Individual skill
python-mock-isolation-design
Use Python mocks and fakes as a design tool without losing behavioral confidence. Use when testing external dependencies, choosing monkeypatch versus unittest.mock.patch, isolating slow boundaries, avoiding mock-heavy tests, interpreting mock call assertions, or refactoring toward clearer dependency seams.
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# Python Mock Isolation Design
Use this skill when replacing a dependency in a Python test changes what the test proves. Mocks are useful at slow, nondeterministic, or external boundaries; they become harmful when they replace the behavior the test is supposed to validate.
## Source Traceability
Primary source: Harry Percival, *Test-Driven Development with Python*, 3rd ed. Guidance is transformed and paraphrased from chapters 20, 21, 27, and Appendix A, especially manual monkeypatching, `unittest.mock.patch`, mock coupling, call argument inspection, test isolation, and the architectural route out of mock-heavy suites.
## Workflow
1. Name the boundary.
- External service, email, clock, filesystem, network, framework adapter, or expensive side effect.
- If the dependency is local domain logic, prefer real code.
