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. Finally run the full test suite to check for regressions before refactoring.

Red means writing a failing test that defines the desired behavior. Green means writing the minimal code to make it pass, even hardcoding values. Refactor means cleaning up duplication and names while keeping all tests green.

A test that passes immediately proves nothing, since it may test existing behavior or the wrong thing. Watching it fail confirms the test actually exercises the missing feature and would catch the bug.

Exceptions are limited to throwaway prototypes, generated code, and configuration files, and only with explicit user permission. Exploration is allowed, but the exploratory code must be discarded and rewritten test-first.

Delete the code and start over with a failing test. Keeping it as reference or adapting it while writing tests is effectively testing after implementation, which biases tests toward what was built rather than what is required.