Designing Tests Skill

This skill provides patterns and techniques for creating effective test suites.

Testing Pyramid

           /\
          /  \      E2E (10%)
         /----\
        /      \    Integration (20%)
       /--------\
      /          \  Unit (70%)
     /------------\

Test Design Patterns

1. Arrange-Act-Assert (AAA)

def test_user_registration():
    # Arrange - Set up test data and dependencies
    user_data = {"email": "test@example.com", "password": "secure123"}

    # Act - Execute the code under test
    result = register_user(user_data)

    # Assert - Verify the outcome
    assert result.success is True
    assert result.user.email == "test@example.com"

2. Given-When-Then (BDD)

def test_discount_for_premium_users():
    # Given a premium user with items in cart
    user = create_premium_user()
    cart = create_cart(user, items=[item(price=100)])

    # When calculating the total
    total = cart.calculate_total()

    # Then a 20% discount is applied
    assert total == 80

3. Test Fixtures

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

@pytest.fixture
def authenticated_user(db_session):
    """Provide an authenticated user."""
    user = User.create(email="test@example.com")
    db_session.add(user)
    db_session.commit()
    return user

4. Parameterized Tests

@pytest.mark.parametrize("input,expected", [
    ("hello", "HELLO"),
    ("World", "WORLD"),
    ("", ""),
    ("123abc", "123ABC"),
])
def test_uppercase(input, expected):
    assert uppercase(input) == expected

Edge Cases Checklist

Input Validation

• Empty/null inputs
• Boundary values (0, -1, MAX_INT)
• Invalid types
• Special characters
• Unicode strings
• Very long inputs
• Whitespace handling

State Management

• Initial state
• After successful operation
• After failed operation
• Concurrent modifications
• Recovery from errors

External Dependencies

• Network timeout
• Service unavailable
• Invalid response format
• Rate limiting
• Partial failures

Time-Based

• Timezone handling
• Daylight saving transitions
• Leap years
• Date boundaries

Mocking Strategies

When to Mock

• External APIs
• Database calls (for unit tests)
• Time-dependent operations
• Random number generation
• File system operations

When NOT to Mock

• The code under test
• Simple value objects
• Pure functions
• Internal implementation details

Mock Examples

# Mock external API
@patch('services.payment.stripe_client')
def test_payment_processing(mock_stripe):
    mock_stripe.charge.return_value = {"id": "ch_123", "status": "succeeded"}
    result = process_payment(amount=100)
    assert result.success is True

# Mock time
@freeze_time("2024-01-15")
def test_subscription_expiry():
    sub = Subscription(expires_at=datetime(2024, 1, 14))
    assert sub.is_expired() is True

Test Organization

File Structure

tests/
├── unit/
│   ├── test_models.py
│   ├── test_services.py
│   └── test_utils.py
├── integration/
│   ├── test_api.py
│   └── test_database.py
├── e2e/
│   └── test_user_flows.py
├── fixtures/
│   └── conftest.py
└── factories/
    └── user_factory.py

Naming Conventions

# Pattern: test_[action]_[condition]_[expected_result]
def test_login_with_valid_credentials_returns_token():
def test_login_with_invalid_password_returns_401():
def test_login_with_locked_account_raises_AccountLockedException():

Coverage Guidelines

Component	Target
Business logic	90%+
Utilities	85%+
API endpoints	80%+
Configuration	60%+

Anti-Patterns to Avoid

• Flaky tests - Tests that sometimes pass, sometimes fail
• Slow tests - Unit tests should be < 100ms
• Test interdependence - Tests that rely on other tests
• Over-mocking - Mocking everything including the thing you're testing
• Assert-less tests - Tests without meaningful assertions
• Duplicate tests - Testing the same thing multiple ways