# Pytest Patterns > Pytest patterns for fixtures, parametrize, mocking, async testing, and factory_boy. Use when writing or reviewing Python tests, setting up test infrastructure, or debugging test failures. Use when this capability is needed. - Skill: `tomevault-io/pytest-patterns-4` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/pytest-patterns-4` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/pytest-patterns-4/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/pytest-patterns-4 --- # Pytest Patterns Patterns and conventions for writing Python tests with pytest. Follow the principle of plain test functions over class-based tests, leverage fixtures for setup and teardown, and prefer integration tests over excessive mocking. ## Core Rules - **Plain test functions** -- Never use `TestCase` classes. Write standalone `test_` functions - **Fixtures for setup** -- Use fixtures for all test setup; shared fixtures go in `conftest.py` - **Run with poetry** -- Execute tests using `workspace/.venv` environment and poetry: `poetry run pytest` - **`| None` over `Optional`** -- Use modern union syntax in all test code - **100-character line limit** -- Applies to test files as well - **No docstrings unless necessary** -- Test function names should be self-explanatory ## Running Tests ```bash # Run all tests poetry run pytest # Run with verbose output poetry run pytest -v # Run a specific file poetry run pytest tests/test_orders.py # Run a specific test poetry run pytest tests/test_orders.py::test_create_order_with_valid_data # Run tests matching a keyword poetry run pytest -k "order and not cancel" # Run with coverage poetry run pytest --cov=src --cov-report=term-missing ``` ## Key Patterns Overview ### Fixtures Fixtures provide reusable test setup with automatic cleanup. Define fixtures in `conftest.py` for sharing across test modules. Use fixture scopes (`function`, `module`, `session`) to control lifecycle. Fixture factories return callables for parameterized object creation. ### Parametrize `@pytest.mark.parametrize` runs the same test with different inputs. Combine multiple parameter sets, use `indirect` to pass parameters through fixtures, and use `pytest.param` with `id` for readable test IDs. ### Mocking Use `pytest-mock`'s `mocker` fixture over raw `unittest.mock.patch`. Patch at the import site, not the definition site. Prefer `spec=True` for type-safe mocks. Use `side_effect` for conditional returns or exceptions. ### Async Testing Use `pytest-asyncio` with `asyncio_mode = "auto"` in `pyproject.toml` to avoid decorating every async test. Use `@pytest_asyncio.fixture` for async fixtures. Test FastAPI with `httpx.AsyncClient` and `ASGITransport`. ### Factory Boy Use `factory_boy` with `DjangoModelFactory` for test data generation. Define `SubFactory` for related models, `LazyAttribute` for computed fields, and `Trait` for common variations. ## Reference Documents - [fixtures-parametrize.md](./references/fixtures-parametrize.md) -- Fixture scopes, factories, conftest hierarchy, parametrize patterns, factory_boy, monkeypatch, markers, and configuration - [mocking.md](./references/mocking.md) -- unittest.mock, pytest-mock, patching strategies, spec-based mocks, external API mocking, Django/FastAPI test clients - [async-testing.md](./references/async-testing.md) -- pytest-asyncio, async fixtures, FastAPI async testing, Celery task testing, WebSocket testing, anyio, event loop configuration ## When to Consult References - **Setting up test fixtures**: Read [fixtures-parametrize.md](./references/fixtures-parametrize.md) for scope management, conftest hierarchy, and fixture factories - **Writing parametrized tests**: Read [fixtures-parametrize.md](./references/fixtures-parametrize.md) for single/multiple params, indirect parametrize, and factory_boy - **Mocking dependencies**: Read [mocking.md](./references/mocking.md) for patch targets, mock types, and framework-specific test clients - **Testing async code**: Read [async-testing.md](./references/async-testing.md) for pytest-asyncio setup, async fixtures, and framework integration - **Testing external APIs**: Read [mocking.md](./references/mocking.md) for responses library, httpx mock, and when to prefer integration tests --- > Source: [weorbitant/compound-engineering-feat-python-plugin](https://github.com/weorbitant/compound-engineering-feat-python-plugin) — distributed by [TomeVault](https://tomevault.io).