# Interface Design for Testability

Good interfaces make testing natural:

1. **Accept dependencies, don't create them**
```python
# 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**
```python
# 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