pytest / Testing Skill

Test generation and coverage improvement - an Agent Skill for Python 3.14 and pytest.

What This Skill Does

Audits a module and adds tests: tests/ layout, conftest.py fixtures, parametrized edge cases, pytest.mark integration/slow, httpx.MockTransport or pytest-httpx for HTTP, database transaction rollbacks, and CI pytest --cov with sensible threshold.

When to Invoke

• New service with zero tests before first deploy
• PR adding complex branch logic without coverage
• Flaky test cleanup after CI noise
• Converting unittest suite to pytest idioms

Inputs

Outputs

• tests/conftest.py with shared fixtures
• Parametrized tests for edge cases from Gotchas docs
• pyproject.toml [tool.pytest.ini_options]
• coverage omit patterns for if TYPE_CHECKING
• CI snippet matching local uv run pytest command

Guardrails

1. Test behavior, not implementation - assert outputs and side effects.
2. One assertion concept per test - clear failure messages.
3. Freeze time with monkeypatch - not sleep in tests.
4. No network in unit tests - mock or VCR cassettes explicit.
5. Do not chase 100% coverage - focus risk-weighted modules.
6. Mark integration tests - keep default pytest -m "not integration" fast.

Recipe

uv sync --extra dev
uv run pytest -q
uv run pytest --cov=app --cov-report=term-missing --cov-fail-under=80
uv run ruff check tests/

Working Example (parametrize)

import pytest
 
from app.pricing import discount
 
 
@pytest.mark.parametrize(
    "amount,coupon,expected",
    [
        (100, "SAVE10", 90),
        (100, "INVALID", 100),
        (0, "SAVE10", 0),
    ],
)
def test_discount(amount, coupon, expected):
    assert discount(amount, coupon) == expected

FAQs

Related

• pytest Setup - fundamentals
• Fixtures - fixture patterns
• Testing FastAPI - API tests
• BDD & Gherkin - acceptance layer

Stack versions: This page was written for Python 3.14.0 (stable 3.14, maintenance 3.13), FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.