Testing — First Principles

1. The Principles

Tests document what the code should do. Reading the tests is reading the contract.

Tests do not depend on each other. Order does not matter. Parallelism is the default.

Same input, same output, every time. No time, randomness, network, or filesystem in the test path unless explicitly modeled.

Tests run in seconds, not minutes. Slow tests are skipped, then deleted.

Cover what the code does, not what it is. Lines covered is not the goal. Behaviors exercised is the goal.

A failing test names the file, the function, the input, the expectation, and the actual. A test that fails unhelpfully is broken.

Test data resembles production data in shape, distribution, and edge cases. Toy data hides bugs.

Tests are first-class code. They are read, reviewed, and refactored. Test code is not throwaway.

Boundaries, nulls, empty sets, maximums, minimums, and invalid inputs are tested. The middle of the range is the easy part.

A test that cannot fail is not a test. A test that asserts nothing is a lie. Tests earn their place by being able to catch real bugs.