test-driven-development

Write one minimal failing test for a single behavior, run it with pytest to confirm it fails for the expected reason, then write the simplest code to pass. Re-run the specific test plus the full suite to check for regressions before refactoring.

RED means writing a failing test that defines desired behavior, GREEN means writing minimal code to make it pass, and REFACTOR means cleaning up duplication and names while keeping tests green. Each cycle covers one behavior at a time.

No. Writing all tests first is horizontal slicing and produces brittle tests designed before the implementation teaches you the real interface. Use vertical tracer bullets: alternate one failing test with one minimal implementation.

Only for throwaway prototypes, generated code, or configuration files, and only after asking the user first. Exploration is fine, but the exploratory code must be discarded and the real implementation restarted test-first.

An immediately passing test means you are testing existing behavior, not new functionality, so it proves nothing. Fix the test to target the missing feature and confirm it fails before writing any implementation code.

Write a failing test that reproduces the bug first, then implement the minimal fix and verify the test passes. The test proves the fix works and prevents the bug from regressing later.