test-driven-development

Write one minimal failing test for the desired behavior, run it with pytest to confirm it fails for the expected reason, then write the simplest code to make it pass. Finally refactor while keeping all tests green, and repeat the cycle for each new behavior.

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

Tests written after code pass immediately, which proves nothing about whether they test the right thing. Watching a test fail first confirms it actually exercises the missing behavior and catches real bugs.

Exceptions include throwaway prototypes, generated code, and configuration files, but only with explicit user permission. Exploration is allowed, but the exploratory code must be discarded and reimplemented with TDD.

A test that passes immediately is testing existing behavior and proves nothing. Fix the test so it targets the missing feature, verify it fails for the expected reason, then implement the code to make it pass.