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Python test-writing best practices covering test structure, fixtures, parametrization, and mocking with pytest. Use when writing, reviewing, or refactoring tests, adding test coverage, or working in tests directories and conftest.py.
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Python coding standards and best practices covering error handling, performance optimization, async patterns, SOLID design principles, documentation, data validation, and object-oriented programming. Use when writing, reviewing, or refactoring Python code, or when the user asks about Python style, design, or best practices.
Python development tooling configuration and best practices for ruff, mypy, pytest, pyscn, and uv. Use when setting up a Python project, configuring linters, formatters, type checkers, or test frameworks, or when editing pyproject.toml or tool config files.
| name | testing |
| description | Python test-writing best practices covering test structure, fixtures, parametrization, and mocking with pytest. Use when writing, reviewing, or refactoring tests, adding test coverage, or working in tests directories and conftest.py. |
| paths | ["tests/**","**/test_*.py","**/*_test.py","**/conftest.py"] |
A collection of test-writing best practices for pytest. Designed for AI agents and LLMs to write readable, maintainable, and trustworthy tests.
Mock external boundaries only, and keep mocks faithful to real interfaces.
| Rule | Description |
|---|---|
| mock-boundaries | Mock external boundaries, not internal logic |
| mock-autospec | Use autospec to keep mocks faithful to real signatures |
| mock-monkeypatch | Use monkeypatch for env vars and attributes |
Structure tests so failures are easy to diagnose.
| Rule | Description |
|---|---|
| struct-aaa-pattern | Structure tests as Arrange-Act-Assert |
| struct-test-naming | Name tests after the behavior they verify |
| struct-one-behavior | Test one behavior per test |
| struct-no-logic | Keep conditionals and loops out of tests |
Share setup without hiding it.
| Rule | Description |
|---|---|
| fixture-scope | Use the narrowest fixture scope that works |
| fixture-conftest | Place fixtures in the nearest conftest.py |
| fixture-factory | Use factory fixtures for varied test data |
Eliminate duplicated tests without hiding cases.
| Rule | Description |
|---|---|
| param-parametrize | Replace duplicated tests with parametrize |
| param-ids | Give parametrized cases readable ids |
def test_expired_token_is_rejected() -> None:
# Arrange
token = make_token(expires_at=YESTERDAY)
# Act
result = validate(token)
# Assert
assert result.valid is False
# Narrow scope by default
@pytest.fixture
def user() -> User:
return User(name="alice", email="alice@example.com")
# Factory fixture for varied data
@pytest.fixture
def make_user() -> Callable[..., User]:
def _make(name: str = "alice", active: bool = True) -> User:
return User(name=name, email=f"{name}@example.com", active=active)
return _make
@pytest.mark.parametrize(
("email", "valid"),
[
pytest.param("a@example.com", True, id="simple-address"),
pytest.param("a@sub.example.com", True, id="subdomain"),
pytest.param("no-at-sign", False, id="missing-at"),
pytest.param("", False, id="empty-string"),
],
)
def test_email_validation(email: str, valid: bool) -> None:
assert is_valid_email(email) is valid
# Mock the external boundary with a faithful interface.
# `mocker` requires the pytest-mock plugin; with stdlib only,
# use unittest.mock.patch as a decorator or context manager.
def test_notifies_on_signup(mocker: MockerFixture) -> None:
send = mocker.patch("app.signup.email_client.send", autospec=True)
signup(email="alice@example.com")
send.assert_called_once_with(to="alice@example.com", template="welcome")
# monkeypatch for environment variables
def test_reads_api_key(monkeypatch: pytest.MonkeyPatch) -> None:
monkeypatch.setenv("API_KEY", "test-key")
assert load_config().api_key == "test-key"