mutation-testinglisted

# Mutation Testing Mutation testing verifies whether your tests *detect* bugs, not just whether they *execute* code. A "mutant" is a small deliberate change (flip `>` to `>=`, delete a `return`) applied to the source. If your tests pass with the mutant, they would not have caught that bug. Deeper reference: `patterns/mutation-testing/MUTATION_TESTING.md` (full operator list, equivalent mutants, cost management strategies). --- ## When to use it - After reaching line/branch coverage targets but before shipping critical business logic (payment, auth, data validation). - When a production bug slips through a test suite that claims high coverage. - As a periodic quality audit (weekly/monthly) rather than a per-commit gate. **When to skip:** Prototype and Internal tiers; generated code; pure data classes with no conditional logic; large legacy codebases where the cost of triaging hundreds of mutants outweighs the benefit. --- ## Mutation score $$\text{mutation score} = \frac{\text{killed}}{\text{total} - \text{equivalent}} \times 100\%$$ | Score | Verdict | |---|---| | ≥ 80% | Strong; surviving mutants warrant individual inspection | | 60–79% | Adequate; identify surviving categories and add targeted tests | | Below 60% | Test suite quality is insufficient; line/branch percentages alone are misleading | --- ## Python — mutmut ```bash pip install mutmut mutmut run --paths-to-mutate src/billing.py mutmut results # list surviving mutants mutmut show 15