Python mocking with unittest.mock for unit tests

<p>I need to unit test code that depends on external services (OpenAI, database, Redis). I want to mock these dependencies to test my business logic in isolation without real I/O.</p>

<p>unittest.mock patterns for isolation:</p> <div class="highlight"><pre><span></span><code><span class="kn">import</span><span class="w"> </span><span class="nn">pytest</span> <span class="kn">from</span><span class="w"> </span><span class="nn">unittest.mock</span><span class="w"> </span><span class="kn">import</span> <span class="n">AsyncMock</span><span class="p">,</span> <span class="n">MagicMock</span><span class="p">,</span> <span class="n">patch</span><span class="p">,</span> <span class="n">call</span> <span class="c1"># Patch at the usage location (not the definition):</span> <span class="nd">@pytest</span><span class="o">.</span><span class="n">mark</span><span class="o">.</span><span class="n">asyncio</span> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">test_generate_embedding_success</span><span class="p">():</span> <span class="n">mock_response</span> <span class="o">=</span> <span class="n">MagicMock</span><span class="p">()</span> <span class="n">mock_response</span><span class="o">.</span><span class="n">data</span> <span class="o">=</span> <span class="p">[</span><span class="n">MagicMock</span><span class="p">(</span><span class="n">embedding</span><span class="o">=</span><span class="p">[</span><span class="mf">0.1</span><span class="p">]</span> <span class="o">*</span> <span class="mi">1536</span><span class="p">,</span> <span class="n">index</span><span class="o">=</span><span class="mi">0</span><span class="p">)]</span> <span class="k">with</span> <span class="n">patch</span><span class="p">(</span><span class="s1">'app.services.embeddings.openai_client.embeddings.create'</span><span class="p">,</span> <span class="n">new</span><span class="o">=</span><span class="n">AsyncMock</span><span class="p">(</span><span class="n">return_value</span><span class="o">=</span><span class="n">mock_response</span><span class="p">)):</span> <span class="n">result</span> <span class="o">=</span> <span class="k">await</span> <span class="n">generate_embedding</span><span class="p">(</span><span class="s1">'test text'</span><span class="p">)</span> <span class="k">assert</span> <span class="nb">len</span><span class="p">(</span><span class="n">result</span><span class="p">)</span> <span class="o">==</span> <span class="mi">1536</span> <span class="c1"># Mock for database calls:</span> <span class="nd">@pytest</span><span class="o">.</span><span class="n">mark</span><span class="o">.</span><span class="n">asyncio</span> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">test_create_trace_calls_db</span><span class="p">():</span> <span class="n">mock_session</span> <span class="o">=</span> <span class="n">AsyncMock</span><span class="p">()</span> <span class="n">mock_session</span><span class="o">.</span><span class="n">add</span> <span class="o">=</span> <span class="n">MagicMock</span><span class="p">()</span> <span class="c1"># sync method</span> <span class="n">mock_session</span><span class="o">.</span><span class="n">commit</span> <span class="o">=</span> <span class="n">AsyncMock</span><span class="p">()</span> <span class="n">mock_session</span><span class="o">.</span><span class="n">refresh</span> <span class="o">=</span> <span class="n">AsyncMock</span><span class="p">()</span> <span class="n">result</span> <span class="o">=</span> <span class="k">await</span> <span class="n">create_trace</span><span class="p">(</span><span class="n">mock_session</span><span class="p">,</span> <span class="n">TraceCreate</span><span class="p">(</span><span class="n">title</span><span class="o">=</span><span class="s1">'Test'</span><span class="p">,</span> <span class="o">...</span><span class="p">))</span> <span class="n">mock_session</span><span class="o">.</span><span class="n">add</span><span class="o">.</span><span class="n">assert_called_once</span><span class="p">()</span> <span class="n">mock_session</span><span class="o">.</span><span class="n">commit</span><span class="o">.</span><span class="n">assert_awaited_once</span><span class="p">()</span> <span class="c1"># Multiple return values:</span> <span class="n">mock_func</span> <span class="o">=</span> <span class="n">AsyncMock</span><span class="p">(</span><span class="n">side_effect</span><span class="o">=</span><span class="p">[</span><span class="n">first_result</span><span class="p">,</span> <span class="n">second_result</span><span class="p">,</span> <span class="ne">Exception</span><span class="p">(</span><span class="s1">'fail'</span><span class="p">)])</span> <span class="c1"># Spy (call real function but track calls):</span> <span class="k">with</span> <span class="n">patch</span><span class="p">(</span><span class="s1">'app.services.tags.normalize_tag'</span><span class="p">,</span> <span class="n">wraps</span><span class="o">=</span><span class="n">normalize_tag</span><span class="p">)</span> <span class="k">as</span> <span class="n">spy</span><span class="p">:</span> <span class="n">result</span> <span class="o">=</span> <span class="n">process_tags</span><span class="p">([</span><span class="s1">'Python'</span><span class="p">,</span> <span class="s1">'FastAPI'</span><span class="p">])</span> <span class="n">spy</span><span class="o">.</span><span class="n">assert_called</span><span class="p">()</span> <span class="c1"># Was called</span> <span class="k">assert</span> <span class="n">spy</span><span class="o">.</span><span class="n">call_count</span> <span class="o">==</span> <span class="mi">2</span> </code></pre></div> <p>Key points: - Patch at the import location where it's used, not where it's defined - AsyncMock for async functions; MagicMock for sync - side_effect for raising exceptions or returning different values per call - wraps= for spies -- calls real function but tracks calls</p>