Tdd Practitioner

TDD Practitioner

Purpose

Write better, more reliable code by letting tests drive the implementation through the red-green-refactor cycle.

The TDD Cycle

Red → Green → Refactor

1. RED:    Write a failing test for the next small piece of behavior
2. GREEN:  Write the minimum code to make it pass
3. REFACTOR: Clean up without breaking tests
Repeat...

1️⃣ Writing a Good Unit Test (AAA Pattern)

def test_order_calculates_total_with_discount():
    # ARRANGE: Set up the system under test
    items = [
        OrderItem(product_id="A", quantity=2, price=Money("25.00")),
        OrderItem(product_id="B", quantity=1, price=Money("50.00")),
    ]
    discount = PercentageDiscount(rate=Decimal("0.10"))  # 10% off
    order = Order(items=items, discount=discount)
    
    # ACT: Perform the action
    total = order.calculate_total()
    
    # ASSERT: Verify the outcome
    assert total == Money("90.00")  # (25*2 + 50) * 0.9 = 90

2️⃣ Test Quality Checklist

• Tests one thing (single assertion concept)
• Test name describes behavior: test_[method]_[scenario]_[expected_result]
• Independent — no test depends on another
• Fast — no I/O, no external calls
• Deterministic — same result every run
• Tests behavior, not implementation details

3️⃣ Test Doubles

# Mock (verify interactions)
mock_email = Mock()
service = UserService(email_sender=mock_email)
service.register(email="test@example.com")
mock_email.send.assert_called_once_with(
    to="test@example.com",
    subject="Welcome!"
)

# Stub (provide canned responses)
stub_repo = Mock()
stub_repo.find_by_email.return_value = User(id=1, email="test@example.com")

# Fake (simplified working implementation)
class InMemoryOrderRepository:
    def __init__(self): self._store = {}
    def save(self, order): self._store[order.id] = order
    def find(self, id): return self._store.get(id)

4️⃣ Outside-In TDD (Starting with Acceptance Tests)

1. Write failing acceptance test (high level behavior)
2. Write failing unit tests for components needed
3. Implement until unit tests pass
4. Run acceptance test — keep going until it passes
5. Refactor

Outputs

1. Test suite for specified feature
2. Mock/stub configuration
3. Test helpers and builders
4. Coverage report analysis
5. TDD workflow documentation