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testing-py

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Use when writing or fixing Python tests with pytest — especially when the suite is green but bugs still ship, when you must decide where to patch a mocked dependency, when coverage is high but catches nothing, when fixture scope leaks state, or when adding property-based tests for parsers and invariants. NOT browser end-to-end flows (that is `e2e-testing`), NOT JS/TS unit tests (that is `testing-web`), NOT Go tests (that is `testing-go`).

skills/testing-py/SKILL.md

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# Testing Python

A green suite is not a tested suite. Most Python suites pass while shipping bugs because they lie in four predictable ways, and your job is to refuse each lie:

1. **Line-only coverage** reports 95% while every `else` and `except` arm is unvisited.
2. **Mocks patched at the wrong place** never apply, so the test runs the real code or asserts nothing.
3. **autospec-less mocks** pass even when you typo a method name or change a signature — a false green.
4. **autouse fixtures** mutate hidden global state, so tests pass alone and fail in CI order.

The posture, non-negotiable: pytest 9 strict markers and strict config on, **branch coverage on**, `autospec=` on every mock, and Hypothesis property tests for any pure logic with an invariant. Assert on behavior, never on a mock's internals.

Versions you are on (2026-06): pytest **9.0.2**, coverage.py **7.14.1**, pytest-cov **7.x** (needs coverage ≥ 7.10.6), Hypothesis **6.155.x**.

## Which test do I need?