Python Test Design Skill

Design Python tests under the shared rules in /python-engineering:standards-for-python-development. Repository requirements and established project gates take precedence. This skill adds Python/pytest-specific boundary and fixture guidance; it does not create a second testing philosophy.

Decision order

1. Establish the supported behavior, contract, callers/consumers, existing protection, and relevant failure consequence before deciding what test to retain.
2. Separate direct validation of the current change from permanent regression protection.
3. Prefer the highest stable contract boundary that still discriminates the consequential failure with acceptable cost and diagnostics.
4. Keep unit tests TDD-sized: one small behavioral slice that survives behavior-preserving refactors.
5. Do not mirror private helpers, branches, constants, incidental call counts, or decomposition unless those details are themselves authoritative contracts.
6. Preserve an existing repository coverage/mutation gate. When none exists, do not invent one.