Python Testing Best Practices: Fixtures, Parametrize, and Avoiding Bad Patterns

Writing many tests in Python is easy, but writing effective ones requires deliberate use of pytest's core tools like fixtures, parametrize, and monkeypatch. The test pyramid guides effort distribution across unit, integration, and end-to-end tests, each serving a distinct purpose in catching different categories of bugs. Unit tests should validate observable behavior rather than internal implementation details, ensuring refactors do not cause false failures. The parametrize decorator eliminates copy-pasting by running the same assertion across multiple inputs, especially at boundary conditions like empty strings or invalid numbers. Integration tests using real database containers catch semantic errors at system boundaries that pure unit tests cannot detect.