Mutation Testing

A second-order test-quality signal. Line coverage tells you a line executed; mutation testing tells you whether any assertion would fail if that line were wrong. Use it to audit the strength of an existing suite — most valuable on logic-dense code, applied on-demand, never as a default blocking CI gate.

What it does

A mutation tool makes small automatic edits ("mutants") to your source, e.g.:

• < -> <= (boundary / off-by-one)
• and -> or (predicate logic)
• == -> != (negated condition)
• return x -> return None, + -> -, deleting a statement
• removing a list/dict entry (allowlist/denylist coverage)

For each mutant it re-runs the test suite: