then <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> can make it

  925. easier to handle various situations that can’t be handled directly in a

  926. <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement.</p>

  927. </section>

  928. <section id="cleaning-up-in-an-enter-implementation">

  929. <h3>Cleaning up in an <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> implementation<a class="headerlink" href="#cleaning-up-in-an-enter-implementation" title="Link to this heading">¶</a></h3>

  930. <p>As noted in the documentation of <a class="reference internal" href="#contextlib.ExitStack.push" title="contextlib.ExitStack.push"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ExitStack.push()</span></code></a>, this

  931. method can be useful in cleaning up an already allocated resource if later

  932. steps in the <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> implementation fail.</p>

  933. <p>Here’s an example of doing this for a context manager that accepts resource

  934. acquisition and release functions, along with an optional validation function,

  935. and maps them to the context management protocol:</p>

  936. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">contextmanager</span><span class="p">,</span> <span class="n">AbstractContextManager</span><span class="p">,</span> <span class="n">ExitStack</span>

  937. 

  938. <span class="k">class</span> <span class="nc">ResourceManager</span><span class="p">(</span><span class="n">AbstractContextManager</span><span class="p">):</span>

  939. 

  940.     <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">acquire_resource</span><span class="p">,</span> <span class="n">release_resource</span><span class="p">,</span> <span class="n">check_resource_ok</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>

  941.         <span class="bp">self</span><span class="o">.</span><span class="n">acquire_resource</span> <span class="o">=</span> <span class="n">acquire_resource</span>

  942.         <span class="bp">self</span><span class="o">.</span><span class="n">release_resource</span> <span class="o">=</span> <span class="n">release_resource</span>

  943.         <span class="k">if</span> <span class="n">check_resource_ok</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>

  944.             <span class="k">def</span> <span class="nf">check_resource_ok</span><span class="p">(</span><span class="n">resource</span><span class="p">):</span>

  945.                 <span class="k">return</span> <span class="kc">True</span>

  946.         <span class="bp">self</span><span class="o">.</span><span class="n">check_resource_ok</span> <span class="o">=</span> <span class="n">check_resource_ok</span>

  947. 

  948.     <span class="nd">@contextmanager</span>

  949.     <span class="k">def</span> <span class="nf">_cleanup_on_error</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  950.         <span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>

  951.             <span class="n">stack</span><span class="o">.</span><span class="n">push</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>

  952.             <span class="k">yield</span>

  953.             <span class="c1"># The validation check passed and didn&#39;t raise an exception</span>

  954.             <span class="c1"># Accordingly, we want to keep the resource, and pass it</span>

  955.             <span class="c1"># back to our caller</span>

  956.             <span class="n">stack</span><span class="o">.</span><span class="n">pop_all</span><span class="p">()</span>

  957. 

  958.     <span class="k">def</span> <span class="fm">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  959.         <span class="n">resource</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">acquire_resource</span><span class="p">()</span>

  960.         <span class="k">with</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cleanup_on_error</span><span class="p">():</span>

  961.             <span class="k">if</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">check_resource_ok</span><span class="p">(</span><span class="n">resource</span><span class="p">):</span>

  962.                 <span class="n">msg</span> <span class="o">=</span> <span class="s2">&quot;Failed validation for </span><span class="si">{!r}</span><span class="s2">&quot;</span>

  963.                 <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="n">msg</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">resource</span><span class="p">))</span>

  964.         <span class="k">return</span> <span class="n">resource</span>

  965. 

  966.     <span class="k">def</span> <span class="fm">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">*</span><span class="n">exc_details</span><span class="p">):</span>

  967.         <span class="c1"># We don&#39;t need to duplicate any of our resource release logic</span>

  968.         <span class="bp">self</span><span class="o">.</span><span class="n">release_resource</span><span class="p">()</span>

  969. </pre></div>

  970. </div>

  971. </section>

  972. <section id="replacing-any-use-of-try-finally-and-flag-variables">

  973. <h3>Replacing any use of <code class="docutils literal notranslate"><span class="pre">try-finally</span></code> and flag variables<a class="headerlink" href="#replacing-any-use-of-try-finally-and-flag-variables" title="Link to this heading">¶</a></h3>

  974. <p>A pattern you will sometimes see is a <code class="docutils literal notranslate"><span class="pre">try-finally</span></code> statement with a flag

  975. variable to indicate whether or not the body of the <code class="docutils literal notranslate"><span class="pre">finally</span></code> clause should

  976. be executed. In its simplest form (that can’t already be handled just by

  977. using an <code class="docutils literal notranslate"><span class="pre">except</span></code> clause instead), it looks something like this:</p>

  978. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">cleanup_needed</span> <span class="o">=</span> <span class="kc">True</span>

  979. <span class="k">try</span><span class="p">:</span>

  980.     <span class="n">result</span> <span class="o">=</span> <span class="n">perform_operation</span><span class="p">()</span>

  981.     <span class="k">if</span> <span class="n">result</span><span class="p">:</span>

  982.         <span class="n">cleanup_needed</span> <span class="o">=</span> <span class="kc">False</span>

  983. <span class="k">finally</span><span class="p">:</span>

  984.     <span class="k">if</span> <span class="n">cleanup_needed</span><span class="p">:</span>

  985.         <span class="n">cleanup_resources</span><span class="p">()</span>

  986. </pre></div>

  987. </div>

  988. <p>As with any <code class="docutils literal notranslate"><span class="pre">try</span></code> statement based code, this can cause problems for

  989. development and review, because the setup code and the cleanup code can end

  990. up being separated by arbitrarily long sections of code.</p>

  991. <p><a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> makes it possible to instead register a callback for

  992. execution at the end of a <code class="docutils literal notranslate"><span class="pre">with</span></code> statement, and then later decide to skip

  993. executing that callback:</p>

  994. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ExitStack</span>

  995. 

  996. <span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>

  997.     <span class="n">stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="n">cleanup_resources</span><span class="p">)</span>

  998.     <span class="n">result</span> <span class="o">=</span> <span class="n">perform_operation</span><span class="p">()</span>

  999.     <span class="k">if</span> <span class="n">result</span><span class="p">:</span>

  1000.         <span class="n">stack</span><span class="o">.</span><span class="n">pop_all</span><span class="p">()</span>

  1001. </pre></div>

  1002. </div>

  1003. <p>This allows the intended cleanup up behaviour to be made explicit up front,

  1004. rather than requiring a separate flag variable.</p>

  1005. <p>If a particular application uses this pattern a lot, it can be simplified

  1006. even further by means of a small helper class:</p>

  1007. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ExitStack</span>

  1008. 

  1009. <span class="k">class</span> <span class="nc">Callback</span><span class="p">(</span><span class="n">ExitStack</span><span class="p">):</span>

  1010.     <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">callback</span><span class="p">,</span> <span class="o">/</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">):</span>

  1011.         <span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="fm">__init__</span><span class="p">()</span>

  1012.         <span class="bp">self</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="n">callback</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">)</span>

  1013. 

  1014.     <span class="k">def</span> <span class="nf">cancel</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  1015.         <span class="bp">self</span><span class="o">.</span><span class="n">pop_all</span><span class="p">()</span>

  1016. 

  1017. <span class="k">with</span> <span class="n">Callback</span><span class="p">(</span><span class="n">cleanup_resources</span><span class="p">)</span> <span class="k">as</span> <span class="n">cb</span><span class="p">:</span>

  1018.     <span class="n">result</span> <span class="o">=</span> <span class="n">perform_operation</span><span class="p">()</span>

  1019.     <span class="k">if</span> <span class="n">result</span><span class="p">:</span>

  1020.         <span class="n">cb</span><span class="o">.</span><span class="n">cancel</span><span class="p">()</span>

  1021. </pre></div>

  1022. </div>

  1023. <p>If the resource cleanup isn’t already neatly bundled into a standalone

  1024. function, then it is still possible to use the decorator form of

  1025. <a class="reference internal" href="#contextlib.ExitStack.callback" title="contextlib.ExitStack.callback"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ExitStack.callback()</span></code></a> to declare the resource cleanup in

  1026. advance:</p>

  1027. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ExitStack</span>

  1028. 

  1029. <span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">stack</span><span class="p">:</span>

  1030.     <span class="nd">@stack</span><span class="o">.</span><span class="n">callback</span>

  1031.     <span class="k">def</span> <span class="nf">cleanup_resources</span><span class="p">():</span>

  1032.         <span class="o">...</span>

  1033.     <span class="n">result</span> <span class="o">=</span> <span class="n">perform_operation</span><span class="p">()</span>

  1034.     <span class="k">if</span> <span class="n">result</span><span class="p">:</span>

  1035.         <span class="n">stack</span><span class="o">.</span><span class="n">pop_all</span><span class="p">()</span>

  1036. </pre></div>

  1037. </div>

  1038. <p>Due to the way the decorator protocol works, a callback function

  1039. declared this way cannot take any parameters. Instead, any resources to

  1040. be released must be accessed as closure variables.</p>

  1041. </section>

  1042. <section id="using-a-context-manager-as-a-function-decorator">

  1043. <h3>Using a context manager as a function decorator<a class="headerlink" href="#using-a-context-manager-as-a-function-decorator" title="Link to this heading">¶</a></h3>

  1044. <p><a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextDecorator</span></code></a> makes it possible to use a context manager in

  1045. both an ordinary <code class="docutils literal notranslate"><span class="pre">with</span></code> statement and also as a function decorator.</p>

  1046. <p>For example, it is sometimes useful to wrap functions or groups of statements

  1047. with a logger that can track the time of entry and time of exit.  Rather than

  1048. writing both a function decorator and a context manager for the task,

  1049. inheriting from <a class="reference internal" href="#contextlib.ContextDecorator" title="contextlib.ContextDecorator"><code class="xref py py-class docutils literal notranslate"><span class="pre">ContextDecorator</span></code></a> provides both capabilities in a

  1050. single definition:</p>

  1051. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ContextDecorator</span>

  1052. <span class="kn">import</span> <span class="nn">logging</span>

  1053. 

  1054. <span class="n">logging</span><span class="o">.</span><span class="n">basicConfig</span><span class="p">(</span><span class="n">level</span><span class="o">=</span><span class="n">logging</span><span class="o">.</span><span class="n">INFO</span><span class="p">)</span>

  1055. 

  1056. <span class="k">class</span> <span class="nc">track_entry_and_exit</span><span class="p">(</span><span class="n">ContextDecorator</span><span class="p">):</span>

  1057.     <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>

  1058.         <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>

  1059. 

  1060.     <span class="k">def</span> <span class="fm">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  1061.         <span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">&#39;Entering: </span><span class="si">%s</span><span class="s1">&#39;</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>

  1062. 

  1063.     <span class="k">def</span> <span class="fm">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">exc_type</span><span class="p">,</span> <span class="n">exc</span><span class="p">,</span> <span class="n">exc_tb</span><span class="p">):</span>

  1064.         <span class="n">logging</span><span class="o">.</span><span class="n">info</span><span class="p">(</span><span class="s1">&#39;Exiting: </span><span class="si">%s</span><span class="s1">&#39;</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>

  1065. </pre></div>

  1066. </div>

  1067. <p>Instances of this class can be used as both a context manager:</p>

  1068. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">track_entry_and_exit</span><span class="p">(</span><span class="s1">&#39;widget loader&#39;</span><span class="p">):</span>

  1069.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Some time consuming activity goes here&#39;</span><span class="p">)</span>

  1070.     <span class="n">load_widget</span><span class="p">()</span>

  1071. </pre></div>

  1072. </div>

  1073. <p>And also as a function decorator:</p>

  1074. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nd">@track_entry_and_exit</span><span class="p">(</span><span class="s1">&#39;widget loader&#39;</span><span class="p">)</span>

  1075. <span class="k">def</span> <span class="nf">activity</span><span class="p">():</span>

  1076.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Some time consuming activity goes here&#39;</span><span class="p">)</span>

  1077.     <span class="n">load_widget</span><span class="p">()</span>

  1078. </pre></div>

  1079. </div>

  1080. <p>Note that there is one additional limitation when using context managers

  1081. as function decorators: there’s no way to access the return value of

  1082. <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a>. If that value is needed, then it is still necessary to use

  1083. an explicit <code class="docutils literal notranslate"><span class="pre">with</span></code> statement.</p>

  1084. <div class="admonition seealso">

  1085. <p class="admonition-title">See also</p>

  1086. <dl class="simple">

  1087. <dt><span class="target" id="index-0"></span><a class="pep reference external" href="https://peps.python.org/pep-0343/"><strong>PEP 343</strong></a> - The “with” statement</dt><dd><p>The specification, background, and examples for the Python <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>

  1088. statement.</p>

  1089. </dd>

  1090. </dl>

  1091. </div>

  1092. </section>

  1093. </section>

  1094. <section id="single-use-reusable-and-reentrant-context-managers">

  1095. <span id="single-use-reusable-and-reentrant-cms"></span><h2>Single use, reusable and reentrant context managers<a class="headerlink" href="#single-use-reusable-and-reentrant-context-managers" title="Link to this heading">¶</a></h2>

  1096. <p>Most context managers are written in a way that means they can only be

  1097. used effectively in a <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement once. These single use

  1098. context managers must be created afresh each time they’re used -

  1099. attempting to use them a second time will trigger an exception or

  1100. otherwise not work correctly.</p>

  1101. <p>This common limitation means that it is generally advisable to create

  1102. context managers directly in the header of the <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement

  1103. where they are used (as shown in all of the usage examples above).</p>

  1104. <p>Files are an example of effectively single use context managers, since

  1105. the first <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement will close the file, preventing any

  1106. further IO operations using that file object.</p>

  1107. <p>Context managers created using <a class="reference internal" href="#contextlib.contextmanager" title="contextlib.contextmanager"><code class="xref py py-func docutils literal notranslate"><span class="pre">contextmanager()</span></code></a> are also single use

  1108. context managers, and will complain about the underlying generator failing

  1109. to yield if an attempt is made to use them a second time:</p>

  1110. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">contextmanager</span>

  1111. <span class="gp">&gt;&gt;&gt; </span><span class="nd">@contextmanager</span>

  1112. <span class="gp">... </span><span class="k">def</span> <span class="nf">singleuse</span><span class="p">():</span>

  1113. <span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Before&quot;</span><span class="p">)</span>

  1114. <span class="gp">... </span>    <span class="k">yield</span>

  1115. <span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;After&quot;</span><span class="p">)</span>

  1116. <span class="gp">...</span>

  1117. <span class="gp">&gt;&gt;&gt; </span><span class="n">cm</span> <span class="o">=</span> <span class="n">singleuse</span><span class="p">()</span>

  1118. <span class="gp">&gt;&gt;&gt; </span><span class="k">with</span> <span class="n">cm</span><span class="p">:</span>

  1119. <span class="gp">... </span>    <span class="k">pass</span>

  1120. <span class="gp">...</span>

  1121. <span class="go">Before</span>

  1122. <span class="go">After</span>

  1123. <span class="gp">&gt;&gt;&gt; </span><span class="k">with</span> <span class="n">cm</span><span class="p">:</span>

  1124. <span class="gp">... </span>    <span class="k">pass</span>

  1125. <span class="gp">...</span>

  1126. <span class="gt">Traceback (most recent call last):</span>

  1127. <span class="w">    </span><span class="o">...</span>

  1128. <span class="gr">RuntimeError</span>: <span class="n">generator didn&#39;t yield</span>

  1129. </pre></div>

  1130. </div>

  1131. <section id="reentrant-context-managers">

  1132. <span id="reentrant-cms"></span><h3>Reentrant context managers<a class="headerlink" href="#reentrant-context-managers" title="Link to this heading">¶</a></h3>

  1133. <p>More sophisticated context managers may be “reentrant”. These context

  1134. managers can not only be used in multiple <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statements,

  1135. but may also be used <em>inside</em> a <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code> statement that is already

  1136. using the same context manager.</p>

  1137. <p><a class="reference internal" href="threading.html#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.RLock</span></code></a> is an example of a reentrant context manager, as are

  1138. <a class="reference internal" href="#contextlib.suppress" title="contextlib.suppress"><code class="xref py py-func docutils literal notranslate"><span class="pre">suppress()</span></code></a>, <a class="reference internal" href="#contextlib.redirect_stdout" title="contextlib.redirect_stdout"><code class="xref py py-func docutils literal notranslate"><span class="pre">redirect_stdout()</span></code></a>, and <a class="reference internal" href="#contextlib.chdir" title="contextlib.chdir"><code class="xref py py-func docutils literal notranslate"><span class="pre">chdir()</span></code></a>. Here’s a very

  1139. simple example of reentrant use:</p>

  1140. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">redirect_stdout</span>

  1141. <span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">io</span> <span class="kn">import</span> <span class="n">StringIO</span>

  1142. <span class="gp">&gt;&gt;&gt; </span><span class="n">stream</span> <span class="o">=</span> <span class="n">StringIO</span><span class="p">()</span>

  1143. <span class="gp">&gt;&gt;&gt; </span><span class="n">write_to_stream</span> <span class="o">=</span> <span class="n">redirect_stdout</span><span class="p">(</span><span class="n">stream</span><span class="p">)</span>

  1144. <span class="gp">&gt;&gt;&gt; </span><span class="k">with</span> <span class="n">write_to_stream</span><span class="p">:</span>

  1145. <span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;This is written to the stream rather than stdout&quot;</span><span class="p">)</span>

  1146. <span class="gp">... </span>    <span class="k">with</span> <span class="n">write_to_stream</span><span class="p">:</span>

  1147. <span class="gp">... </span>        <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;This is also written to the stream&quot;</span><span class="p">)</span>

  1148. <span class="gp">...</span>

  1149. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="s2">&quot;This is written directly to stdout&quot;</span><span class="p">)</span>

  1150. <span class="go">This is written directly to stdout</span>

  1151. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">stream</span><span class="o">.</span><span class="n">getvalue</span><span class="p">())</span>

  1152. <span class="go">This is written to the stream rather than stdout</span>

  1153. <span class="go">This is also written to the stream</span>

  1154. </pre></div>

  1155. </div>

  1156. <p>Real world examples of reentrancy are more likely to involve multiple

  1157. functions calling each other and hence be far more complicated than this

  1158. example.</p>

  1159. <p>Note also that being reentrant is <em>not</em> the same thing as being thread safe.

  1160. <a class="reference internal" href="#contextlib.redirect_stdout" title="contextlib.redirect_stdout"><code class="xref py py-func docutils literal notranslate"><span class="pre">redirect_stdout()</span></code></a>, for example, is definitely not thread safe, as it

  1161. makes a global modification to the system state by binding <a class="reference internal" href="sys.html#sys.stdout" title="sys.stdout"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.stdout</span></code></a>

  1162. to a different stream.</p>

  1163. </section>

  1164. <section id="reusable-context-managers">

  1165. <span id="reusable-cms"></span><h3>Reusable context managers<a class="headerlink" href="#reusable-context-managers" title="Link to this heading">¶</a></h3>

  1166. <p>Distinct from both single use and reentrant context managers are “reusable”

  1167. context managers (or, to be completely explicit, “reusable, but not

  1168. reentrant” context managers, since reentrant context managers are also

  1169. reusable). These context managers support being used multiple times, but

  1170. will fail (or otherwise not work correctly) if the specific context manager

  1171. instance has already been used in a containing with statement.</p>

  1172. <p><a class="reference internal" href="threading.html#threading.Lock" title="threading.Lock"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Lock</span></code></a> is an example of a reusable, but not reentrant,

  1173. context manager (for a reentrant lock, it is necessary to use

  1174. <a class="reference internal" href="threading.html#threading.RLock" title="threading.RLock"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.RLock</span></code></a> instead).</p>

  1175. <p>Another example of a reusable, but not reentrant, context manager is

  1176. <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a>, as it invokes <em>all</em> currently registered callbacks

  1177. when leaving any with statement, regardless of where those callbacks

  1178. were added:</p>

  1179. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ExitStack</span>

  1180. <span class="gp">&gt;&gt;&gt; </span><span class="n">stack</span> <span class="o">=</span> <span class="n">ExitStack</span><span class="p">()</span>

  1181. <span class="gp">&gt;&gt;&gt; </span><span class="k">with</span> <span class="n">stack</span><span class="p">:</span>

  1182. <span class="gp">... </span>    <span class="n">stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="nb">print</span><span class="p">,</span> <span class="s2">&quot;Callback: from first context&quot;</span><span class="p">)</span>

  1183. <span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Leaving first context&quot;</span><span class="p">)</span>

  1184. <span class="gp">...</span>

  1185. <span class="go">Leaving first context</span>

  1186. <span class="go">Callback: from first context</span>

  1187. <span class="gp">&gt;&gt;&gt; </span><span class="k">with</span> <span class="n">stack</span><span class="p">:</span>

  1188. <span class="gp">... </span>    <span class="n">stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="nb">print</span><span class="p">,</span> <span class="s2">&quot;Callback: from second context&quot;</span><span class="p">)</span>

  1189. <span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Leaving second context&quot;</span><span class="p">)</span>

  1190. <span class="gp">...</span>

  1191. <span class="go">Leaving second context</span>

  1192. <span class="go">Callback: from second context</span>

  1193. <span class="gp">&gt;&gt;&gt; </span><span class="k">with</span> <span class="n">stack</span><span class="p">:</span>

  1194. <span class="gp">... </span>    <span class="n">stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="nb">print</span><span class="p">,</span> <span class="s2">&quot;Callback: from outer context&quot;</span><span class="p">)</span>

  1195. <span class="gp">... </span>    <span class="k">with</span> <span class="n">stack</span><span class="p">:</span>

  1196. <span class="gp">... </span>        <span class="n">stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="nb">print</span><span class="p">,</span> <span class="s2">&quot;Callback: from inner context&quot;</span><span class="p">)</span>

  1197. <span class="gp">... </span>        <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Leaving inner context&quot;</span><span class="p">)</span>

  1198. <span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Leaving outer context&quot;</span><span class="p">)</span>

  1199. <span class="gp">...</span>

  1200. <span class="go">Leaving inner context</span>

  1201. <span class="go">Callback: from inner context</span>

  1202. <span class="go">Callback: from outer context</span>

  1203. <span class="go">Leaving outer context</span>

  1204. </pre></div>

  1205. </div>

  1206. <p>As the output from the example shows, reusing a single stack object across

  1207. multiple with statements works correctly, but attempting to nest them

  1208. will cause the stack to be cleared at the end of the innermost with

  1209. statement, which is unlikely to be desirable behaviour.</p>

  1210. <p>Using separate <a class="reference internal" href="#contextlib.ExitStack" title="contextlib.ExitStack"><code class="xref py py-class docutils literal notranslate"><span class="pre">ExitStack</span></code></a> instances instead of reusing a single

  1211. instance avoids that problem:</p>

  1212. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">ExitStack</span>

  1213. <span class="gp">&gt;&gt;&gt; </span><span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">outer_stack</span><span class="p">:</span>

  1214. <span class="gp">... </span>    <span class="n">outer_stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="nb">print</span><span class="p">,</span> <span class="s2">&quot;Callback: from outer context&quot;</span><span class="p">)</span>

  1215. <span class="gp">... </span>    <span class="k">with</span> <span class="n">ExitStack</span><span class="p">()</span> <span class="k">as</span> <span class="n">inner_stack</span><span class="p">:</span>

  1216. <span class="gp">... </span>        <span class="n">inner_stack</span><span class="o">.</span><span class="n">callback</span><span class="p">(</span><span class="nb">print</span><span class="p">,</span> <span class="s2">&quot;Callback: from inner context&quot;</span><span class="p">)</span>

  1217. <span class="gp">... </span>        <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Leaving inner context&quot;</span><span class="p">)</span>

  1218. <span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Leaving outer context&quot;</span><span class="p">)</span>

  1219. <span class="gp">...</span>

  1220. <span class="go">Leaving inner context</span>

  1221. <span class="go">Callback: from inner context</span>

  1222. <span class="go">Leaving outer context</span>

  1223. <span class="go">Callback: from outer context</span>

  1224. </pre></div>

  1225. </div>

  1226. </section>

  1227. </section>

  1228. </section>

  1229. 

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  1239.     <ul>

  1240. <li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextlib</span></code> — Utilities for <code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code>-statement contexts</a><ul>

  1241. <li><a class="reference internal" href="#utilities">Utilities</a></li>

  1242. <li><a class="reference internal" href="#examples-and-recipes">Examples and Recipes</a><ul>

  1243. <li><a class="reference internal" href="#supporting-a-variable-number-of-context-managers">Supporting a variable number of context managers</a></li>

  1244. <li><a class="reference internal" href="#catching-exceptions-from-enter-methods">Catching exceptions from <code class="docutils literal notranslate"><span class="pre">__enter__</span></code> methods</a></li>

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  1246. <li><a class="reference internal" href="#replacing-any-use-of-try-finally-and-flag-variables">Replacing any use of <code class="docutils literal notranslate"><span class="pre">try-finally</span></code> and flag variables</a></li>

  1247. <li><a class="reference internal" href="#using-a-context-manager-as-a-function-decorator">Using a context manager as a function decorator</a></li>

  1248. </ul>

  1249. </li>

  1250. <li><a class="reference internal" href="#single-use-reusable-and-reentrant-context-managers">Single use, reusable and reentrant context managers</a><ul>

  1251. <li><a class="reference internal" href="#reentrant-context-managers">Reentrant context managers</a></li>

  1252. <li><a class="reference internal" href="#reusable-context-managers">Reusable context managers</a></li>

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