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<li class="nav-item nav-item-this"><a href="">Using <code class="docutils literal notranslate"><span class="pre">async</span></code> and <code class="docutils literal notranslate"><span class="pre">await</span></code></a></li>
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<section id="using-async-and-await">
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<span id="async-await"></span><h1>Using <code class="docutils literal notranslate"><span class="pre">async</span></code> and <code class="docutils literal notranslate"><span class="pre">await</span></code><a class="headerlink" href="#using-async-and-await" title="Link to this heading">¶</a></h1>
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<details class="changelog">
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<summary>Changelog</summary><div class="versionadded">
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<p><span class="versionmodified added">Added in version 2.0.</span></p>
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</div>
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</details><p>Routes, error handlers, before request, after request, and teardown
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functions can all be coroutine functions if Flask is installed with the
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<code class="docutils literal notranslate"><span class="pre">async</span></code> extra (<code class="docutils literal notranslate"><span class="pre">pip</span> <span class="pre">install</span> <span class="pre">flask[async]</span></code>). This allows views to be
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defined with <code class="docutils literal notranslate"><span class="pre">async</span> <span class="pre">def</span></code> and use <code class="docutils literal notranslate"><span class="pre">await</span></code>.</p>
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<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="nd">@app</span><span class="o">.</span><span class="n">route</span><span class="p">(</span><span class="s2">"/get-data"</span><span class="p">)</span>
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<span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">get_data</span><span class="p">():</span>
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<span class="n">data</span> <span class="o">=</span> <span class="k">await</span> <span class="n">async_db_query</span><span class="p">(</span><span class="o">...</span><span class="p">)</span>
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<span class="k">return</span> <span class="n">jsonify</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
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</pre></div>
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</div>
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<p>Pluggable class-based views also support handlers that are implemented as
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coroutines. This applies to the <a class="reference internal" href="api.html#flask.views.View.dispatch_request" title="flask.views.View.dispatch_request"><code class="xref py py-meth docutils literal notranslate"><span class="pre">dispatch_request()</span></code></a>
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method in views that inherit from the <a class="reference internal" href="api.html#flask.views.View" title="flask.views.View"><code class="xref py py-class docutils literal notranslate"><span class="pre">flask.views.View</span></code></a> class, as
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well as all the HTTP method handlers in views that inherit from the
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<a class="reference internal" href="api.html#flask.views.MethodView" title="flask.views.MethodView"><code class="xref py py-class docutils literal notranslate"><span class="pre">flask.views.MethodView</span></code></a> class.</p>
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<div class="admonition-using-async-with-greenlet admonition">
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<p class="admonition-title">Using <code class="docutils literal notranslate"><span class="pre">async</span></code> with greenlet</p>
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<p>When using gevent or eventlet to serve an application or patch the
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runtime, greenlet>=1.0 is required. When using PyPy, PyPy>=7.3.7 is
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required.</p>
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</div>
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<section id="performance">
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<h2>Performance<a class="headerlink" href="#performance" title="Link to this heading">¶</a></h2>
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<p>Async functions require an event loop to run. Flask, as a WSGI
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application, uses one worker to handle one request/response cycle.
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When a request comes in to an async view, Flask will start an event loop
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in a thread, run the view function there, then return the result.</p>
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<p>Each request still ties up one worker, even for async views. The upside
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is that you can run async code within a view, for example to make
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multiple concurrent database queries, HTTP requests to an external API,
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etc. However, the number of requests your application can handle at one
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time will remain the same.</p>
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<p><strong>Async is not inherently faster than sync code.</strong> Async is beneficial
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when performing concurrent IO-bound tasks, but will probably not improve
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CPU-bound tasks. Traditional Flask views will still be appropriate for
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most use cases, but Flask’s async support enables writing and using
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code that wasn’t possible natively before.</p>
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</section>
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<section id="background-tasks">
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<h2>Background tasks<a class="headerlink" href="#background-tasks" title="Link to this heading">¶</a></h2>
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<p>Async functions will run in an event loop until they complete, at
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which stage the event loop will stop. This means any additional
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spawned tasks that haven’t completed when the async function completes
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will be cancelled. Therefore you cannot spawn background tasks, for
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example via <code class="docutils literal notranslate"><span class="pre">asyncio.create_task</span></code>.</p>
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<p>If you wish to use background tasks it is best to use a task queue to
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trigger background work, rather than spawn tasks in a view
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function. With that in mind you can spawn asyncio tasks by serving
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Flask with an ASGI server and utilising the asgiref WsgiToAsgi adapter
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as described in <a class="reference internal" href="deploying/asgi.html"><span class="doc">ASGI</span></a>. This works as the adapter creates
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an event loop that runs continually.</p>
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</section>
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<section id="when-to-use-quart-instead">
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<h2>When to use Quart instead<a class="headerlink" href="#when-to-use-quart-instead" title="Link to this heading">¶</a></h2>
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<p>Flask’s async support is less performant than async-first frameworks due
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to the way it is implemented. If you have a mainly async codebase it
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would make sense to consider <a class="reference external" href="https://github.com/pallets/quart">Quart</a>. Quart is a reimplementation of
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Flask based on the <a class="reference external" href="https://asgi.readthedocs.io/en/latest/">ASGI</a> standard instead of WSGI. This allows it to
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handle many concurrent requests, long running requests, and websockets
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without requiring multiple worker processes or threads.</p>
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<p>It has also already been possible to run Flask with Gevent or Eventlet
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to get many of the benefits of async request handling. These libraries
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patch low-level Python functions to accomplish this, whereas <code class="docutils literal notranslate"><span class="pre">async</span></code>/
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<code class="docutils literal notranslate"><span class="pre">await</span></code> and ASGI use standard, modern Python capabilities. Deciding
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whether you should use Flask, Quart, or something else is ultimately up
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to understanding the specific needs of your project.</p>
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</section>
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<section id="extensions">
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<h2>Extensions<a class="headerlink" href="#extensions" title="Link to this heading">¶</a></h2>
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<p>Flask extensions predating Flask’s async support do not expect async views.
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If they provide decorators to add functionality to views, those will probably
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not work with async views because they will not await the function or be
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awaitable. Other functions they provide will not be awaitable either and
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will probably be blocking if called within an async view.</p>
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<p>Extension authors can support async functions by utilising the
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<a class="reference internal" href="api.html#flask.Flask.ensure_sync" title="flask.Flask.ensure_sync"><code class="xref py py-meth docutils literal notranslate"><span class="pre">flask.Flask.ensure_sync()</span></code></a> method. For example, if the extension
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provides a view function decorator add <code class="docutils literal notranslate"><span class="pre">ensure_sync</span></code> before calling
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the decorated function,</p>
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<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span><span class="w"> </span><span class="nf">extension</span><span class="p">(</span><span class="n">func</span><span class="p">):</span>
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<span class="nd">@wraps</span><span class="p">(</span><span class="n">func</span><span class="p">)</span>
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<span class="k">def</span><span class="w"> </span><span class="nf">wrapper</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
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<span class="o">...</span> <span class="c1"># Extension logic</span>
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<span class="k">return</span> <span class="n">current_app</span><span class="o">.</span><span class="n">ensure_sync</span><span class="p">(</span><span class="n">func</span><span class="p">)(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
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<span class="k">return</span> <span class="n">wrapper</span>
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</pre></div>
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</div>
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<p>Check the changelog of the extension you want to use to see if they’ve
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implemented async support, or make a feature request or PR to them.</p>
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</section>
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<section id="other-event-loops">
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<h2>Other event loops<a class="headerlink" href="#other-event-loops" title="Link to this heading">¶</a></h2>
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<p>At the moment Flask only supports <a class="reference external" href="https://docs.python.org/3/library/asyncio.html#module-asyncio" title="(in Python v3.13)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">asyncio</span></code></a>. It’s possible to
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override <a class="reference internal" href="api.html#flask.Flask.ensure_sync" title="flask.Flask.ensure_sync"><code class="xref py py-meth docutils literal notranslate"><span class="pre">flask.Flask.ensure_sync()</span></code></a> to change how async functions
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are wrapped to use a different library.</p>
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</section>
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<h3>Contents</h3>
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<ul>
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<li><a class="reference internal" href="#">Using <code class="docutils literal notranslate"><span class="pre">async</span></code> and <code class="docutils literal notranslate"><span class="pre">await</span></code></a><ul>
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<li><a class="reference internal" href="#performance">Performance</a></li>
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<li><a class="reference internal" href="#background-tasks">Background tasks</a></li>
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<li><a class="reference internal" href="#when-to-use-quart-instead">When to use Quart instead</a></li>
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<li><a class="reference internal" href="#extensions">Extensions</a></li>
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<li><a class="reference internal" href="#other-event-loops">Other event loops</a></li>
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