test-driven-development

Test-driven development prevents regressions by requiring a failing test that defines expected behavior before writing any production code. You implement the smallest change to pass the test, ensuring repeatable verification and regression safety across the codebase.

The red-green-refactor cycle is a test-driven development discipline where you write a minimal failing test (RED), implement the smallest code change to pass it (GREEN), then clean up without changing behavior (REFACTOR). You repeat this for each new feature.

Yes, you can use pytest for test-driven development on an existing codebase. It supports applying the red-green-refactor discipline to new features, bug fixes, and refactors, providing repeatable verification and regression safety for your current code.

You should write behavior-first tests when you need to focus on real outcomes and edge cases rather than implementation details or mock interactions. This approach ensures tests remain valid during refactoring and verify actual functional requirements.

To ensure a failing test fails for the right reason, run it immediately after writing it to confirm the failure is due to missing production behavior, not a syntax or setup error. This verification is a core step before implementing the minimal code to pass.