Test-Driven Development (TDD)
Comprehensive TDD patterns and practices for all programming languages. This skill eliminates ~500-800 lines of redundant testing guidance per agent.
When to Use
Apply TDD for:
- New feature implementation
- Bug fixes (test the bug first)
- Code refactoring (tests ensure behavior preservation)
- API development (test contracts)
- Complex business logic
TDD Workflow (Red-Green-Refactor)
1. Red Phase: Write Failing Test
Write a test that:
- Describes the desired behavior
- Fails for the right reason (not due to syntax errors)
- Is focused on a single behavior
2. Green Phase: Make It Pass
Write the minimum code to:
- Pass the test
- Not introduce regressions
- Follow existing patterns
3. Refactor Phase: Improve Code
While keeping tests green:
- Remove duplication
- Improve naming
- Simplify logic
- Extract functions/classes
Test Structure Patterns
Arrange-Act-Assert (AAA)
// Arrange: Set up test data and conditions
const user = createTestUser({ role: 'admin' });
// Act: Perform the action being tested
const result = await authenticateUser(user);
// Assert: Verify the outcome
expect(result.isAuthenticated).toBe(true);
expect(result.permissions).toContain('admin');
Given-When-Then (BDD Style)
Given: A user with admin privileges
When: They attempt to access protected resource
Then: Access is granted with appropriate permissions
Test Naming Conventions
Pattern: test_should_<expected_behavior>_when_<condition>
Examples:
test_should_return_user_when_id_exists()test_should_raise_error_when_user_not_found()test_should_validate_email_format_when_creating_account()
Language-Specific Conventions
Python (pytest):
def test_should_calculate_total_when_items_added():
cart = ShoppingCart()
cart.add_item(Item("Book", 10.00))
cart.add_item(Item("Pen", 1.50))
total = cart.calculate_total()
assert total == 11.50
JavaScript (Jest):
describe('ShoppingCart', () => {
test('should calculate total when items added', () => {
const cart = new ShoppingCart();
cart.addItem({ name: 'Book', price: 10.00 });
cart.addItem({ name: 'Pen', price: 1.50 });
const total = cart.calculateTotal();
expect(total).toBe(11.50);
});
});
Go:
func TestShouldCalculateTotalWhenItemsAdded(t *testing.T) {
cart := NewShoppingCart()
cart.AddItem(Item{Name: "Book", Price: 10.00})
cart.AddItem(Item{Name: "Pen", Price: 1.50})
total := cart.CalculateTotal()
if total != 11.50 {
t.Errorf("Expected 11.50, got %f", total)
}
}
Test Types and Scope
Unit Tests
- Scope: Single function/method
- Dependencies: Mocked
- Speed: Fast (< 10ms per test)
- Coverage: 80%+ of code paths
Integration Tests
- Scope: Multiple components
- Dependencies: Real or test doubles
- Speed: Moderate (< 1s per test)
- Coverage: Critical paths and interfaces
End-to-End Tests
- Scope: Full user workflows
- Dependencies: Real (in test environment)
- Speed: Slow (seconds to minutes)
- Coverage: Core user journeys
Mocking and Test Doubles
When to Mock
- External APIs and services
- Database operations (for unit tests)
- File system operations
- Time-dependent operations
- Random number generation
Mock Types
Stub: Returns predefined data
def get_user_stub(user_id):
return User(id=user_id, name="Test User")
Mock: Verifies interactions
mock_service = Mock()
service.process_payment(payment_data)
mock_service.process_payment.assert_called_once_with(payment_data)
Fake: Working implementation (simplified)
class FakeDatabase:
def __init__(self):
self.data = {}
def save(self, key, value):
self.data[key] = value
def get(self, key):
return self.data.get(key)
Test Coverage Guidelines
Target Coverage Levels
- Critical paths: 100%
- Business logic: 95%+
- Overall project: 80%+
- UI components: 70%+
What to Test
- ✅ Business logic and algorithms
- ✅ Edge cases and boundary conditions
- ✅ Error handling and validation
- ✅ State transitions
- ✅ Public APIs and interfaces
What NOT to Test
- ❌ Framework internals
- ❌ Third-party libraries
- ❌ Trivial getters/setters
- ❌ Generated code
- ❌ Configuration files
Testing Best Practices
1. One Assertion Per Test (When Possible)
def test_should_validate_email_format():
assert is_valid_email("user@example.com") is True
def test_validation():
assert is_valid_email("user@example.com") is True
assert is_valid_phone("123-456-7890") is True
2. Test Independence
def test_user_creation():
user = create_user("test@example.com")
assert user.email == "test@example.com"
shared_user = None
def test_create_user():
global shared_user
shared_user = create_user("test@example.com")
def test_update_user():
shared_user.name = "Updated"
3. Descriptive Test Failures
assert result.status == 200, f"Expected 200, got {result.status}: {result.body}"
assert result.status == 200
4. Test Data Builders
def create_test_user(**overrides):
defaults = {
'email': 'test@example.com',
'name': 'Test User',
'role': 'user'
}
return User(**{**defaults, **overrides})
admin = create_test_user(role='admin')
guest = create_test_user(email='guest@example.com')
Testing Anti-Patterns to Avoid
❌ Testing Implementation Details
def test_user_storage():
user = User("test@example.com")
assert user._internal_cache is not None
❌ Fragile Tests
assert user.to_json() == '{"name":"John","email":"john@example.com"}'
data = json.loads(user.to_json())
assert data['name'] == "John"
assert data['email'] == "john@example.com"
❌ Slow Tests in Unit Test Suite
def test_api_integration():
response = requests.get("https://api.example.com/users")
assert response.status_code == 200
❌ Testing Everything Through UI
def test_calculation():
browser.click("#input1")
browser.type("5")
browser.click("#input2")
browser.type("3")
browser.click("#calculate")
assert browser.find("#result").text == "8"
def test_calculation():
assert calculate(5, 3) == 8
Quick Reference by Language
Python (pytest)
@pytest.fixture
def database():
db = create_test_database()
yield db
db.cleanup()
@pytest.mark.parametrize("input,expected", [
("user@example.com", True),
("invalid-email", False),
])
def test_email_validation(input, expected):
assert is_valid_email(input) == expected
JavaScript (Jest)
beforeEach(() => {
database = createTestDatabase();
});
afterEach(() => {
database.cleanup();
});
test('should fetch user data', async () => {
const user = await fetchUser(1);
expect(user.name).toBe('John');
});
Go
func TestEmailValidation(t *testing.T) {
tests := []struct {
input string
expected bool
}{
{"user@example.com", true},
{"invalid-email", false},
}
for _, tt := range tests {
result := IsValidEmail(tt.input)
if result != tt.expected {
t.Errorf("IsValidEmail(%s) = %v, want %v",
tt.input, result, tt.expected)
}
}
}
TDD Benefits Realized
- Design Improvement: Tests drive better API design
- Documentation: Tests serve as executable documentation
- Confidence: Refactoring becomes safe
- Debugging: Tests isolate issues quickly
- Coverage: Ensures comprehensive test coverage
- Regression Prevention: Catches bugs before deployment
Related Skills
When using Test Driven Development, these skills enhance your workflow:
- systematic-debugging: Debug-first methodology when tests fail unexpectedly
- react: Testing React components, hooks, and context
- django: Testing Django models, views, and forms
- fastapi-local-dev: Testing FastAPI endpoints and dependency injection
[Full documentation available in these skills if deployed in your bundle]
