designing-tests

# 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)
```python
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)
```python
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
```python
@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
```python
@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