backend-python-testing

pytest enables test execution for FastAPI endpoints through fixtures that set up test clients and dependency injection. Write test functions using the AAA pattern—arrange test data, act by calling endpoints, assert expected responses—then run pytest from the command line to execute all tests and report results.

pytest fixtures provide setup and teardown mechanisms that create isolated test environments. Define fixtures for database connections, mock services, and client initialization, then inject them into test functions. This ensures each test runs independently without state leakage from previous tests.

Mocking replaces real external service calls with controlled test doubles during testing. pytest integrates with mocking libraries to stub API responses, database queries, and third-party integrations. This isolates the code under test and prevents test failures from external service outages.

pytest supports async testing through plugins and async fixtures that handle coroutine execution. Test async FastAPI endpoints and database operations by writing async test functions and fixtures, enabling comprehensive coverage of async code paths alongside synchronous logic.

Test-driven development writes tests before implementation code. Define test cases that specify expected endpoint behavior, then implement FastAPI routes incrementally to pass those tests. This approach ensures testable design, clear requirements, and confidence that features work as specified.

Meaningful test coverage focuses on critical paths and business logic rather than line counts. pytest integrates with coverage tools to measure execution. Aim to cover unit tests for components, integration tests for endpoint interactions, and performance tests for CI/CD pipelines to validate both correctness and reliability.