claude-copilot: Skill for Claude Code

Pytest Patterns

Modern pytest testing patterns, anti-patterns, and quality rules for Python.

pytest_smell.py detects structural test smells deterministically. The prose sections below cover judgment-level guidance that the script cannot evaluate. Never re-derive smell detection by eye when the script can do it; always run the script for consistent, auditable findings.

Core Principles

Patterns vs Anti-Patterns

Fixtures

# GOOD: Fixtures for test dependencies
@pytest.fixture
def db_session():
    """Provide a clean database session."""
    session = create_session()
    yield session
    session.rollback()
    session.close()

@pytest.fixture
def sample_user(db_session):
    """Create a sample user for tests."""
    user = User(name="John", email="[email protected]")
    db_session.add(user)
    db_session.commit()
    return user

def test_get_user(db_session, sample_user):
    result = get_user_by_id(db_session, sample_user.id)
    assert result.name == "John"

# BAD: Setup in each test
def test_get_user():
    session = create_session()
    user = User(name="John", email="[email protected]")
    session.add(user)
    session.commit()
    # ... test code
    session.rollback()  # Easy to forget cleanup!

Fixture Scopes

# GOOD: Appropriate fixture scopes
@pytest.fixture(scope="session")
def docker_db():
    """Start database container once per test session."""
    container = start_postgres_container()
    yield container
    container.stop()

@pytest.fixture(scope="module")
def api_client():
    """Create API client once per test module."""
    return APIClient(base_url="http://test.local")

@pytest.fixture  # Default: function scope
def clean_data(db_session):
    """Fresh data for each test."""
    yield
    db_session.query(User).delete()

# BAD: Wrong scope causes test pollution
@pytest.fixture(scope="session")  # DANGER!
def user():
    return User(name="John")  # Shared across all tests!