FastAPI lifespan event for startup and shutdown tasks

<p>I need to initialize resources (database connection pool, Redis client, HTTP client) when my FastAPI app starts and clean them up when it shuts down. The old @app.on_event('startup') pattern is deprecated in newer FastAPI versions.</p>

<p>Use the <code>lifespan</code> context manager pattern (FastAPI 0.93+):</p> <div class="highlight"><pre><span></span><code><span class="kn">from</span><span class="w"> </span><span class="nn">contextlib</span><span class="w"> </span><span class="kn">import</span> <span class="n">asynccontextmanager</span> <span class="kn">from</span><span class="w"> </span><span class="nn">fastapi</span><span class="w"> </span><span class="kn">import</span> <span class="n">FastAPI</span> <span class="kn">import</span><span class="w"> </span><span class="nn">httpx</span> <span class="nd">@asynccontextmanager</span> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">lifespan</span><span class="p">(</span><span class="n">app</span><span class="p">:</span> <span class="n">FastAPI</span><span class="p">):</span> <span class="c1"># Startup: initialize resources</span> <span class="n">app</span><span class="o">.</span><span class="n">state</span><span class="o">.</span><span class="n">http_client</span> <span class="o">=</span> <span class="n">httpx</span><span class="o">.</span><span class="n">AsyncClient</span><span class="p">(</span><span class="n">timeout</span><span class="o">=</span><span class="mf">10.0</span><span class="p">)</span> <span class="n">app</span><span class="o">.</span><span class="n">state</span><span class="o">.</span><span class="n">redis</span> <span class="o">=</span> <span class="k">await</span> <span class="n">create_redis_connection</span><span class="p">()</span> <span class="k">yield</span> <span class="c1"># Shutdown: close resources</span> <span class="k">await</span> <span class="n">app</span><span class="o">.</span><span class="n">state</span><span class="o">.</span><span class="n">http_client</span><span class="o">.</span><span class="n">aclose</span><span class="p">()</span> <span class="k">await</span> <span class="n">app</span><span class="o">.</span><span class="n">state</span><span class="o">.</span><span class="n">redis</span><span class="o">.</span><span class="n">aclose</span><span class="p">()</span> <span class="n">app</span> <span class="o">=</span> <span class="n">FastAPI</span><span class="p">(</span><span class="n">lifespan</span><span class="o">=</span><span class="n">lifespan</span><span class="p">)</span> </code></pre></div> <p>Access in dependencies:</p> <div class="highlight"><pre><span></span><code><span class="kn">from</span><span class="w"> </span><span class="nn">fastapi</span><span class="w"> </span><span class="kn">import</span> <span class="n">Request</span> <span class="k">def</span><span class="w"> </span><span class="nf">get_http_client</span><span class="p">(</span><span class="n">request</span><span class="p">:</span> <span class="n">Request</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">httpx</span><span class="o">.</span><span class="n">AsyncClient</span><span class="p">:</span> <span class="k">return</span> <span class="n">request</span><span class="o">.</span><span class="n">app</span><span class="o">.</span><span class="n">state</span><span class="o">.</span><span class="n">http_client</span> </code></pre></div> <p>Key points: - Everything before <code>yield</code> runs at startup, after <code>yield</code> at shutdown - Resources on <code>app.state</code> are accessible from any route via <code>request.app.state</code> - Lifespan replaces both <code>@app.on_event('startup')</code> and <code>@app.on_event('shutdown')</code> - FastAPI wraps the lifespan in an asynccontextmanager automatically</p>