python-pytest

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Pytest discipline for writing, reviewing, and fixing Python tests — what deserves to be a committed test, deterministic concurrency testing, fixture isolation, mocking boundaries, and suite structure. Triggers on writing or reviewing pytest tests, designing fixtures, parametrizing cases, mocking or patching, debugging flaky or order-dependent tests, xfail/skip decisions, and pytest configuration.

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Sourcenathan-gage/python-skills

Added on2026-10-18

NPX Install

npx skill4agent add nathan-gage/python-skills python-pytest

SKILL.md Content

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Python Pytest

Guidelines for writing and reviewing pytest suites. 15 rules across 5 categories, prioritized by impact.

A rule match is a signal, not a verdict. Most rules are design preferences for new tests — check the rule's impact level before flagging in review or churning a stable suite.

Quick-reference lines are triggers, not licenses: before applying a rule as a review finding or a transformation, open the rule file and check its counter-signal — the marker-opened paragraph (

**When ...**

/

**Scope:**

/

**Exception ...**

) saying when NOT to apply it.

When to Apply

Writing new tests or fixtures
Reviewing test files for value, isolation, or determinism
Debugging flaky, order-dependent, or falsely-green tests
Configuring pytest and its plugin surface

The unit of judgment is the contract a test defends — every rule here traces back to whether a failure would mean something.

Impact Levels

```
HIGH
```
— the test defends nothing or lies (probes, self-oracles, timing bets, masked flakes). Fix when found.
```
MEDIUM-HIGH
```
— isolation and mocking failures that surface as order-dependence or tautology.
```
MEDIUM
```
— structure and granularity; apply to new tests.
```
LOW-MEDIUM
```
— suite-level hygiene; apply when configuring or composing suites.

Rule Categories by Priority


value-
determinism-
fixtures-
mock-
structure-

Priority	Category	Impact	Prefix
1	Test Value	HIGH	`value-`
2	Determinism	HIGH	`determinism-`
3	Fixtures & Isolation	MEDIUM-HIGH	`fixtures-`
4	Mocking	MEDIUM-HIGH	`mock-`
5	Structure & Execution	MEDIUM	`structure-`

Quick Reference

Test Value (

value-

)

```
value-observable-contracts
```
— A committed test protects a named behavior; delete execution probes
```
value-independent-oracles
```
— Expected values derived independently, never by re-running the implementation's formula
```
value-one-behavior-per-test
```
— Split independently-failing phases; merge duplicate coverage

Determinism (

determinism-

)

```
determinism-sync-not-sleep
```
— Events/barriers with failure-bound timeouts; never sleeps or elapsed-time thresholds
```
determinism-no-flake-masking
```
— No rerun plugins, widened timeouts, or intermittent xfail; fix the race

determinism-strict-xfail

—

xfail(strict=True)

so an unexpected pass fails the run

Fixtures & Isolation (

fixtures-

)

```
fixtures-narrowest-scope
```
— Function scope by default; widen only expensive immutable setup
```
fixtures-restore-global-state
```
—
```
monkeypatch
```
env/cwd/attrs; restore absent vars too; cleanup runs on failure
```
fixtures-canonical-objects
```
— Production constructors for fixtures, not hand-guessed dicts; raw inputs when the constructor is under test

Mocking (

mock-

)

```
mock-stable-boundaries
```
— Fake the transport you don't own; run the code you do; no mock tautologies
```
mock-patch-where-used
```
— Patch the namespace where the name is looked up, not where it's defined
```
mock-assert-wire-format
```
— Assert serialized bytes/strings at compatibility boundaries, not Python intent

Structure & Execution (

structure-

)

```
structure-parametrize-partitions
```
— Cases are contract-distinct partitions with independent expecteds; guard generated case sets
```
structure-unique-module-names
```
— Collision-free test module import identities across composed suites
```
structure-plugin-hygiene
```
— Autoload is a fine default; on observed plugin drift, make the surface explicit

How to Use

Read individual rule files for detail:

rules/value-observable-contracts.md
rules/mock-patch-where-used.md

Each rule has:

Impact level in frontmatter
Brief explanation
Incorrect example
Correct example
Optional note on edge cases

For the full compiled guide with all rules expanded:

AGENTS.md

.

python-pytest

NPX Install

Tags

SKILL.md Content

Python Pytest

When to Apply

Impact Levels

Rule Categories by Priority

Quick Reference

Test Value (
`value-`
)

Determinism (
`determinism-`
)

Fixtures & Isolation (
`fixtures-`
)

Mocking (
`mock-`
)

Structure & Execution (
`structure-`
)

How to Use

python-pytest

NPX Install

Tags

SKILL.md Content

Python Pytest

When to Apply

Impact Levels

Rule Categories by Priority

Quick Reference

Test Value (value-)

Determinism (determinism-)

Fixtures & Isolation (fixtures-)

Mocking (mock-)

Structure & Execution (structure-)

How to Use

Test Value (
`value-`
)

Determinism (
`determinism-`
)

Fixtures & Isolation (
`fixtures-`
)

Mocking (
`mock-`
)

Structure & Execution (
`structure-`
)