structlog structured logging setup for FastAPI

<p>I need structured JSON logging in my FastAPI application. I want request IDs correlated across log lines, log levels configurable via env var, and logs formatted as JSON in production but human-readable in development.</p>

<p>Configure structlog for structured logging:</p> <div class="highlight"><pre><span></span><code><span class="c1"># app/logging.py</span> <span class="kn">import</span><span class="w"> </span><span class="nn">logging</span> <span class="kn">import</span><span class="w"> </span><span class="nn">sys</span> <span class="kn">import</span><span class="w"> </span><span class="nn">structlog</span> <span class="k">def</span><span class="w"> </span><span class="nf">configure_logging</span><span class="p">(</span><span class="n">log_level</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s1">'INFO'</span><span class="p">,</span> <span class="n">json_logs</span><span class="p">:</span> <span class="nb">bool</span> <span class="o">=</span> <span class="kc">False</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="kc">None</span><span class="p">:</span> <span class="n">shared_processors</span> <span class="o">=</span> <span class="p">[</span> <span class="n">structlog</span><span class="o">.</span><span class="n">contextvars</span><span class="o">.</span><span class="n">merge_contextvars</span><span class="p">,</span> <span class="n">structlog</span><span class="o">.</span><span class="n">processors</span><span class="o">.</span><span class="n">add_log_level</span><span class="p">,</span> <span class="n">structlog</span><span class="o">.</span><span class="n">processors</span><span class="o">.</span><span class="n">TimeStamper</span><span class="p">(</span><span class="n">fmt</span><span class="o">=</span><span class="s1">'iso'</span><span class="p">),</span> <span class="p">]</span> <span class="k">if</span> <span class="n">json_logs</span><span class="p">:</span> <span class="n">renderer</span> <span class="o">=</span> <span class="n">structlog</span><span class="o">.</span><span class="n">processors</span><span class="o">.</span><span class="n">JSONRenderer</span><span class="p">()</span> <span class="k">else</span><span class="p">:</span> <span class="n">renderer</span> <span class="o">=</span> <span class="n">structlog</span><span class="o">.</span><span class="n">dev</span><span class="o">.</span><span class="n">ConsoleRenderer</span><span class="p">()</span> <span class="n">structlog</span><span class="o">.</span><span class="n">configure</span><span class="p">(</span> <span class="n">processors</span><span class="o">=</span><span class="n">shared_processors</span> <span class="o">+</span> <span class="p">[</span><span class="n">renderer</span><span class="p">],</span> <span class="n">logger_factory</span><span class="o">=</span><span class="n">structlog</span><span class="o">.</span><span class="n">PrintLoggerFactory</span><span class="p">(),</span> <span class="n">cache_logger_on_first_use</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="p">)</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">log_level</span><span class="p">,</span> <span class="n">stream</span><span class="o">=</span><span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="p">,</span> <span class="nb">format</span><span class="o">=</span><span class="s1">'</span><span class="si">%(message)s</span><span class="s1">'</span><span class="p">)</span> <span class="c1"># Middleware to inject request_id into every log in a request:</span> <span class="kn">from</span><span class="w"> </span><span class="nn">starlette.middleware.base</span><span class="w"> </span><span class="kn">import</span> <span class="n">BaseHTTPMiddleware</span> <span class="kn">import</span><span class="w"> </span><span class="nn">uuid</span> <span class="k">class</span><span class="w"> </span><span class="nc">RequestIDMiddleware</span><span class="p">(</span><span class="n">BaseHTTPMiddleware</span><span class="p">):</span> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">dispatch</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">request</span><span class="p">,</span> <span class="n">call_next</span><span class="p">):</span> <span class="n">request_id</span> <span class="o">=</span> <span class="nb">str</span><span class="p">(</span><span class="n">uuid</span><span class="o">.</span><span class="n">uuid4</span><span class="p">())</span> <span class="n">structlog</span><span class="o">.</span><span class="n">contextvars</span><span class="o">.</span><span class="n">bind_contextvars</span><span class="p">(</span><span class="n">request_id</span><span class="o">=</span><span class="n">request_id</span><span class="p">)</span> <span class="n">response</span> <span class="o">=</span> <span class="k">await</span> <span class="n">call_next</span><span class="p">(</span><span class="n">request</span><span class="p">)</span> <span class="n">structlog</span><span class="o">.</span><span class="n">contextvars</span><span class="o">.</span><span class="n">unbind_contextvars</span><span class="p">(</span><span class="s1">'request_id'</span><span class="p">)</span> <span class="n">response</span><span class="o">.</span><span class="n">headers</span><span class="p">[</span><span class="s1">'X-Request-ID'</span><span class="p">]</span> <span class="o">=</span> <span class="n">request_id</span> <span class="k">return</span> <span class="n">response</span> <span class="c1"># Usage:</span> <span class="n">log</span> <span class="o">=</span> <span class="n">structlog</span><span class="o">.</span><span class="n">get_logger</span><span class="p">(</span><span class="vm">__name__</span><span class="p">)</span> <span class="k">await</span> <span class="n">log</span><span class="o">.</span><span class="n">ainfo</span><span class="p">(</span><span class="s1">'trace_created'</span><span class="p">,</span> <span class="n">trace_id</span><span class="o">=</span><span class="nb">str</span><span class="p">(</span><span class="n">trace</span><span class="o">.</span><span class="n">id</span><span class="p">),</span> <span class="n">user_id</span><span class="o">=</span><span class="nb">str</span><span class="p">(</span><span class="n">user_id</span><span class="p">))</span> </code></pre></div> <p>Key points: - <code>merge_contextvars</code> injects request-scoped data (request_id) into every log line - <code>cache_logger_on_first_use=True</code> for performance in production - Set <code>json_logs=True</code> in production for log aggregation tools (Datadog, CloudWatch) - Use <code>structlog.get_logger()</code> not <code>logging.getLogger()</code> for structured output</p>