Skill: pytest

Best practices for writing and organizing tests with pytest including fixtures, parametrize, and plugins.

When to Use

Apply this skill when writing and organizing tests with pytest — fixtures, parametrize, markers, plugins, and test structure.

Test Organization

• Place tests in a tests/ directory mirroring the source structure.
• Name test files test<module>.py and test functions test<behavior>().
• Group related tests in classes only when they share fixtures/setup.

Fixtures

• Define fixtures at the narrowest scope needed (function > class > module > session).
• Use conftest.py for shared fixtures; put it at the appropriate directory level.
• Prefer factory fixtures over complex fixture inheritance.
• Use yield fixtures for setup/teardown; prefer tmp_path over tempfile.

Parametrize

• Use @pytest.mark.parametrize for data-driven tests with multiple inputs.
• Give test IDs (ids=...) for readable test output.
• Combine parametrize with fixtures for cross-product testing.

Assertions

• Use plain assert statements — pytest rewrites them for clear failure messages.
• Use pytest.raises(ExceptionType, match=...) for exception testing.
• Use pytest.approx() for floating-point comparisons.

Plugins

• Common plugins: pytest-cov, pytest-mock, pytest-asyncio, pytest-xdist, pytest-timeout.
• Use pytest-mock's mocker fixture over raw unittest.mock.patch.

Pitfalls

• Don't use session-scoped fixtures for mutable state.
• Don't assert on implementation details — test observable behavior.
• Avoid test interdependence; each test should be runnable in isolation.