principle-test-behavior-not-implementation

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Test behaviour, not implementation

A test checks what the code's users observe: return values, errors, readable state, and outside effects such as a sent request or a written row. Internal helpers and call order are not behaviour.

Two imagined edits:

• Break the behaviour: return a wrong value, skip an effect or do it twice. If the test still passes, it proves nothing.
• Change the internals and keep every observable result. If the test fails, it is pinned to the implementation.

Keep a test that catches the first edit and survives the second; rewrite or delete others.

Usual causes: an assertion looser than the contract, an expected value from the code under test, a restated constant or prompt string, a fixture asserting itself, an exact message, not the error type that callers branch on.

The fix. Drive the code through its users' interface with a concrete input. Derive the expected value from the requirement; assert exactly what the contract promises. Pair an absence (no call, an empty list) with a case that shows the presence. A payload sent to an external service is behaviour; an internal helper call is not.

Keep invariant tests: a key in two tables, or a compile-time type check.