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class="bodywrapper">221 <div class="body" role="main">222 223 <section id="developing-with-asyncio">224<span id="asyncio-dev"></span><h1>Developing with asyncio<a class="headerlink" href="#developing-with-asyncio" title="Link to this heading">¶</a></h1>225<p>Asynchronous programming is different from classic “sequential”226programming.</p>227<p>This page lists common mistakes and traps and explains how228to avoid them.</p>229<section id="debug-mode">230<span id="asyncio-debug-mode"></span><h2>Debug Mode<a class="headerlink" href="#debug-mode" title="Link to this heading">¶</a></h2>231<p>By default asyncio runs in production mode. In order to ease232the development asyncio has a <em>debug mode</em>.</p>233<p>There are several ways to enable asyncio debug mode:</p>234<ul class="simple">235<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>236<li><p>Using the <a class="reference internal" href="devmode.html#devmode"><span class="std std-ref">Python Development Mode</span></a>.</p></li>237<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>238<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>239</ul>240<p>In addition to enabling the debug mode, consider also:</p>241<ul>242<li><p>setting the log level of the <a class="reference internal" href="#asyncio-logger"><span class="std std-ref">asyncio logger</span></a> to243<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 code244can be run at startup of the application:</p>245<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>246</pre></div>247</div>248</li>249<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 display250<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 by251using 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>252</ul>253<p>When the debug mode is enabled:</p>254<ul class="simple">255<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> and256<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 called257from a wrong thread.</p></li>258<li><p>The execution time of the I/O selector is logged if it takes too long to259perform an I/O operation.</p></li>260<li><p>Callbacks taking longer than 100 milliseconds are logged. The261<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 the262minimum execution duration in seconds that is considered “slow”.</p></li>263</ul>264</section>265<section id="concurrency-and-multithreading">266<span id="asyncio-multithreading"></span><h2>Concurrency and Multithreading<a class="headerlink" href="#concurrency-and-multithreading" title="Link to this heading">¶</a></h2>267<p>An event loop runs in a thread (typically the main thread) and executes268all callbacks and Tasks in its thread. While a Task is running in the269event loop, no other Tasks can run in the same thread. When a Task270executes an <code class="docutils literal notranslate"><span class="pre">await</span></code> expression, the running Task gets suspended, and271the event loop executes the next Task.</p>272<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, the273<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>274<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>275</pre></div>276</div>277<p>Almost all asyncio objects are not thread safe, which is typically278not a problem unless there is code that works with them from outside279of a Task or a callback. If there’s a need for such code to call a280low-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> method281should be used, e.g.:</p>282<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>283</pre></div>284</div>285<p>To schedule a coroutine object from a different OS thread, the286<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 a287<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>288<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>289 <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>290 291<span class="c1"># Later in another OS thread:</span>292 293<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>294<span class="c1"># Wait for the result:</span>295<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>296</pre></div>297</div>298<p>To handle signals the event loop must be299run in the main thread.</p>300<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 a301<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> or302<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 execute303blocking code in a different OS thread without blocking the OS thread304that the event loop runs in.</p>305<p>There is currently no way to schedule coroutines or callbacks directly306from a different process (such as one started with307<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>308section lists APIs that can read from pipes and watch file descriptors309without blocking the event loop. In addition, asyncio’s310<a class="reference internal" href="asyncio-subprocess.html#asyncio-subprocess"><span class="std std-ref">Subprocess</span></a> APIs provide a way to start a311process and communicate with it from the event loop. Lastly, the312aforementioned <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 used313with 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 execute314code in a different process.</p>315</section>316<section id="running-blocking-code">317<span id="asyncio-handle-blocking"></span><h2>Running Blocking Code<a class="headerlink" href="#running-blocking-code" title="Link to this heading">¶</a></h2>318<p>Blocking (CPU-bound) code should not be called directly. For example,319if a function performs a CPU-intensive calculation for 1 second,320all concurrent asyncio Tasks and IO operations would be delayed321by 1 second.</p>322<p>An executor can be used to run a task in a different thread,323including in a different interpreter, or even in324a different process to avoid blocking the OS thread with the325event 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 more326details.</p>327</section>328<section id="logging">329<span id="asyncio-logger"></span><h2>Logging<a class="headerlink" href="#logging" title="Link to this heading">¶</a></h2>330<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 performed331via the <code class="docutils literal notranslate"><span class="pre">"asyncio"</span></code> logger.</p>332<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 easily333adjusted:</p>334<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>335</pre></div>336</div>337<p>Network logging can block the event loop. It is recommended to use338a separate thread for handling logs or use non-blocking IO. For example,339see <a class="reference internal" href="../howto/logging-cookbook.html#blocking-handlers"><span class="std std-ref">Dealing with handlers that block</span></a>.</p>340</section>341<section id="detect-never-awaited-coroutines">342<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>343<p>When a coroutine function is called, but not awaited344(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>)345or 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>, asyncio346will 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>347<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>348 349<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">test</span><span class="p">():</span>350 <span class="nb">print</span><span class="p">(</span><span class="s2">"never scheduled"</span><span class="p">)</span>351 352<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>353 <span class="n">test</span><span class="p">()</span>354 355<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>356</pre></div>357</div>358<p>Output:</p>359<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>360 <span class="n">test</span><span class="p">()</span>361</pre></div>362</div>363<p>Output in debug mode:</p>364<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>365<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>366 <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>367 <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>368 369 <span class="o"><</span> <span class="o">..</span> <span class="o">></span>370 371 <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>372 <span class="n">test</span><span class="p">()</span>373 <span class="n">test</span><span class="p">()</span>374</pre></div>375</div>376<p>The usual fix is to either await the coroutine or call the377<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>378<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>379 <span class="k">await</span> <span class="n">test</span><span class="p">()</span>380</pre></div>381</div>382</section>383<section id="detect-never-retrieved-exceptions">384<h2>Detect never-retrieved exceptions<a class="headerlink" href="#detect-never-retrieved-exceptions" title="Link to this heading">¶</a></h2>385<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 is386never awaited on, the exception would never be propagated to the387user code. In this case, asyncio would emit a log message when the388Future object is garbage collected.</p>389<p>Example of an unhandled exception:</p>390<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>391 392<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">bug</span><span class="p">():</span>393 <span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s2">"not consumed"</span><span class="p">)</span>394 395<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">main</span><span class="p">():</span>396 <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>397 398<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>399</pre></div>400</div>401<p>Output:</p>402<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>403<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>404 <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>405 406<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>407 <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>408 <span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s2">"not consumed"</span><span class="p">)</span>409<span class="ne">Exception</span><span class="p">:</span> <span class="ow">not</span> <span class="n">consumed</span>410</pre></div>411</div>412<p><a class="reference internal" href="#asyncio-debug-mode"><span class="std std-ref">Enable the debug mode</span></a> to get the413traceback where the task was created:</p>414<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>415</pre></div>416</div>417<p>Output in debug mode:</p>418<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>419<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>420 <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>421 422<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>423 <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>424 <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>425 426<span class="o"><</span> <span class="o">..</span> <span class="o">></span>427 428<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>429 <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>430 <span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s2">"not consumed"</span><span class="p">)</span>431<span class="ne">Exception</span><span class="p">:</span> <span class="ow">not</span> <span class="n">consumed</span>432</pre></div>433</div>434</section>435</section>436 437 438 <div class="clearer"></div>439 </div>440 </div>441 </div>442 <div class="sphinxsidebar" role="navigation" aria-label="Main">443 <div class="sphinxsidebarwrapper">444 <div>445 <h3><a href="../contents.html">Table of Contents</a></h3>446 <ul>447<li><a class="reference internal" href="#">Developing with asyncio</a><ul>448<li><a class="reference internal" href="#debug-mode">Debug Mode</a></li>449<li><a class="reference internal" href="#concurrency-and-multithreading">Concurrency and Multithreading</a></li>450<li><a class="reference internal" href="#running-blocking-code">Running Blocking Code</a></li>451<li><a class="reference internal" href="#logging">Logging</a></li>452<li><a class="reference 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