FastAPI dependency injection with typed dependencies

<p>I want to use FastAPI's dependency injection system cleanly across route handlers. I have database sessions, current user extraction from JWT, and Redis clients that need to be injected. I want to use type aliases to keep route signatures readable.</p>

<p>Use <code>Annotated</code> type aliases for clean dependency injection:</p> <div class="highlight"><pre><span></span><code><span class="kn">from</span><span class="w"> </span><span class="nn">typing</span><span class="w"> </span><span class="kn">import</span> <span class="n">Annotated</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">Depends</span><span class="p">,</span> <span class="n">HTTPException</span> <span class="kn">from</span><span class="w"> </span><span class="nn">sqlalchemy.ext.asyncio</span><span class="w"> </span><span class="kn">import</span> <span class="n">AsyncSession</span> <span class="kn">from</span><span class="w"> </span><span class="nn">app.database</span><span class="w"> </span><span class="kn">import</span> <span class="n">get_db</span> <span class="kn">from</span><span class="w"> </span><span class="nn">app.auth</span><span class="w"> </span><span class="kn">import</span> <span class="n">get_current_user</span> <span class="c1"># Type aliases — these are the core pattern</span> <span class="n">DbSession</span> <span class="o">=</span> <span class="n">Annotated</span><span class="p">[</span><span class="n">AsyncSession</span><span class="p">,</span> <span class="n">Depends</span><span class="p">(</span><span class="n">get_db</span><span class="p">)]</span> <span class="n">CurrentUser</span> <span class="o">=</span> <span class="n">Annotated</span><span class="p">[</span><span class="n">User</span><span class="p">,</span> <span class="n">Depends</span><span class="p">(</span><span class="n">get_current_user</span><span class="p">)]</span> <span class="c1"># Sub-dependencies compose naturally</span> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">get_admin_user</span><span class="p">(</span><span class="n">user</span><span class="p">:</span> <span class="n">CurrentUser</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">User</span><span class="p">:</span> <span class="k">if</span> <span class="ow">not</span> <span class="n">user</span><span class="o">.</span><span class="n">is_admin</span><span class="p">:</span> <span class="k">raise</span> <span class="n">HTTPException</span><span class="p">(</span><span class="mi">403</span><span class="p">,</span> <span class="s1">'Admin required'</span><span class="p">)</span> <span class="k">return</span> <span class="n">user</span> <span class="n">AdminUser</span> <span class="o">=</span> <span class="n">Annotated</span><span class="p">[</span><span class="n">User</span><span class="p">,</span> <span class="n">Depends</span><span class="p">(</span><span class="n">get_admin_user</span><span class="p">)]</span> <span class="c1"># Route handlers stay clean:</span> <span class="nd">@router</span><span class="o">.</span><span class="n">post</span><span class="p">(</span><span class="s1">'/traces'</span><span class="p">)</span> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">create_trace</span><span class="p">(</span><span class="n">body</span><span class="p">:</span> <span class="n">TraceCreate</span><span class="p">,</span> <span class="n">db</span><span class="p">:</span> <span class="n">DbSession</span><span class="p">,</span> <span class="n">user</span><span class="p">:</span> <span class="n">CurrentUser</span><span class="p">):</span> <span class="n">trace</span> <span class="o">=</span> <span class="n">Trace</span><span class="p">(</span><span class="n">contributor_id</span><span class="o">=</span><span class="n">user</span><span class="o">.</span><span class="n">id</span><span class="p">,</span> <span class="o">...</span><span class="p">)</span> <span class="n">db</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">trace</span><span class="p">)</span> <span class="k">await</span> <span class="n">db</span><span class="o">.</span><span class="n">commit</span><span class="p">()</span> <span class="k">return</span> <span class="n">trace</span> </code></pre></div> <p>Key points: - <code>Annotated[T, Depends(fn)]</code> is the v2 pattern — cleaner than <code>= Depends(fn)</code> defaults - Type aliases are importable, so define them in <code>app/dependencies.py</code> - FastAPI caches dependencies per request by default (same instance within one request)</p>