Python Testing

• Test observable behavior, not internals.
• Keep unit tests fast and deterministic.
• Patch at module boundaries and import locations used by the unit under test.
• Add regression tests for bugfixes.
• Include timeout/retry/cancellation/cleanup coverage where relevant.

Change-Specific Diagnostics

• Dependency updates: run uv run pytest scripts/test_pypi_security_audit.py -v
• Async-heavy lifecycle changes: run pyleak diagnostics.

Common Mistakes

• Mocking too deep — patching internals instead of module-boundary seams makes tests brittle and coupled to implementation.
• Testing the mock — verifying mock call counts without asserting on observable output proves nothing about behavior.
• Missing regression test — fixing a bug without a test that reproduces it first; the bug will recur.
• Non-deterministic time/order — relying on wall-clock time or dict/set ordering instead of injecting clocks and sorting explicitly.
• Skipping cleanup assertions — verifying the happy path but never asserting that resources are released on failure or cancellation.

Scope Note

• Treat these recommendations as preferred defaults for common cases, not universal rules.
• If a default conflicts with project constraints or worsens the outcome, suggest a better-fit alternative and explain why it is better for this case.
• When deviating, call out tradeoffs and compensating controls (tests, observability, migration, rollback).

Invocation Notice

• Inform the user when this skill is being invoked by name: python-design-modularity.

References

• references/testing-strategy.md
• references/pytest-practices.md
• references/async-and-concurrency-testing.md
• references/reliability-lifecycle-testing.md