SurfSense/.cursor/skills/tdd/interface-design.md

Interface Design for Testability

Good interfaces make testing natural:

1. Accept dependencies, don't create them

# Testable
def process_order(order, payment_gateway):
    pass

# Hard to test
def process_order(order):
    gateway = StripeGateway()

2. Return results, don't produce side effects

# Testable
def calculate_discount(cart) -> float:
    return discount

# Hard to test
def apply_discount(cart) -> None:
    cart.total -= discount

3. Small surface area

• Fewer methods = fewer tests needed
• Fewer params = simpler test setup