Python Testing With Pytest

• Write test functions, not unittest.TestCase classes, and use plain assert
• Name tests test_<subject>_<condition>_<expected> so a failure name explains itself
• Mirror the package layout under tests/, and keep test files importable without side effects
• Use @pytest.mark.parametrize instead of looping inside a test, so each case reports separately
• Use fixtures for setup, put shared ones in conftest.py, and keep their scope as narrow as possible
• Use tmp_path for filesystem work and monkeypatch for environment and attribute patching
• Assert on behaviour and public output, not on internal calls, unless the call is the behaviour
• Use pytest.raises with match= to pin the error type and message
• Mock only at the process boundary: network, clock, randomness, subprocess
• Never weaken an assertion or add a skip to get a green run, fix the code or report the failure