tdd-workflow

Test-Driven Development

Write a failing test → make it pass simply → refactor. The tests are a design tool that produce a regression net as a by-product.

The loop (2–10 minutes per cycle)

1. Test list: brainstorm behaviors/edge cases; pick the simplest next one.
2. RED: write one small failing test; run it; see it fail for the right reason (message says what you expect).
3. GREEN: write the least code to pass — fake it, hard-code, triangulate with a second example.
4. REFACTOR: remove duplication, improve names, keep all green. No new behavior here.
5. Commit; pick the next item. Update the list with anything learned.

RED   test_fizzbuzz_returns_1_for_1       → NameError
GREEN def fizzbuzz(n): return "1"
RED   test_returns_2_for_2                → "1" != "2"
GREEN def fizzbuzz(n): return str(n)      # triangulated

Two schools

	Chicago / classicist	London / mockist
Direction	Inside-out, state-based	Outside-in, interaction-based
Doubles	Real collaborators where cheap	Mock collaborators to discover roles
Good for	Algorithms, domain logic	Layered apps, discovering interfaces
Risk	Slow to find seams	Tests coupled to implementation
Blend: outside-in acceptance test (bdd-gherkin-testing) driving inside-out unit TDD.

Heuristics

Baby steps · one assertion idea per test · test names as sentences · don't test private methods · transformation priority premise (constant→variable→conditional→loop) · keep the bar green when refactoring · delete tests that duplicate coverage.

TDD with an AI coding agent

1. You write (or approve) the failing test first; agent implements to green. 2. Ask agent to run the suite and show red before green. 3. Forbid editing tests to pass (diff review). 4. Have it propose the next 5 test-list items and edge cases. 5. Verify with mutation testing that the tests bite.

Legacy code

Find a seam (constructor/param injection, subclass-and-override, wrapper), pin behavior with characterization tests (snapshot-golden-testing), then refactor and TDD new behavior (Feathers' Working Effectively with Legacy Code).

Smells

Test after code "TDD" · huge red phase · brittle mocks · tests that never fail · skipping refactor.

Related

unit-testing, test-design-techniques, mutation-testing, property-based-testing