The Matt Pages

FIRST Principles (Testing)

Glossary
<testing/><unit-testing/>

FIRST is a set of five properties that a good unit test should have, coined by Brett Schuchert and Tim Ottinger and popularized by Robert C. Martin’s Clean Code. The acronym is a checklist: a test that is slow, order-dependent, environment-sensitive, manually-checked, or written too late has given up one of the properties that make a suite worth running.

The properties are not independent goals; they describe one well-built test seen from five angles. A test that owns its state is both Independent and Repeatable; removing its I/O makes it both Fast and repeatable. Together they are what let a suite run on every change and be trusted when it does.

The five properties are:

  • Fast – a test runs in milliseconds, so the whole suite runs in seconds and is run constantly. Slow tests get run less often, and a test that is not run catches nothing.
  • Independent (also Isolated) – tests do not depend on each other or on execution order; each sets up its own state and leaves nothing behind, so any test can run alone or in parallel.
  • Repeatable – the same test yields the same result on every run, in any environment. A test that relies on the clock, the network, or random data is a flaky test; a test that is a pure function of its inputs is idempotent across runs.
  • Self-validating (also Self-verifying) – a test decides pass or fail on its own, through assertions, with no manual inspection of output or logs.
  • Timely – tests are written at the right time, ideally just before the production code they cover (as in test-driven development), so the design stays testable and the tests actually get written.

References