Python testing strategies using pytest, TDD methodology, fixtures, mocking, parametrization, and coverage requirements. Use when writing pytest tests — fixtures, mocks, parametrization, or coverage.

Python Testing Patterns Comprehensive testing strategies for Python applications using pytest, TDD methodology, and best practices. When to Activate Writing new Python code (follow TDD: red, green, refactor) Designing test suites for Python projects Reviewing Python test coverage Setting up testing infrastructure Core Testing Philosophy Test Driven Development (TDD) Always follow the TDD cycle: 1. RED : Write a failing test for the desired behavior 2. GREEN : Write minimal code to make the test pass 3. REFACTOR : Improve code while keeping tests green Coverage Requirements Target : 80%+ code coverage Critical paths : 100% coverage required Use pytest cov to measure coverage pytest Fundamentals Basic Test Structure Assertions Fixtures Basic Fixture Usage Fixture with Setup/Teardown Fixture Scopes Fixture with Parameters Using Multiple Fixtures Autouse Fixtures Conftest.py for Shared Fixtures Parametrization Basic Parametrization Multiple Parameters Parametrize with IDs Parametrized Fixtures Markers and Test Selection Custom Markers Run Specific Tests Configure Markers in pytest.ini Mocking and Patching Mocking Functions Mocking Return Values Mocking Exceptions Mocking Context Managers Using Autospec Mock Class Instances Mock Property Testing Async Code Async Tests with pytest asyncio Async Fixture Mocking Async Functions Testing Exceptions Testing Expected Exceptions Testing Exception Attributes Testing Side Effects Testing File Operations Testing with pytest's tmp path Fixture Testing with tmpdir Fixture Test Organization Directory Structure Test Classes Best Practices DO Follow TDD : Write tests before code (red green refactor) Test one thing : Each test should verify a single behavior Use descriptive names : test user login with invalid credentials fails Use fixtures : Eliminate duplication with fixtures Mock external dependencies : Don't depend on external services Test edge cases : Empty inputs, None values, boundary conditions Aim for 80%+ coverage : Focus on critical paths Keep tests fast : Use marks to separate slow tests DON'T Don't test implementation : Test behavior, not internals Don't use complex conditionals in tests : Keep tests simple Don't ignore test failures : All tests must pass Don't test third party code : Trust libraries to work Don't share state between tests : Tests should be independent Don't catch exceptions in tests : Use pytest.raises Don't use print statements : Use assertions and pytest output Don't write tests that are too brittle : Avoid over specific mocks Common Patterns Testing API Endpoints (FastAPI/Flask) Testing Database Operations Testing Class Methods pytest Configuration pytest.ini pyproject.toml Running Tests Quick Reference Pattern Usage pytest.raises() Test expected exceptions @pytest.fixture() Create reusable test fixtures @pytest.mark.parametrize() Run tests with multiple inputs @pytest.mark.slow Mark slow tests pytest m "not slow" Skip slow tests @patch() Mock functions and classes tmp path fixture Automatic temp directory pytest cov Generate coverage report assert Simple and readable assertions Remember : Tests are code too. Keep them clean, readable, and maintainable. Good tests catch bugs; great tests prevent them.

python-testing