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<div class="section" id="developing-with-asyncio">
<span id="asyncio-dev"></span><h1>Developing with asyncio<a class="headerlink" href="#developing-with-asyncio" title="Permalink to this headline">¶</a></h1>
<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>
<div class="section" id="debug-mode">
<span id="asyncio-debug-mode"></span><h2>Debug Mode<a class="headerlink" href="#debug-mode" title="Permalink to this headline">¶</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="../using/cmdline.html#id5"><code class="xref std std-option docutils literal notranslate"><span class="pre">-X</span></code></a> <code class="docutils literal notranslate"><span class="pre">dev</span></code> Python command line option.</p></li>
<li><p>Passing <code class="docutils literal notranslate"><span class="pre">debug=True</span></code> to <a class="reference internal" href="asyncio-task.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>
</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
<code class="xref py py-data docutils literal notranslate"><span class="pre">logging.DEBUG</span></code>, 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>
</div>
</li>
<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>
</ul>
<p>When the debug mode is enabled:</p>
<ul class="simple">
<li><p>asyncio checks for <a class="reference internal" href="#asyncio-coroutine-not-scheduled"><span class="std std-ref">coroutines that were not awaited</span></a> and logs them; this mitigates
the “forgotten await” pitfall.</p></li>
<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 100ms are logged. The
<code class="xref py py-attr docutils literal notranslate"><span class="pre">loop.slow_callback_duration</span></code> attribute can be used to set the
minimum execution duration in seconds that is considered “slow”.</p></li>
</ul>
</div>
<div class="section" id="concurrency-and-multithreading">
<span id="asyncio-multithreading"></span><h2>Concurrency and Multithreading<a class="headerlink" href="#concurrency-and-multithreading" title="Permalink to this headline">¶</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 callback from a different 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="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>
</div>
<p>To handle signals and to execute subprocesses, 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> to execute
blocking code in a different OS thread without blocking the OS thread
that the event loop runs in.</p>
</div>
<div class="section" id="running-blocking-code">
<span id="asyncio-handle-blocking"></span><h2>Running Blocking Code<a class="headerlink" href="#running-blocking-code" title="Permalink to this headline">¶</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 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>
</div>
<div class="section" id="logging">
<span id="asyncio-logger"></span><h2>Logging<a class="headerlink" href="#logging" title="Permalink to this headline">¶</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 <code class="xref py py-data docutils literal notranslate"><span class="pre">logging.INFO</span></code>, 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>
</div>
<div class="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="Permalink to this headline">¶</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="nn">asyncio</span>
<span class="k">async</span> <span class="k">def</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="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="nf">main</span><span class="p">():</span>
<span class="k">await</span> <span class="n">test</span><span class="p">()</span>
</pre></div>
</div>
</div>
<div class="section" id="detect-never-retrieved-exceptions">
<h2>Detect never-retrieved exceptions<a class="headerlink" href="#detect-never-retrieved-exceptions" title="Permalink to this headline">¶</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="nn">asyncio</span>
<span class="k">async</span> <span class="k">def</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="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>
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