程序：购物车程序

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，打印已购买商品和余额

   product_list = [
       ("Iphone",5800),
       ("Mac Pro",9800),
       ("Bike",800),
       ("Watch",10600),
       ("Coffee",31),
       ("book",120),
   ]
   shopping_list = []
   salary = input("Input your salary:")
   if salary.isdigit():
       salary = int(salary)
       while True:
          '''
           for item in product_list:
               print(product_list.index(item),item)
           break
          '''#方法一，其中break为test时跳出循环
          for index,item in enumerate(product_list):
              print(index,item)
          user_choice = input("选择要买吗？>>>>")
          if user_choice.isdigit():
              user_choice = int(user_choice)
              if user_choice < len(product_list) and user_choice >=0:
                  p_item = product_list[user_choice]
                  if p_item[1] <= salary: #买得起
                      shopping_list.append(p_item)
                      salary -= p_item[1]
                      print("Add %s into shopping cart, your current balance is \033[31;1m%s\033[0m"%(p_item,salary))
                  else:
                      print("\033[41;1m你的余额只剩[%s]啦，还买个毛线\033[0m"% salary)
              else:
                  print("product code [%s] is not exist! " % user_choice)
          elif user_choice == 'q':
              print("-------shopping list--------")
              for p in shopping_list:
                  print(p)
              print("Your current balance:",salary)
              exit()
          else:
              print("invalid option")