Test Fixture

A test fixture is the prepared input a test needs: a sample record, a rendered document, a seeded database, a drawn image. Fixtures are where a surprising number of tests go quietly wrong, because a fixture chosen for convenience can make an assertion meaningless.

The failure has a general shape: the fixture already satisfies the property being asserted, for reasons unrelated to the code. A layout test using an image wider than its container passes whether or not the layout code works, because a shrink-to-fit box and a spanning box produce the same width in that case. Drawing a deliberately narrow fixture makes the two outcomes different, and the assertion real. A scroll test in a container with no height constraint passes vacuously. See Vacuous Truth.

Two further notes. Fixtures should be local and deterministic where the assertion is about behaviour — a remote image introduces network variance and decode noise into what should be a pure comparison. And a helper that merges overrides into a base fixture must merge before pruning explicitly-absent keys, or every attempt to blank a field silently keeps the base value and every negative-path test becomes a happy path.