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  <section id="developing-with-asyncio">
<span id="asyncio-dev"></span><h1>Developing with asyncio<a class="headerlink" href="#developing-with-asyncio" title="Link to this heading">¶</a></h1>
<p>Asynchronous programming is different from classic &quot;sequential&quot;
programming.</p>
<p>This page lists common mistakes and traps and explains how
to avoid them.</p>
<section id="debug-mode">
<span id="asyncio-debug-mode"></span><h2>Debug Mode<a class="headerlink" href="#debug-mode" title="Link to this heading">¶</a></h2>
<p>By default asyncio runs in production mode.  In order to ease
the development asyncio has a <em>debug mode</em>.</p>
<p>There are several ways to enable asyncio debug mode:</p>
<ul class="simple">
<li><p>Setting the <span class="target" id="index-0"></span><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONASYNCIODEBUG"><code class="xref std std-envvar docutils literal notranslate"><span class="pre">PYTHONASYNCIODEBUG</span></code></a> environment variable to <code class="docutils literal notranslate"><span class="pre">1</span></code>.</p></li>
<li><p>Using the <a class="reference internal" href="devmode.html#devmode"><span class="std std-ref">Python Development Mode</span></a>.</p></li>
<li><p>Passing <code class="docutils literal notranslate"><span class="pre">debug=True</span></code> to <a class="reference internal" href="asyncio-runner.html#asyncio.run" title="asyncio.run"><code class="xref py py-func docutils literal notranslate"><span class="pre">asyncio.run()</span></code></a>.</p></li>
<li><p>Calling <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.set_debug" title="asyncio.loop.set_debug"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.set_debug()</span></code></a>.</p></li>
<p>In addition to enabling the debug mode, consider also:</p>
<li><p>setting the log level of the <a class="reference internal" href="#asyncio-logger"><span class="std std-ref">asyncio logger</span></a> to
<a class="reference internal" href="logging.html#logging.DEBUG" title="logging.DEBUG"><code class="xref py py-const docutils literal notranslate"><span class="pre">logging.DEBUG</span></code></a>, for example the following snippet of code
can be run at startup of the application:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><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">DEBUG</span><span class="p">)</span>
</pre></div>
<li><p>configuring the <a class="reference internal" href="warnings.html#module-warnings" title="warnings: Issue warning messages and control their disposition."><code class="xref py py-mod docutils literal notranslate"><span class="pre">warnings</span></code></a> module to display
<a class="reference internal" href="exceptions.html#ResourceWarning" title="ResourceWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ResourceWarning</span></code></a> warnings.  One way of doing that is by
using the <a class="reference internal" href="../using/cmdline.html#cmdoption-W"><code class="xref std std-option docutils literal notranslate"><span class="pre">-W</span></code></a> <code class="docutils literal notranslate"><span class="pre">default</span></code> command line option.</p></li>
<p>When the debug mode is enabled:</p>
<ul class="simple">
<li><p>Many non-threadsafe asyncio APIs (such as <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.call_soon" title="asyncio.loop.call_soon"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.call_soon()</span></code></a> and
<a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.call_at" title="asyncio.loop.call_at"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.call_at()</span></code></a> methods) raise an exception if they are called
from a wrong thread.</p></li>
<li><p>The execution time of the I/O selector is logged if it takes too long to
perform an I/O operation.</p></li>
<li><p>Callbacks taking longer than 100 milliseconds are logged.  The
<a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.slow_callback_duration" title="asyncio.loop.slow_callback_duration"><code class="xref py py-attr docutils literal notranslate"><span class="pre">loop.slow_callback_duration</span></code></a> attribute can be used to set the
minimum execution duration in seconds that is considered &quot;slow&quot;.</p></li>
<section id="concurrency-and-multithreading">
<span id="asyncio-multithreading"></span><h2>Concurrency and Multithreading<a class="headerlink" href="#concurrency-and-multithreading" title="Link to this heading">¶</a></h2>
<p>An event loop runs in a thread (typically the main thread) and executes
all callbacks and Tasks in its thread.  While a Task is running in the
event loop, no other Tasks can run in the same thread.  When a Task
executes an <code class="docutils literal notranslate"><span class="pre">await</span></code> expression, the running Task gets suspended, and
the event loop executes the next Task.</p>
<p>To schedule a <a class="reference internal" href="../glossary.html#term-callback"><span class="xref std std-term">callback</span></a> from another OS thread, the
<a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.call_soon_threadsafe" title="asyncio.loop.call_soon_threadsafe"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.call_soon_threadsafe()</span></code></a> method should be used. Example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">loop</span><span class="o">.</span><span class="n">call_soon_threadsafe</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>
</pre></div>
<p>Almost all asyncio objects are not thread safe, which is typically
not a problem unless there is code that works with them from outside
of a Task or a callback.  If there's a need for such code to call a
low-level asyncio API, the <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.call_soon_threadsafe" title="asyncio.loop.call_soon_threadsafe"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.call_soon_threadsafe()</span></code></a> method
should be used, e.g.:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">loop</span><span class="o">.</span><span class="n">call_soon_threadsafe</span><span class="p">(</span><span class="n">fut</span><span class="o">.</span><span class="n">cancel</span><span class="p">)</span>
</pre></div>
<p>To schedule a coroutine object from a different OS thread, the
<a class="reference internal" href="asyncio-task.html#asyncio.run_coroutine_threadsafe" title="asyncio.run_coroutine_threadsafe"><code class="xref py py-func docutils literal notranslate"><span class="pre">run_coroutine_threadsafe()</span></code></a> function should be used. It returns a
<a class="reference internal" href="concurrent.futures.html#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">concurrent.futures.Future</span></code></a> to access the result:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">coro_func</span><span class="p">():</span>
     <span class="k">return</span> <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">42</span><span class="p">)</span>
<span class="c1"># Later in another OS thread:</span>
<span class="n">future</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">run_coroutine_threadsafe</span><span class="p">(</span><span class="n">coro_func</span><span class="p">(),</span> <span class="n">loop</span><span class="p">)</span>
<span class="c1"># Wait for the result:</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">future</span><span class="o">.</span><span class="n">result</span><span class="p">()</span>
</pre></div>
<p>To handle signals the event loop must be
run in the main thread.</p>
<p>The <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.run_in_executor" title="asyncio.loop.run_in_executor"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.run_in_executor()</span></code></a> method can be used with a
<a class="reference internal" href="concurrent.futures.html#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">concurrent.futures.ThreadPoolExecutor</span></code></a> or
<a class="reference internal" href="concurrent.futures.html#concurrent.futures.InterpreterPoolExecutor" title="concurrent.futures.InterpreterPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">InterpreterPoolExecutor</span></code></a> to execute
blocking code in a different OS thread without blocking the OS thread
that the event loop runs in.</p>
<p>There is currently no way to schedule coroutines or callbacks directly
from a different process (such as one started with
<a class="reference internal" href="multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a>). The <a class="reference internal" href="asyncio-eventloop.html#asyncio-event-loop-methods"><span class="std std-ref">Event loop methods</span></a>
section lists APIs that can read from pipes and watch file descriptors
without blocking the event loop. In addition, asyncio's
<a class="reference internal" href="asyncio-subprocess.html#asyncio-subprocess"><span class="std std-ref">Subprocess</span></a> APIs provide a way to start a
process and communicate with it from the event loop. Lastly, the
aforementioned <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.run_in_executor" title="asyncio.loop.run_in_executor"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.run_in_executor()</span></code></a> method can also be used
with a <a class="reference internal" href="concurrent.futures.html#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">concurrent.futures.ProcessPoolExecutor</span></code></a> to execute
code in a different process.</p>
<section id="running-blocking-code">
<span id="asyncio-handle-blocking"></span><h2>Running Blocking Code<a class="headerlink" href="#running-blocking-code" title="Link to this heading">¶</a></h2>
<p>Blocking (CPU-bound) code should not be called directly.  For example,
if a function performs a CPU-intensive calculation for 1 second,
all concurrent asyncio Tasks and IO operations would be delayed
by 1 second.</p>
<p>An executor can be used to run a task in a different thread,
including in a different interpreter, or even in
a different process to avoid blocking the OS thread with the
event loop.  See the <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.run_in_executor" title="asyncio.loop.run_in_executor"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.run_in_executor()</span></code></a> method for more
details.</p>
<section id="logging">
<span id="asyncio-logger"></span><h2>Logging<a class="headerlink" href="#logging" title="Link to this heading">¶</a></h2>
<p>asyncio uses the <a class="reference internal" href="logging.html#module-logging" title="logging: Flexible event logging system for applications."><code class="xref py py-mod docutils literal notranslate"><span class="pre">logging</span></code></a> module and all logging is performed
via the <code class="docutils literal notranslate"><span class="pre">&quot;asyncio&quot;</span></code> logger.</p>
<p>The default log level is <a class="reference internal" href="logging.html#logging.INFO" title="logging.INFO"><code class="xref py py-const docutils literal notranslate"><span class="pre">logging.INFO</span></code></a>, which can be easily
adjusted:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">logging</span><span class="o">.</span><span class="n">getLogger</span><span class="p">(</span><span class="s2">"asyncio"</span><span class="p">)</span><span class="o">.</span><span class="n">setLevel</span><span class="p">(</span><span class="n">logging</span><span class="o">.</span><span class="n">WARNING</span><span class="p">)</span>
</pre></div>
<p>Network logging can block the event loop. It is recommended to use
a separate thread for handling logs or use non-blocking IO. For example,
see <a class="reference internal" href="../howto/logging-cookbook.html#blocking-handlers"><span class="std std-ref">Dealing with handlers that block</span></a>.</p>
<section id="detect-never-awaited-coroutines">
<span id="asyncio-coroutine-not-scheduled"></span><h2>Detect never-awaited coroutines<a class="headerlink" href="#detect-never-awaited-coroutines" title="Link to this heading">¶</a></h2>
<p>When a coroutine function is called, but not awaited
(e.g. <code class="docutils literal notranslate"><span class="pre">coro()</span></code> instead of <code class="docutils literal notranslate"><span class="pre">await</span> <span class="pre">coro()</span></code>)
or the coroutine is not scheduled with <a class="reference internal" href="asyncio-task.html#asyncio.create_task" title="asyncio.create_task"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asyncio.create_task()</span></code></a>, asyncio
will emit a <a class="reference internal" href="exceptions.html#RuntimeWarning" title="RuntimeWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeWarning</span></code></a>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">test</span><span class="p">():</span>
    <span class="nb">print</span><span class="p">(</span><span class="s2">"never scheduled"</span><span class="p">)</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
    <span class="n">test</span><span class="p">()</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>
</pre></div>
<p>Output:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">test</span><span class="o">.</span><span class="n">py</span><span class="p">:</span><span class="mi">7</span><span class="p">:</span> <span class="ne">RuntimeWarning</span><span class="p">:</span> <span class="n">coroutine</span> <span class="s1">'test'</span> <span class="n">was</span> <span class="n">never</span> <span class="n">awaited</span>
  <span class="n">test</span><span class="p">()</span>
</pre></div>
<p>Output in debug mode:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">test</span><span class="o">.</span><span class="n">py</span><span class="p">:</span><span class="mi">7</span><span class="p">:</span> <span class="ne">RuntimeWarning</span><span class="p">:</span> <span class="n">coroutine</span> <span class="s1">'test'</span> <span class="n">was</span> <span class="n">never</span> <span class="n">awaited</span>
<span class="n">Coroutine</span> <span class="n">created</span> <span class="n">at</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">)</span>
  <span class="n">File</span> <span class="s2">"../t.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">9</span><span class="p">,</span> <span class="ow">in</span> <span class="o">&lt;</span><span class="n">module</span><span class="o">&gt;</span>
    <span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">(),</span> <span class="n">debug</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
  <span class="o">&lt;</span> <span class="o">..</span> <span class="o">&gt;</span>
  <span class="n">File</span> <span class="s2">"../t.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">7</span><span class="p">,</span> <span class="ow">in</span> <span class="n">main</span>
    <span class="n">test</span><span class="p">()</span>
  <span class="n">test</span><span class="p">()</span>
</pre></div>
<p>The usual fix is to either await the coroutine or call the
<a class="reference internal" href="asyncio-task.html#asyncio.create_task" title="asyncio.create_task"><code class="xref py py-meth docutils literal notranslate"><span class="pre">asyncio.create_task()</span></code></a> function:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
    <span class="k">await</span> <span class="n">test</span><span class="p">()</span>
</pre></div>
<section id="detect-never-retrieved-exceptions">
<h2>Detect never-retrieved exceptions<a class="headerlink" href="#detect-never-retrieved-exceptions" title="Link to this heading">¶</a></h2>
<p>If a <a class="reference internal" href="asyncio-future.html#asyncio.Future.set_exception" title="asyncio.Future.set_exception"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Future.set_exception()</span></code></a> is called but the Future object is
never awaited on, the exception would never be propagated to the
user code.  In this case, asyncio would emit a log message when the
Future object is garbage collected.</p>
<p>Example of an unhandled exception:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">bug</span><span class="p">():</span>
    <span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s2">"not consumed"</span><span class="p">)</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
    <span class="n">asyncio</span><span class="o">.</span><span class="n">create_task</span><span class="p">(</span><span class="n">bug</span><span class="p">())</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>
</pre></div>
<p>Output:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">Task</span> <span class="n">exception</span> <span class="n">was</span> <span class="n">never</span> <span class="n">retrieved</span>
<span class="n">future</span><span class="p">:</span> <span class="o">&lt;</span><span class="n">Task</span> <span class="n">finished</span> <span class="n">coro</span><span class="o">=&lt;</span><span class="n">bug</span><span class="p">()</span> <span class="n">done</span><span class="p">,</span> <span class="n">defined</span> <span class="n">at</span> <span class="n">test</span><span class="o">.</span><span class="n">py</span><span class="p">:</span><span class="mi">3</span><span class="o">&gt;</span>
  <span class="n">exception</span><span class="o">=</span><span class="ne">Exception</span><span class="p">(</span><span class="s1">'not consumed'</span><span class="p">)</span><span class="o">&gt;</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
  <span class="n">File</span> <span class="s2">"test.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">4</span><span class="p">,</span> <span class="ow">in</span> <span class="n">bug</span>
    <span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s2">"not consumed"</span><span class="p">)</span>
<span class="ne">Exception</span><span class="p">:</span> <span class="ow">not</span> <span class="n">consumed</span>
</pre></div>
<p><a class="reference internal" href="#asyncio-debug-mode"><span class="std std-ref">Enable the debug mode</span></a> to get the
traceback where the task was created:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">(),</span> <span class="n">debug</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
</pre></div>
<p>Output in debug mode:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">Task</span> <span class="n">exception</span> <span class="n">was</span> <span class="n">never</span> <span class="n">retrieved</span>
<span class="n">future</span><span class="p">:</span> <span class="o">&lt;</span><span class="n">Task</span> <span class="n">finished</span> <span class="n">coro</span><span class="o">=&lt;</span><span class="n">bug</span><span class="p">()</span> <span class="n">done</span><span class="p">,</span> <span class="n">defined</span> <span class="n">at</span> <span class="n">test</span><span class="o">.</span><span class="n">py</span><span class="p">:</span><span class="mi">3</span><span class="o">&gt;</span>
    <span class="n">exception</span><span class="o">=</span><span class="ne">Exception</span><span class="p">(</span><span class="s1">'not consumed'</span><span class="p">)</span> <span class="n">created</span> <span class="n">at</span> <span class="n">asyncio</span><span class="o">/</span><span class="n">tasks</span><span class="o">.</span><span class="n">py</span><span class="p">:</span><span class="mi">321</span><span class="o">&gt;</span>
<span class="n">source_traceback</span><span class="p">:</span> <span class="n">Object</span> <span class="n">created</span> <span class="n">at</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
  <span class="n">File</span> <span class="s2">"../t.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">9</span><span class="p">,</span> <span class="ow">in</span> <span class="o">&lt;</span><span class="n">module</span><span class="o">&gt;</span>
    <span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">(),</span> <span class="n">debug</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<span class="o">&lt;</span> <span class="o">..</span> <span class="o">&gt;</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
  <span class="n">File</span> <span class="s2">"../t.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">4</span><span class="p">,</span> <span class="ow">in</span> <span class="n">bug</span>
    <span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s2">"not consumed"</span><span class="p">)</span>
<span class="ne">Exception</span><span class="p">:</span> <span class="ow">not</span> <span class="n">consumed</span>
</pre></div>
<section id="asynchronous-generators-best-practices">
<h2>Asynchronous generators best practices<a class="headerlink" href="#asynchronous-generators-best-practices" title="Link to this heading">¶</a></h2>
<p>Writing correct and efficient asyncio code requires awareness of certain pitfalls.
This section outlines essential best practices that can save you hours of debugging.</p>
<section id="close-asynchronous-generators-explicitly">
<h3>Close asynchronous generators explicitly<a class="headerlink" href="#close-asynchronous-generators-explicitly" title="Link to this heading">¶</a></h3>
<p>It is recommended to manually close the
<a class="reference internal" href="../glossary.html#term-asynchronous-generator-iterator"><span class="xref std std-term">asynchronous generator</span></a>. If a generator
exits early - for example, due to an exception raised in the body of
an <code class="docutils literal notranslate"><span class="pre">async</span> <span class="pre">for</span></code> loop - its asynchronous cleanup code may run in an
unexpected context. This can occur after the tasks it depends on have completed,
or during the event loop shutdown when the async-generator's garbage collection
hook is called.</p>
<p>To avoid this, explicitly close the generator by calling its
<a class="reference internal" href="../reference/expressions.html#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate"><span class="pre">aclose()</span></code></a> method, or use the <a class="reference internal" href="contextlib.html#contextlib.aclosing" title="contextlib.aclosing"><code class="xref py py-func docutils literal notranslate"><span class="pre">contextlib.aclosing()</span></code></a>
context manager:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="kn">import</span><span class="w"> </span><span class="nn">contextlib</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">gen</span><span class="p">():</span>
  <span class="k">yield</span> <span class="mi">1</span>
  <span class="k">yield</span> <span class="mi">2</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">func</span><span class="p">():</span>
  <span class="k">async</span> <span class="k">with</span> <span class="n">contextlib</span><span class="o">.</span><span class="n">aclosing</span><span class="p">(</span><span class="n">gen</span><span class="p">())</span> <span class="k">as</span> <span class="n">g</span><span class="p">:</span>
    <span class="k">async</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">g</span><span class="p">:</span>
      <span class="k">break</span>  <span class="c1"># Don't iterate until the end</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">func</span><span class="p">())</span>
</pre></div>
<p>As noted above, the cleanup code for these asynchronous generators is deferred.
The following example demonstrates that the finalization of an asynchronous
generator can occur in an unexpected order:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="n">work_done</span> <span class="o">=</span> <span class="kc">False</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">cursor</span><span class="p">():</span>
    <span class="k">try</span><span class="p">:</span>
        <span class="k">yield</span> <span class="mi">1</span>
    <span class="k">finally</span><span class="p">:</span>
        <span class="k">assert</span> <span class="n">work_done</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">rows</span><span class="p">():</span>
    <span class="k">global</span> <span class="n">work_done</span>
    <span class="k">try</span><span class="p">:</span>
        <span class="k">yield</span> <span class="mi">2</span>
    <span class="k">finally</span><span class="p">:</span>
        <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.1</span><span class="p">)</span> <span class="c1"># immitate some async work</span>
        <span class="n">work_done</span> <span class="o">=</span> <span class="kc">True</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
    <span class="k">async</span> <span class="k">for</span> <span class="n">c</span> <span class="ow">in</span> <span class="n">cursor</span><span class="p">():</span>
        <span class="k">async</span> <span class="k">for</span> <span class="n">r</span> <span class="ow">in</span> <span class="n">rows</span><span class="p">():</span>
            <span class="k">break</span>
        <span class="k">break</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>
</pre></div>
<p>For this example, we get the following output:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">unhandled</span> <span class="n">exception</span> <span class="n">during</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">()</span> <span class="n">shutdown</span>
<span class="n">task</span><span class="p">:</span> <span class="o">&lt;</span><span class="n">Task</span> <span class="n">finished</span> <span class="n">name</span><span class="o">=</span><span class="s1">'Task-3'</span> <span class="n">coro</span><span class="o">=&lt;&lt;</span><span class="n">async_generator_athrow</span> <span class="n">without</span> <span class="vm">__name__</span><span class="o">&gt;</span><span class="p">()</span><span class="o">&gt;</span> <span class="n">exception</span><span class="o">=</span><span class="ne">AssertionError</span><span class="p">()</span><span class="o">&gt;</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
  <span class="n">File</span> <span class="s2">"example.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">6</span><span class="p">,</span> <span class="ow">in</span> <span class="n">cursor</span>
    <span class="k">yield</span> <span class="mi">1</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">exceptions</span><span class="o">.</span><span class="n">CancelledError</span>
<span class="n">During</span> <span class="n">handling</span> <span class="n">of</span> <span class="n">the</span> <span class="n">above</span> <span class="n">exception</span><span class="p">,</span> <span class="n">another</span> <span class="n">exception</span> <span class="n">occurred</span><span class="p">:</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
  <span class="n">File</span> <span class="s2">"example.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">8</span><span class="p">,</span> <span class="ow">in</span> <span class="n">cursor</span>
    <span class="k">assert</span> <span class="n">work_done</span>
           <span class="o">^^^^^^^^^</span>
<span class="ne">AssertionError</span>
</pre></div>
<p>The <code class="docutils literal notranslate"><span class="pre">cursor()</span></code> asynchronous generator was finalized before the <code class="docutils literal notranslate"><span class="pre">rows</span></code>
generator - an unexpected behavior.</p>
<p>The example can be fixed by explicitly closing the
<code class="docutils literal notranslate"><span class="pre">cursor</span></code> and <code class="docutils literal notranslate"><span class="pre">rows</span></code> async-generators:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
    <span class="k">async</span> <span class="k">with</span> <span class="n">contextlib</span><span class="o">.</span><span class="n">aclosing</span><span class="p">(</span><span class="n">cursor</span><span class="p">())</span> <span class="k">as</span> <span class="n">cursor_gen</span><span class="p">:</span>
        <span class="k">async</span> <span class="k">for</span> <span class="n">c</span> <span class="ow">in</span> <span class="n">cursor_gen</span><span class="p">:</span>
            <span class="k">async</span> <span class="k">with</span> <span class="n">contextlib</span><span class="o">.</span><span class="n">aclosing</span><span class="p">(</span><span class="n">rows</span><span class="p">())</span> <span class="k">as</span> <span class="n">rows_gen</span><span class="p">:</span>
                <span class="k">async</span> <span class="k">for</span> <span class="n">r</span> <span class="ow">in</span> <span class="n">rows_gen</span><span class="p">:</span>
                    <span class="k">break</span>
            <span class="k">break</span>
</pre></div>
<section id="create-asynchronous-generators-only-when-the-event-loop-is-running">
<h3>Create asynchronous generators only when the event loop is running<a class="headerlink" href="#create-asynchronous-generators-only-when-the-event-loop-is-running" title="Link to this heading">¶</a></h3>
<p>It is recommended to create
<a class="reference internal" href="../glossary.html#term-asynchronous-generator-iterator"><span class="xref std std-term">asynchronous generators</span></a> only after
the event loop has been created.</p>
<p>To ensure that asynchronous generators close reliably, the event loop uses the
<a class="reference internal" href="sys.html#sys.set_asyncgen_hooks" title="sys.set_asyncgen_hooks"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.set_asyncgen_hooks()</span></code></a> function to register callback functions. These
callbacks update the list of running asynchronous generators to keep it in a
consistent state.</p>
<p>When the <a class="reference internal" href="asyncio-eventloop.html#asyncio.loop.shutdown_asyncgens" title="asyncio.loop.shutdown_asyncgens"><code class="xref py py-meth docutils literal notranslate"><span class="pre">loop.shutdown_asyncgens()</span></code></a>
function is called, the running generators are stopped gracefully and the
list is cleared.</p>
<p>The asynchronous generator invokes the corresponding system hook during its
first iteration. At the same time, the generator records that the hook has
been called and does not call it again.</p>
<p>Therefore, if iteration begins before the event loop is created,
the event loop will not be able to add the generator to its list of active
generators because the hooks are set after the generator attempts to call them.
Consequently, the event loop will not be able to terminate the generator
if necessary.</p>
<p>Consider the following example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">agenfn</span><span class="p">():</span>
    <span class="k">try</span><span class="p">:</span>
        <span class="k">yield</span> <span class="mi">10</span>
    <span class="k">finally</span><span class="p">:</span>
        <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="k">with</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">Runner</span><span class="p">()</span> <span class="k">as</span> <span class="n">runner</span><span class="p">:</span>
    <span class="n">agen</span> <span class="o">=</span> <span class="n">agenfn</span><span class="p">()</span>
    <span class="nb">print</span><span class="p">(</span><span class="n">runner</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">anext</span><span class="p">(</span><span class="n">agen</span><span class="p">)))</span>
    <span class="k">del</span> <span class="n">agen</span>
</pre></div>
<p>Output:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="mi">10</span>
<span class="ne">Exception</span> <span class="n">ignored</span> <span class="k">while</span> <span class="n">closing</span> <span class="n">generator</span> <span class="o">&lt;</span><span class="n">async_generator</span> <span class="nb">object</span> <span class="n">agenfn</span> <span class="n">at</span> <span class="mh">0x000002F71CD10D70</span><span class="o">&gt;</span><span class="p">:</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
  <span class="n">File</span> <span class="s2">"example.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">13</span><span class="p">,</span> <span class="ow">in</span> <span class="o">&lt;</span><span class="n">module</span><span class="o">&gt;</span>
    <span class="k">del</span> <span class="n">agen</span>
        <span class="o">^^^^</span>
<span class="ne">RuntimeError</span><span class="p">:</span> <span class="k">async</span> <span class="n">generator</span> <span class="n">ignored</span> <span class="ne">GeneratorExit</span>
</pre></div>
<p>This example can be fixed as follows:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">agenfn</span><span class="p">():</span>
    <span class="k">try</span><span class="p">:</span>
        <span class="k">yield</span> <span class="mi">10</span>
    <span class="k">finally</span><span class="p">:</span>
        <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>
    <span class="n">agen</span> <span class="o">=</span> <span class="n">agenfn</span><span class="p">()</span>
    <span class="nb">print</span><span class="p">(</span><span class="k">await</span> <span class="n">anext</span><span class="p">(</span><span class="n">agen</span><span class="p">))</span>
    <span class="k">del</span> <span class="n">agen</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>
</pre></div>
<section id="avoid-concurrent-iteration-and-closure-of-the-same-generator">
<h3>Avoid concurrent iteration and closure of the same generator<a class="headerlink" href="#avoid-concurrent-iteration-and-closure-of-the-same-generator" title="Link to this heading">¶</a></h3>
<p>Async generators may be reentered while another
<a class="reference internal" href="../reference/expressions.html#agen.__anext__" title="agen.__anext__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__anext__()</span></code></a> / <a class="reference internal" href="../reference/expressions.html#agen.athrow" title="agen.athrow"><code class="xref py py-meth docutils literal notranslate"><span class="pre">athrow()</span></code></a> / <a class="reference internal" href="../reference/expressions.html#agen.aclose" title="agen.aclose"><code class="xref py py-meth docutils literal notranslate"><span class="pre">aclose()</span></code></a> call is in
progress. This may lead to an inconsistent state of the async generator and can
cause errors.</p>
<p>Let's consider the following example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span><span class="w"> </span><span class="nn">asyncio</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">consumer</span><span class="p">():</span>
    <span class="k">for</span> <span class="n">idx</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">100</span><span class="p">):</span>
        <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
        <span class="n">message</span> <span class="o">=</span> <span class="k">yield</span> <span class="n">idx</span>
        <span class="nb">print</span><span class="p">(</span><span class="s1">'received'</span><span class="p">,</span> <span class="n">message</span><span class="p">)</span>
<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">amain</span><span class="p">():</span>
    <span class="n">agenerator</span> <span class="o">=</span> <span class="n">consumer</span><span class="p">()</span>
    <span class="k">await</span> <span class="n">agenerator</span><span class="o">.</span><span class="n">asend</span><span class="p">(</span><span class="kc">None</span><span class="p">)</span>
    <span class="n">fa</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">create_task</span><span class="p">(</span><span class="n">agenerator</span><span class="o">.</span><span class="n">asend</span><span class="p">(</span><span class="s1">'A'</span><span class="p">))</span>
    <span class="n">fb</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">create_task</span><span class="p">(</span><span class="n">agenerator</span><span class="o">.</span><span class="n">asend</span><span class="p">(</span><span class="s1">'B'</span><span class="p">))</span>
    <span class="k">await</span> <span class="n">fa</span>
    <span class="k">await</span> <span class="n">fb</span>
<span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">amain</span><span class="p">())</span>
</pre></div>
<p>Output:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">received</span> <span class="n">A</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
  <span class="n">File</span> <span class="s2">"test.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">38</span><span class="p">,</span> <span class="ow">in</span> <span class="o">&lt;</span><span class="n">module</span><span class="o">&gt;</span>
    <span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">amain</span><span class="p">())</span>
    <span class="o">~~~~~~~~~~~^^^^^^^^^</span>
  <span class="n">File</span> <span class="s2">"Lib/asyncio/runners.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">204</span><span class="p">,</span> <span class="ow">in</span> <span class="n">run</span>
    <span class="k">return</span> <span class="n">runner</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">)</span>
           <span class="o">~~~~~~~~~~^^^^^^</span>
  <span class="n">File</span> <span class="s2">"Lib/asyncio/runners.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">127</span><span class="p">,</span> <span class="ow">in</span> <span class="n">run</span>
    <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_loop</span><span class="o">.</span><span class="n">run_until_complete</span><span class="p">(</span><span class="n">task</span><span class="p">)</span>
           <span class="o">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^</span>
  <span class="n">File</span> <span class="s2">"Lib/asyncio/base_events.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">719</span><span class="p">,</span> <span class="ow">in</span> <span class="n">run_until_complete</span>
    <span class="k">return</span> <span class="n">future</span><span class="o">.</span><span class="n">result</span><span class="p">()</span>
           <span class="o">~~~~~~~~~~~~~^^</span>
  <span class="n">File</span> <span class="s2">"test.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">36</span><span class="p">,</span> <span class="ow">in</span> <span class="n">amain</span>
    <span class="k">await</span> <span class="n">fb</span>
<span class="ne">RuntimeError</span><span class="p">:</span> <span class="n">anext</span><span class="p">():</span> <span class="n">asynchronous</span> <span class="n">generator</span> <span class="ow">is</span> <span class="n">already</span> <span class="n">running</span>
</pre></div>
<p>Therefore, it is recommended to avoid using asynchronous generators in parallel
tasks or across multiple event loops.</p>
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