python-testing git:20260919.4491844A

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---
name: python-testing
description: "Write Python tests with pytest, fixtures, property tests, and test-driven development."
license: MIT
metadata:
  kind: task
  author: Médéric HURIER (Fmind)
  source: github.com/fmind/dot/tree/main/skills/python-testing
  created: "2026-08-08"
  updated: "2026-09-19"
---

# Python Testing

Prove a change with an honest red-green-refactor cycle: a failing test that detects the missing or broken behavior, then the smallest trustworthy change; [quality-assurance](../quality-assurance/SKILL.md) owns the broader campaign and [systematic-debugging](../systematic-debugging/SKILL.md) owns failures not yet understood.

For pytest fixture, collection, or assertion maintenance, use [pytest mechanics](references/pytest.md) directly. Use the red-green-refactor workflow below when implementing a behavior change; test-only maintenance does not require inventing a production change.

## Workflow

1. **Discover the harness**: Read repository instructions, existing tests, task definitions, and nearby patterns; find the smallest command that exercises the target behavior.
1. **State the contract**: Name the production change that would make the test pass and a plausible regression that would make it fail.
1. **RED**: Write one minimal test for one observable behavior; run it and confirm it fails for the missing behavior, not for a syntax, fixture, environment, or setup error.
1. **GREEN**: Implement only enough production code to satisfy that test; run the focused test and read the full output.
1. **Protect the neighborhood**: Run the package or subsystem tests; fix production code when the new behavior breaks a valid existing contract and revisit the spec when contracts conflict.
1. **REFACTOR**: Improve names, structure, duplication, and types only while everything stays green; add no behavior.
1. **Repeat**: Take the next smallest behavior, edge case, or failure path through a new red cycle.
1. **Prove the regression test**: For a bug fix, temporarily reverse the fix when safe, confirm the test fails, then restore it and confirm green.
1. **Qualify proportionately**: reuse passing focused and subsystem results while relevant inputs remain unchanged; add affected static checks. Run the full gate only when repository policy or cross-cutting risk requires it. Apply the [dirty-tree rule](../mise/SKILL.md#gotchas) when unrelated work is present.
1. **Report evidence**: summarize the observed red and green outcomes, checks actually run, and remaining limits; do not run additional suites merely to fill the report.

## Gotchas

- **Never weaken an existing test**: do not loosen a type, add a skip, or mock away the defect to manufacture green.
- **Implementation came first**: Never delete or overwrite user work because it preceded the test; preserve it, add a red-capable test, and disclose the sequence.
- **Test-first does not fit**: For a spike, generated code, or configuration-only change, say why and define another failing validation signal; do not call tests written afterward TDD.
- **Test level**: Fast unit or contract tests first; integration, property, concurrency, or browser tests only where the boundary demands them; characterize legacy behavior before changing it.
- **Real collaborators**: Prefer real parsers, databases, filesystems, and HTTP handlers at lightweight boundaries over mocks of the unit under test; fake only paid, destructive, slow, or unreliable systems behind a narrow owned interface.
- **Readable failures**: Keep fixtures readable and assertions on outcomes; a little duplicated setup beats hidden intent, and a failure should explain the broken contract without a debugger.

## References

- [Property tests](references/property-tests.md): read when parsers, codecs, migrations, or state transitions need generated input coverage; use `uv` for project test dependencies.

## Documentation

- Adapted from [Superpowers test-driven-development](https://github.com/obra/superpowers/blob/44c9b2d6e889982ac18c27d05a19fefe335194e1/skills/test-driven-development/SKILL.md), [agent-skills test-driven-development](https://github.com/addyosmani/agent-skills/blob/d2478bf0c73a6357df39a3ed6aff16acaa218843/skills/test-driven-development/SKILL.md).
- Releases: [pytest changelog](https://docs.pytest.org/en/stable/changelog.html)
- Companion skills: [quality-assurance](../quality-assurance/SKILL.md) (risk-based campaign), [systematic-debugging](../systematic-debugging/SKILL.md) (unexplained failure), [implementation-plan](../implementation-plan/SKILL.md) (planned slices), [mise](../mise/SKILL.md) (task vocabulary).