Test-Driven Development (TDD) Basics

# TDD Cycle: Red → Green → Refactor

# Step 1: RED — Write a failing test
def test_calculator_add():
    calc = Calculator()
    assert calc.add(2, 3) == 5

def test_calculator_subtract():
    calc = Calculator()
    assert calc.subtract(10, 4) == 6

def test_calculator_history():
    calc = Calculator()
    calc.add(2, 3)
    calc.subtract(10, 4)
    assert len(calc.history) == 2

# Step 2: GREEN — Write minimal code to pass
class Calculator:
    def __init__(self):
        self.history = []
    
    def add(self, a, b):
        result = a + b
        self.history.append(f"{a} + {b} = {result}")
        return result
    
    def subtract(self, a, b):
        result = a - b
        self.history.append(f"{a} - {b} = {result}")
        return result

# Step 3: REFACTOR — Improve without changing behavior
# (Already clean, but could extract _record method)

# Run TDD tests
print("=== TDD Demo ===")
tests = [test_calculator_add, test_calculator_subtract, test_calculator_history]
for test in tests:
    try:
        test()
        print(f"  ✅ {test.__name__}")
    except Exception as e:
        print(f"  ❌ {test.__name__}: {e}")

# Show history
calc = Calculator()
calc.add(5, 3)
calc.subtract(10, 4)
calc.add(7, 8)
print(f"\nHistory:")
for entry in calc.history:
    print(f"  {entry}")