Giova445 Test-Driven Development
The law
NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST
Wrote the code first? Delete it. Not "keep it as reference", not "adapt it while writing the test" — delete, then implement fresh from the test. Code you are looking at while writing a test contaminates the test into describing what you built rather than what you need.
RED must be proven, not asserted
The value of test-first collapses if the test would have passed anyway. So:
- Write the test.
- Run it at the pre-implementation state.
- Confirm it fails for an assertion reason. An
ImportErroror collection error proves the file does not parse — it proves nothing about the test. - Confirm it fails for the right reason. A test failing on a typo you then "fix" taught you nothing.
Only then write implementation.
Why the third step matters more than it looks
A hook can prove a test failed. It cannot prove it failed for the intended reason. That gap is real and unclosable by tooling — it is why mutation testing and adversarial review exist downstream. Do not treat a red test as self-evidently a good test.
Rationalizations, answered
| Excuse | Reality |
|---|---|
| "I'll test after" | Tests written after pass immediately. You never watched it fail, so you never learned whether it can. |
| "It's too simple to test" | Then the test is one line. Write it. |
| "I already know it works" | Confidence is not evidence. |
| "The test is hard to write" | That is the design telling you something. Listen to it. |
| "I'll delete the code and rewrite from the test" (while looking at it) | Delete means delete. |
Test quality, not test count
A test that passes whether or not the code is correct is worse than no test: it is a false signal that costs runtime forever. Before accepting your own test, name one mutation to the production code that would survive it. If you can, the test is too weak.
Mechanically backed
./bin/verify runs the suite; the Stop hook refuses the turn while it is red. The cheat
scanner diffs for skipped tests, deleted assertions, || true, and retry-to-green — the
documented ways a failing test gets made to disappear instead of fixed.