Python购物车开发：如何限制折扣卡仅可应用一次？

问题根源 #

你的代码无法限制重复添加折扣卡，核心原因是存储已添加折扣卡的集合added_discount_cards被定义在while selection == '1'循环内部，每次进入循环时集合都会被重新初始化（变成空集合），导致之前添加的记录完全丢失，自然无法检测到重复操作。此外代码还存在语法错误和缩进问题。

修正后的代码 #

print("Welcome to the Store!")
print("Enter 1 to start registering your grocery")
print("Enter 2 to close the self-check in register")

def is_valid_string_input(user_input, valid_choices):
    return user_input in valid_choices

# 将已添加折扣卡的集合放在循环外部，保留会话内的记录
added_discount_cards = set()
card_names = {'1': 'Pensioner', '2': 'Weekend', '3': 'Green'}

selection = input("Please, enter your choice: ")

while selection == '1':
    print("Enter 1 to add a discount card.")
    print("Enter 2 to register a new item.")
    print("Enter 3 to compute the total and pay.")

    user_choice = input("Please, enter your choice: ")

    if user_choice == '1':
        print("1. Enter 1, for adding a Pensioner Card")
        print("2. Enter 2, for adding a Weekend Card")
        print("3. Enter 3, for adding a Green Card")

        discount_choice = input("Please, enter your choice: ")

        # 验证折扣卡输入合法性
        if is_valid_string_input(discount_choice, {'1', '2', '3'}):
            # 检查是否已添加该折扣卡
            if discount_choice not in added_discount_cards:
                added_discount_cards.add(discount_choice)
                print(f"The {card_names.get(discount_choice, 'unknown')} Card successfully added!")
            else:
                # 对应已添加的情况，缩进修正
                card_name = card_names.get(discount_choice, 'unknown')
                print(f"Error: {card_name} Card already added.")
        else:
            print("Invalid selection! Please select a Discount Card from the menu.")
    elif user_choice == '2':
        # 这里可以添加登记商品的逻辑
        print("Registering new item...")
    elif user_choice == '3':
        # 这里可以添加计算总价的逻辑
        print("Computing total and processing payment...")
        # 结账后可以重置集合（如果需要每次购物会话独立）
        added_discount_cards.clear()
        selection = '2'  # 退出当前购物循环
    else:
        print("Invalid choice! Please select from the menu.")
    
    # 如果用户没选择结账，重新询问是否继续购物
    if selection != '2':
        selection = input("Enter 1 to continue shopping, 2 to close: ")

关键修改点 #

1. 集合位置调整：将added_discount_cards = set()和card_names移到while循环外部，确保整个会话内的折扣卡添加记录被保留。
2. 语法错误修复：删除未完成的for循环和无效代码，修正else块的缩进，使其正确对应“检查折扣卡是否已添加”的条件。
3. 函数位置优化：将is_valid_string_input函数移到循环外部，避免重复定义。
4. 补充逻辑：添加了user_choice为2和3的分支处理，以及购物流程的循环控制。

运行效果（符合期望输出） #

Welcome to the Store!
Enter 1 to start registering your grocery
Enter 2 to close the self-check in register
Please, enter your choice: 1
Enter 1 to add a discount card.
Enter 2 to register a new item.
Enter 3 to compute the total and pay.
Please, enter your choice: 1
1. Enter 1, for adding a Pensioner Card
2. Enter 2, for adding a Weekend Card
3. Enter 3, for adding a Green Card
Please, enter your choice: 1
The Pensioner Card successfully added!
Enter 1 to continue shopping, 2 to close: 1
Enter 1 to add a discount card.
Enter 2 to register a new item.
Enter 3 to compute the total and pay.
Please, enter your choice: 1
1. Enter 1, for adding a Pensioner Card
2. Enter 2, for adding a Weekend Card
3. Enter 3, for adding a Green Card
Please, enter your choice: 1
Error: Pensioner Card already added.
Enter 1 to continue shopping, 2 to close: 

内容的提问来源于stack exchange，提问作者Noob