Pydantic v2 Settings with environment variable validation

<p>I need to manage configuration for a FastAPI app across local dev, CI, and production environments. I use environment variables and .env files. I want type-safe settings with validation, defaults, and support for secrets stored in env vars.</p>

<p>Use <code>pydantic-settings</code> for type-safe configuration:</p> <div class="highlight"><pre><span></span><code><span class="kn">from</span><span class="w"> </span><span class="nn">pydantic</span><span class="w"> </span><span class="kn">import</span> <span class="n">AnyHttpUrl</span><span class="p">,</span> <span class="n">field_validator</span><span class="p">,</span> <span class="n">model_validator</span> <span class="kn">from</span><span class="w"> </span><span class="nn">pydantic_settings</span><span class="w"> </span><span class="kn">import</span> <span class="n">BaseSettings</span><span class="p">,</span> <span class="n">SettingsConfigDict</span> <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">Self</span> <span class="k">class</span><span class="w"> </span><span class="nc">Settings</span><span class="p">(</span><span class="n">BaseSettings</span><span class="p">):</span> <span class="n">model_config</span> <span class="o">=</span> <span class="n">SettingsConfigDict</span><span class="p">(</span> <span class="n">env_file</span><span class="o">=</span><span class="s1">'.env'</span><span class="p">,</span> <span class="n">env_file_encoding</span><span class="o">=</span><span class="s1">'utf-8'</span><span class="p">,</span> <span class="n">case_sensitive</span><span class="o">=</span><span class="kc">False</span><span class="p">,</span> <span class="p">)</span> <span class="n">database_url</span><span class="p">:</span> <span class="nb">str</span> <span class="n">redis_url</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s1">'redis://localhost:6379'</span> <span class="n">secret_key</span><span class="p">:</span> <span class="nb">str</span> <span class="n">debug</span><span class="p">:</span> <span class="nb">bool</span> <span class="o">=</span> <span class="kc">False</span> <span class="n">allowed_hosts</span><span class="p">:</span> <span class="nb">list</span><span class="p">[</span><span class="nb">str</span><span class="p">]</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'localhost'</span><span class="p">]</span> <span class="n">openai_api_key</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s1">''</span> <span class="nd">@field_validator</span><span class="p">(</span><span class="s1">'database_url'</span><span class="p">)</span> <span class="nd">@classmethod</span> <span class="k">def</span><span class="w"> </span><span class="nf">validate_db_url</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">v</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">str</span><span class="p">:</span> <span class="k">if</span> <span class="ow">not</span> <span class="n">v</span><span class="o">.</span><span class="n">startswith</span><span class="p">(</span><span class="s1">'postgresql+asyncpg://'</span><span class="p">):</span> <span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s1">'database_url must use asyncpg driver'</span><span class="p">)</span> <span class="k">return</span> <span class="n">v</span> <span class="nd">@model_validator</span><span class="p">(</span><span class="n">mode</span><span class="o">=</span><span class="s1">'after'</span><span class="p">)</span> <span class="k">def</span><span class="w"> </span><span class="nf">warn_missing_keys</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">Self</span><span class="p">:</span> <span class="k">if</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">openai_api_key</span><span class="p">:</span> <span class="kn">import</span><span class="w"> </span><span class="nn">warnings</span> <span class="n">warnings</span><span class="o">.</span><span class="n">warn</span><span class="p">(</span><span class="s1">'OPENAI_API_KEY not set — embeddings disabled'</span><span class="p">)</span> <span class="k">return</span> <span class="bp">self</span> <span class="n">settings</span> <span class="o">=</span> <span class="n">Settings</span><span class="p">()</span> <span class="c1"># Singleton — import from here</span> </code></pre></div> <p>Key points: - <code>BaseSettings</code> reads from env vars first, then <code>.env</code> file, then defaults - Validators run AFTER env var parsing — use <code>mode='before'</code> to transform raw strings - Sensitive values use <code>SecretStr</code> type: <code>secret_key: SecretStr</code> (prevents logging) - Use <code>settings = Settings()</code> as module-level singleton to fail fast at import</p>