python购物卡案例代码分析与思考

Python中购物卡案例通常用于演示设计模式中的状态模式或策略模式，这些模式可以帮助处理复杂的业务逻辑，如根据优惠规则更新卡余额。这里我会简单概述一个基于策略模式的购物卡示例，并提供一些思考点。

首先，我们创建一个`Card`基类，包含基本属性如`balance`和方法`redeem`：

class Card:
    def __init__(self, initial_balance):
        self.balance = initial_balance

    def redeem(self, amount, strategy):
        return strategy.apply(amount, self)

# 策略接口
class DiscountStrategy:
    def apply(self, amount, card):
        pass

接着，我们可以定义几种不同的`DiscountStrategy`子类，比如全价、折扣、满减等：

class FullPriceDiscountStrategy(DiscountStrategy):
    def apply(self, amount, card):
        card.balance -= amount
        return f"Redeemed {amount}, new balance: {card.balance}"

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

    def apply(self, amount, card):
        discount = amount * self.discount_rate / 100
        card.balance -= amount - discount
        return f"Redeemed {amount} (with {self.discount_rate}% off), new balance: {card.balance}"

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

    def apply(self, amount, card):
        if card.balance >= self.min_spent:
            discount = (card.balance - self.min_spent) * self.discount_rate / 100
            card.balance -= amount - discount
            return f"Redeemed {amount} (after spending {self.min_spent} with {self.discount_rate}% off), new balance: {card.balance}"
        else:
            return f"Not enough balance for discount, redeemed {amount}, new balance: {card.balance}"

在主程序中，用户可以选择策略并应用到购物卡上：

if __name__ == "__main__":
    card = Card(100)
    print(card.redeem(50, FullPriceDiscountStrategy()))
    card.strategy = PercentageDiscountStrategy(10)
    print(card.redeem(75, card.strategy))
    card.min_spent = 50
    card.strategy = MinimumSpentDiscountStrategy(50, 20)
    print(card.redeem(25, card.strategy))

思考点：
1. 这种设计模式如何简化了代码？是否易于扩展新的优惠策略？
2. 如何处理用户输入的策略不匹配卡类型的情况？
3. 有没有可能优化`apply`方法，减少对`card.balance`的操作？