Pydantic settings with multiple env sources and precedence

<p>Application needs configuration from multiple sources: environment variables override .env file, which overrides defaults. Also need nested settings objects (database config, Redis config) and type coercion from string env vars to typed Python objects.</p>

<p>Use pydantic-settings with customized model_config and nested models:</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">Field</span><span class="p">,</span> <span class="n">SecretStr</span><span class="p">,</span> <span class="n">AnyUrl</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="k">class</span><span class="w"> </span><span class="nc">DatabaseSettings</span><span class="p">(</span><span class="n">BaseSettings</span><span class="p">):</span> <span class="n">host</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s1">'localhost'</span> <span class="n">port</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">5432</span> <span class="n">name</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s1">'myapp'</span> <span class="n">user</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s1">'postgres'</span> <span class="n">password</span><span class="p">:</span> <span class="n">SecretStr</span> <span class="o">=</span> <span class="n">SecretStr</span><span class="p">(</span><span class="s1">''</span><span class="p">)</span> <span class="n">pool_size</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">10</span> <span class="n">max_overflow</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">20</span> <span class="nd">@property</span> <span class="k">def</span><span class="w"> </span><span class="nf">url</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">str</span><span class="p">:</span> <span class="n">pwd</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">password</span><span class="o">.</span><span class="n">get_secret_value</span><span class="p">()</span> <span class="k">return</span> <span class="sa">f</span><span class="s1">'postgresql+asyncpg://</span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">user</span><span class="si">}</span><span class="s1">:</span><span class="si">{</span><span class="n">pwd</span><span class="si">}</span><span class="s1">@</span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">host</span><span class="si">}</span><span class="s1">:</span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">port</span><span class="si">}</span><span class="s1">/</span><span class="si">{</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="si">}</span><span class="s1">'</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">env_nested_delimiter</span><span class="o">=</span><span class="s1">'__'</span><span class="p">,</span> <span class="c1"># DB__HOST=localhost -&gt; database.host</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">app_name</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s1">'MyApp'</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">secret_key</span><span class="p">:</span> <span class="n">SecretStr</span> <span class="o">=</span> <span class="n">Field</span><span class="p">(</span><span class="n">default</span><span class="o">=...</span><span class="p">,</span> <span class="n">description</span><span class="o">=</span><span class="s1">'JWT secret key'</span><span class="p">)</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="c1"># Nested settings</span> <span class="n">database</span><span class="p">:</span> <span class="n">DatabaseSettings</span> <span class="o">=</span> <span class="n">DatabaseSettings</span><span class="p">()</span> <span class="c1"># Validator for derived values</span> <span class="nd">@property</span> <span class="k">def</span><span class="w"> </span><span class="nf">is_production</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">bool</span><span class="p">:</span> <span class="k">return</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">debug</span> <span class="c1"># .env file</span> <span class="c1"># SECRET_KEY=my-secret-key-here</span> <span class="c1"># DB__HOST=prod-postgres.example.com</span> <span class="c1"># DB__PASSWORD=super-secret-password</span> <span class="c1"># ALLOWED_HOSTS=["api.example.com","www.example.com"]</span> <span class="n">settings</span> <span class="o">=</span> <span class="n">Settings</span><span class="p">()</span> <span class="nb">print</span><span class="p">(</span><span class="n">settings</span><span class="o">.</span><span class="n">database</span><span class="o">.</span><span class="n">url</span><span class="p">)</span> <span class="nb">print</span><span class="p">(</span><span class="n">settings</span><span class="o">.</span><span class="n">secret_key</span><span class="o">.</span><span class="n">get_secret_value</span><span class="p">())</span> <span class="c1"># Access secret</span> </code></pre></div> <p><code>env_nested_delimiter='__'</code> allows <code>DB__HOST</code> to set <code>database.host</code>. <code>SecretStr</code> prevents accidental logging — <code>.get_secret_value()</code> is the only way to access it. <code>list[str]</code> env vars accept JSON-encoded arrays: <code>["a","b"]</code>.</p>