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<li><a class="reference internal" href="#debug-mode">Debug Mode</a></li>
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<li><a class="reference internal" href="#avoid-concurrent-iteration-and-closure-of-the-same-generator">Avoid concurrent iteration and closure of the same generator</a></li>
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<section id="developing-with-asyncio">
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<span id="asyncio-dev"></span><h1>Developing with asyncio<a class="headerlink" href="#developing-with-asyncio" title="Link to this heading">¶</a></h1>
|
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<p>Asynchronous programming is different from classic “sequential”
|
||
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>
|
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<p>By default asyncio runs in production mode. In order to ease
|
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the development asyncio has a <em>debug mode</em>.</p>
|
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<p>There are several ways to enable asyncio debug mode:</p>
|
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<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>
|
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<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>
|
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<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>
|
||
</ul>
|
||
<p>In addition to enabling the debug mode, consider also:</p>
|
||
<ul>
|
||
<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>
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</pre></div>
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</div>
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</li>
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<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="../builtins/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>
|
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</ul>
|
||
<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>
|
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<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 “slow”.</p></li>
|
||
</ul>
|
||
</section>
|
||
<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>
|
||
</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>
|
||
</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>
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||
<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>
|
||
</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>
|
||
<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>
|
||
<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">"asyncio"</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>
|
||
</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>
|
||
<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="../builtins/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>
|
||
</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>
|
||
</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"><</span><span class="n">module</span><span class="o">></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"><</span> <span class="o">..</span> <span class="o">></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>
|
||
</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>
|
||
</div>
|
||
</section>
|
||
<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>
|
||
</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"><</span><span class="n">Task</span> <span class="n">finished</span> <span class="n">coro</span><span class="o">=<</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">></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">></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>
|
||
</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>
|
||
</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"><</span><span class="n">Task</span> <span class="n">finished</span> <span class="n">coro</span><span class="o">=<</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">></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">></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"><</span><span class="n">module</span><span class="o">></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"><</span> <span class="o">..</span> <span class="o">></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>
|
||
</div>
|
||
</section>
|
||
<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>
|
||
</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"># imitate 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>
|
||
</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"><</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">=<<</span><span class="n">async_generator_athrow</span> <span class="n">without</span> <span class="vm">__name__</span><span class="o">></span><span class="p">()</span><span class="o">></span> <span class="n">exception</span><span class="o">=</span><span class="ne">AssertionError</span><span class="p">()</span><span class="o">></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>
|
||
</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>
|
||
</div>
|
||
</section>
|
||
<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>
|
||
</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"><</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">></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"><</span><span class="n">module</span><span class="o">></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>
|
||
</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>
|
||
</div>
|
||
</section>
|
||
<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>
|
||
</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"><</span><span class="n">module</span><span class="o">></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>
|
||
</div>
|
||
<p>Therefore, it is recommended to avoid using asynchronous generators in parallel
|
||
tasks or across multiple event loops.</p>
|
||
</section>
|
||
</section>
|
||
</section>
|
||
|
||
|
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<div>
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<h3><a href="../contents.html">Table of Contents</a></h3>
|
||
<ul>
|
||
<li><a class="reference internal" href="#">Developing with asyncio</a><ul>
|
||
<li><a class="reference internal" href="#debug-mode">Debug Mode</a></li>
|
||
<li><a class="reference internal" href="#concurrency-and-multithreading">Concurrency and Multithreading</a></li>
|
||
<li><a class="reference internal" href="#running-blocking-code">Running Blocking Code</a></li>
|
||
<li><a class="reference internal" href="#logging">Logging</a></li>
|
||
<li><a class="reference internal" href="#detect-never-awaited-coroutines">Detect never-awaited coroutines</a></li>
|
||
<li><a class="reference internal" href="#detect-never-retrieved-exceptions">Detect never-retrieved exceptions</a></li>
|
||
<li><a class="reference internal" href="#asynchronous-generators-best-practices">Asynchronous generators best practices</a><ul>
|
||
<li><a class="reference internal" href="#close-asynchronous-generators-explicitly">Close asynchronous generators explicitly</a></li>
|
||
<li><a class="reference internal" href="#create-asynchronous-generators-only-when-the-event-loop-is-running">Create asynchronous generators only when the event loop is running</a></li>
|
||
<li><a class="reference internal" href="#avoid-concurrent-iteration-and-closure-of-the-same-generator">Avoid concurrent iteration and closure of the same generator</a></li>
|
||
</ul>
|
||
</li>
|
||
</ul>
|
||
</li>
|
||
</ul>
|
||
|
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|
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|
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|
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title="previous chapter">Low-level API Index</a></p>
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<h4>Next topic</h4>
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