from abc import ABC, abstractmethod

# 策略介面
class PricingStrategy(ABC):
    @abstractmethod
    def calculate_price(self, price):
        pass

# 具體策略類
class NormalStrategy(PricingStrategy):
    def calculate_price(self, price):
        return price

class DiscountStrategy(PricingStrategy):
    def __init__(self, discount_rate):
        self.discount_rate = discount_rate

    def calculate_price(self, price):
        return price * (1 - self.discount_rate)

# 上下文類
class ShoppingCart:
    def __init__(self, pricing_strategy):
        self.pricing_strategy = pricing_strategy

    def set_pricing_strategy(self, pricing_strategy):
        self.pricing_strategy = pricing_strategy

    def checkout(self, items):
        total_price = sum(items)
        return self.pricing_strategy.calculate_price(total_price)

# 使用範例
cart = ShoppingCart(NormalStrategy())
items = [100, 200, 300]
print("Total price with normal strategy:", cart.checkout(items))  # Output: Total price with normal strategy: 600

cart.set_pricing_strategy(DiscountStrategy(0.2))
print("Total price with discount strategy:", cart.checkout(items))  # Output: Total price with discount strategy: 480