Testing best practices

Testing Best Practices Skill

This skill provides expert knowledge on software testing methodologies, patterns, and best practices.

Core Principles

1. Test Pyramid

• Unit Tests (70%): Fast, isolated tests for individual functions/methods
• Integration Tests (20%): Test interaction between components
• E2E Tests (10%): Full application flow tests

2. Test Characteristics (F.I.R.S.T.)

• Fast: Tests should run quickly
• Independent: Tests shouldn't depend on each other
• Repeatable: Same results every time
• Self-validating: Clear pass/fail without manual verification
• Timely: Written alongside or before production code (TDD)

Testing Patterns

Arrange-Act-Assert (AAA)

def test_user_registration():
    # Arrange: Set up test data and dependencies
    user_data = {"email": "[email protected]", "password": "secure123"}
    user_service = UserService()

    # Act: Execute the behavior being tested
    result = user_service.register(user_data)

    # Assert: Verify the outcome
    assert result.success is True
    assert result.user.email == "[email protected]"

Test Doubles

Mocks: Verify interactions

mock_email_service = Mock()
user_service.register(user_data)
mock_email_service.send_welcome_email.assert_called_once()

Stubs: Provide predetermined responses

stub_database = StubDatabase()
stub_database.get_user.returns(None)  # Simulate user not found

Fakes: Working implementations with shortcuts

fake_cache = InMemoryCache()  # Instead of Redis for testing

Best Practices

1. Descriptive Test Names

# Good: Describes what and why
def test_registration_fails_when_email_already_exists():
    pass

# Bad: Generic, unclear
def test_registration():
    pass

2. One Assertion Per Test (When Possible)

# Good: Focused test
def test_user_email_is_stored_lowercase():
    user = User(email="[email protected]")
    assert user.email == "[email protected]"

def test_user_email_is_validated():
    with pytest.raises(ValueError):
        User(email="invalid-email")

3. Avoid Test Interdependence

# Bad: Tests depend on execution order
def test_create_user():
    global created_user
    created_user = User.create(...)

def test_update_user():
    created_user.update(...)  # Depends on previous test

# Good: Each test is independent
def test_update_user():
    user = User.create(...)  # Create locally
    user.update(...)

4. Use Fixtures for Setup/Teardown

@pytest.fixture
def authenticated_user():
    user = User.create(email="[email protected]")
    user.login()
    yield user
    user.delete()  # Cleanup

def test_user_can_access_dashboard(authenticated_user):
    response = authenticated_user.get_dashboard()
    assert response.status_code == 200

5. Test Edge Cases

• Empty inputs
• Null/None values
• Boundary conditions
• Invalid data
• Concurrent operations
• Error conditions

Coverage Guidelines

• Aim for 80%+ code coverage
• 100% coverage for critical paths
• Focus on behavior coverage, not just line coverage
• Don't sacrifice test quality for coverage metrics

Common Anti-Patterns to Avoid

1. Testing implementation details - Test behavior, not internals
2. Fragile tests - Avoid coupling to UI elements or data structures
3. Slow tests - Optimize for speed, use test doubles
4. Ignored/skipped tests - Fix or delete, don't accumulate technical debt
5. Testing frameworks instead of code - Don't test library behavior

Usage Instructions

When this skill is active:

• Write comprehensive tests following the AAA pattern
• Suggest appropriate test types (unit, integration, E2E)
• Identify edge cases that need coverage
• Recommend test doubles when appropriate
• Ensure tests are fast, isolated, and maintainable