需求： 1.启动程序后，让用户输入工资，然后打印商品列表； 2.允许用户根据商品编号购买商品； 3.用户选择商品后，检测余额是否足够，够就直接扣款，不够就提醒； 4.可随时退出，退出时打印已购买的商品列表。 # Author: Ewan Wang shopping_list = [] goods_list = [     ('Nike shoes',800),     ('Mjstyle trousers',500),     ('Bag',300),     ('Dictionary',100),     ('Cup',80),     ('Skin lotion',60) ] salary = input('Input your salary:') if salary.isdigit():     salary = int(salary)     while True:         for index,item in enumerate(goods_list):          #print(goods_list.index(item),item)           print(index,item)         user_choice = input('The product you want to buy is')         if user_choice.isdigit():             user_choice = int(user_choice)             if user_choice < len(goods_list) and user_choice >=0:                 p_item = goods_list[user_choice]                 if salary >= p_item[1]:                     shopping_list.append(p_item)                     salary -= p_item[1]                     print('Add %s in your shopping cart,your salary balance is %s'%(p_item[0],salary))                 else:                     print('Your balance is insufficient.')             else:                 print('Invalid option.')         elif user_choice == 'q':# 这里是==而非=             print('----------shopping list---------')             for p in shopping_list:                 print(p)             exit()