Test Behavior, Not Implementation

A test calls the code the way its users do and asserts the result they observe against a literal expected value. A test that asserts which calls the code made, or restates a constant the code contains, does neither.

The check: before you keep a test, ask whether it would still pass if every function it imports returned None. If yes, it observes no behavior and cannot fail for a defect. Rewrite the assertion or delete the test.

Why: A test that cannot fail for a defect costs CI time and review attention and catches nothing. A constant pin also fails when someone edits the constant or the prompt it restates, so it prevents that edit.

Five shapes that still pass when every imported function returns None:

• Weak or no assertion. No assert, or only assert result, is not None, isinstance(...), > 0, or a call that only shows nothing raised.
• Mock or absence only. Only mock.assert_called(), assert_not_called(), is None, == [], len(x) == 0, != wrong_value.
• Self-referential. The expected value comes from the code under test: assert f(a) == f(a), assert parsed.url == build_url(...).
• Constant pin. The assertion restates a hand-maintained constant, config default, table row, or prompt string: assert LIMITS.max_tools == 8, assert "You are" in PROMPT.
• Fixture asserts fixture. The assertion reads data the test built or a value a pytest fixture computed, and the subject never runs inside the body.

The fix: call the subject inside the test body with one concrete input and assert the literal output or the observable effect, assert slugify("Hello, World!") == "hello-world". For an absence, assert the presence on the other input in the same test. For a constant, test the mechanism that reads it with one input instead of restating the value. For a mock, assert the payload it received or the state after the call, not that it was called. When no such assertion exists, delete the test.

Firmware: with a fake HAL or register map, assert the value written to the register or the bytes put on the bus, not that the write function was called.

Keep a test of a relation across a table's rows (a key present in two tables, a parent that exists), and a static check the type checker runs (typing.assert_type).