Pytest Best Practices

pytest Best Practices

Description

Guidelines for writing effective pytest tests that are maintainable, fast, and provide clear failure messages.

When to Use

• Writing new unit or integration tests
• Refactoring existing tests
• Setting up test fixtures
• Mocking external dependencies

How to Use

1. Organize tests by convention: Place tests in tests/ directory, mirror source structure
2. Use descriptive names: Test function names should describe expected behavior
3. Follow Arrange-Act-Assert: Set up data, perform action, verify results
4. Use fixtures for shared setup: Extract common setup into fixtures
5. Parametrize for data-driven tests: Test multiple inputs with @pytest.mark.parametrize

Examples

Descriptive Test Names

def test_user_registration_creates_account_with_valid_email():
    ...

def test_api_returns_401_when_token_expired():
    ...

Using Fixtures

@pytest.fixture
def auth_token():
    return generate_test_token(user_id="test-user")

def test_protected_endpoint_requires_auth(client, auth_token):
    response = client.get("/api/user", headers={"Authorization": f"Bearer {auth_token}"})
    assert response.status_code == 200

Parametrized Tests

@pytest.mark.parametrize("input,expected", [
    ("user@example.com", True),
    ("invalid-email", False),
    ("", False),
])
def test_email_validation(input, expected):
    assert validate_email(input) == expected

Using Mocks

from unittest.mock import patch

@patch("mymodule.external_api")
def test_handles_api_failure(mock_api):
    mock_api.get_user.side_effect = APIError("Service unavailable")
    
    result = get_user_profile("user123")
    
    assert result is None
    assert mock_api.get_user.called_once()

Best Practices

• Keep tests independent — no shared state between tests
• Use pytest.ini or pyproject.toml for configuration
• Run tests in isolation: pytest -x stops on first failure
• Use pytest.fixture(scope="session") only when necessary
• Name fixtures descriptively: db_connection not db

Related Skills

• Unit & Integration Testing — Broader testing strategies