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 <section id="developing-with-asyncio">
<h1>Developing with asyncio<a class="headerlink" href="#developing-with-asyncio" title="Permalink to this headline">¶</a></h1>
Asynchronous programming is different from classic “sequential”
programming.
This page lists common mistakes and traps and explains how
to avoid them.
<section id="debug-mode">
<h2>Debug Mode<a class="headerlink" href="#debug-mode" title="Permalink to this headline">¶</a></h2>
By default asyncio runs in production mode. In order to ease
the development asyncio has a debug mode.
There are several ways to enable asyncio debug mode:
<ul class="simple">
<li>Setting the <a class="reference internal" href="../using/cmdline.html#envvar-PYTHONASYNCIODEBUG"><code class="xref std std-envvar docutils literal notranslate">PYTHONASYNCIODEBUG</code></a> environment variable to <code class="docutils literal notranslate">1</code>.</li>
<li>Using the <a class="reference internal" href="devmode.html#devmode">Python Development Mode</a>.</li>
<li>Passing <code class="docutils literal notranslate">debug=True</code> to <a class="reference internal" href="asyncio-task.html#asyncio.run" title="asyncio.run"><code class="xref py py-func docutils literal notranslate">asyncio.run()</code></a>.</li>
<li>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">loop.set_debug()</code></a>.</li>
</ul>
In addition to enabling the debug mode, consider also:
<ul>
<li>setting the log level of the <a class="reference internal" href="#asyncio-logger">asyncio logger</a> to
<code class="xref py py-data docutils literal notranslate">logging.DEBUG</code>, for example the following snippet of code
can be run at startup of the application:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>logging.basicConfig(level=logging.DEBUG)
</pre></div>
</div>
</li>
<li>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">warnings</code></a> module to display
<a class="reference internal" href="exceptions.html#ResourceWarning" title="ResourceWarning"><code class="xref py py-exc docutils literal notranslate">ResourceWarning</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">-W</code></a> <code class="docutils literal notranslate">default</code> command line option.</li>
</ul>
When the debug mode is enabled:
<ul class="simple">
<li>asyncio checks for <a class="reference internal" href="#asyncio-coroutine-not-scheduled">coroutines that were not awaited</a> and logs them; this mitigates
the “forgotten await” pitfall.</li>
<li>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">loop.call_soon()</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">loop.call_at()</code></a> methods) raise an exception if they are called
from a wrong thread.</li>
<li>The execution time of the I/O selector is logged if it takes too long to
perform an I/O operation.</li>
<li>Callbacks taking longer than 100ms are logged. The
<code class="xref py py-attr docutils literal notranslate">loop.slow_callback_duration</code> attribute can be used to set the
minimum execution duration in seconds that is considered “slow”.</li>
</ul>
</section>
<section id="concurrency-and-multithreading">
<h2>Concurrency and Multithreading<a class="headerlink" href="#concurrency-and-multithreading" title="Permalink to this headline">¶</a></h2>
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">await</code> expression, the running Task gets suspended, and
the event loop executes the next Task.
To schedule a <a class="reference internal" href="../glossary.html#term-callback">callback</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">loop.call_soon_threadsafe()</code></a> method should be used. Example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>loop.call_soon_threadsafe(callback, *args)
</pre></div>
</div>
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">loop.call_soon_threadsafe()</code></a> method
should be used, e.g.:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>loop.call_soon_threadsafe(fut.cancel)
</pre></div>
</div>
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">run_coroutine_threadsafe()</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">concurrent.futures.Future</code></a> to access the result:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>async def coro_func():
 return await asyncio.sleep(1, 42)

# Later in another OS thread:

future = asyncio.run_coroutine_threadsafe(coro_func(), loop)
# Wait for the result:
result = future.result()
</pre></div>
</div>
To handle signals and to execute subprocesses, the event loop must be
run in the main thread.
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">loop.run_in_executor()</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">concurrent.futures.ThreadPoolExecutor</code></a> to execute
blocking code in a different OS thread without blocking the OS thread
that the event loop runs in.
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">multiprocessing</code></a>). The <a class="reference internal" href="asyncio-eventloop.html#asyncio-event-loop-methods">Event Loop Methods</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">Subprocess</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">loop.run_in_executor()</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">concurrent.futures.ProcessPoolExecutor</code></a> to execute
code in a different process.
</section>
<section id="running-blocking-code">
<h2>Running Blocking Code<a class="headerlink" href="#running-blocking-code" title="Permalink to this headline">¶</a></h2>
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.
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">loop.run_in_executor()</code></a> method for more
details.
</section>
<section id="logging">
<h2>Logging<a class="headerlink" href="#logging" title="Permalink to this headline">¶</a></h2>
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">logging</code></a> module and all logging is performed
via the <code class="docutils literal notranslate">&quot;asyncio&quot;</code> logger.
The default log level is <code class="xref py py-data docutils literal notranslate">logging.INFO</code>, which can be easily
adjusted:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>logging.getLogger(&quot;asyncio&quot;).setLevel(logging.WARNING)
</pre></div>
</div>
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">Dealing with handlers that block</a>.
</section>
<section id="detect-never-awaited-coroutines">
<h2>Detect never-awaited coroutines<a class="headerlink" href="#detect-never-awaited-coroutines" title="Permalink to this headline">¶</a></h2>
When a coroutine function is called, but not awaited
(e.g. <code class="docutils literal notranslate">coro()</code> instead of <code class="docutils literal notranslate">await coro()</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">asyncio.create_task()</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">RuntimeWarning</code></a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>import asyncio

async def test():
 print(&quot;never scheduled&quot;)

async def main():
 test()

asyncio.run(main())
</pre></div>
</div>
Output:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>test.py:7: RuntimeWarning: coroutine &#39;test&#39; was never awaited
 test()
</pre></div>
</div>
Output in debug mode:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>test.py:7: RuntimeWarning: coroutine &#39;test&#39; was never awaited
Coroutine created at (most recent call last)
 File &quot;../t.py&quot;, line 9, in &lt;module&gt;
 asyncio.run(main(), debug=True)

&lt; .. &gt;

File &quot;../t.py&quot;, line 7, in main
 test()
 test()
</pre></div>
</div>
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">asyncio.create_task()</code></a> function:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>async def main():
 await test()
</pre></div>
</div>
</section>
<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>
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">Future.set_exception()</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.
Example of an unhandled exception:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>import asyncio

async def bug():
 raise Exception(&quot;not consumed&quot;)

async def main():
 asyncio.create_task(bug())

Traceback (most recent call last):
 File &quot;test.py&quot;, line 4, in bug
 raise Exception(&quot;not consumed&quot;)
Exception: not consumed
</pre></div>
</div>
<a class="reference internal" href="#asyncio-debug-mode">Enable the debug mode</a> to get the
traceback where the task was created:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>asyncio.run(main(), debug=True)
</pre></div>
</div>
Output in debug mode:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>Task exception was never retrieved
future: &lt;Task finished coro=&lt;bug() done, defined at test.py:3&gt;
 exception=Exception(&#39;not consumed&#39;) created at asyncio/tasks.py:321&gt;

source_traceback: Object created at (most recent call last):
 File &quot;../t.py&quot;, line 9, in &lt;module&gt;
 asyncio.run(main(), debug=True)

&lt; .. &gt;

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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>
</ul>
</li>
</ul>

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