str.isdigit()

 '''程序：购物车程序

 需求:

 启动程序后，让用户输入工资，然后打印商品列表

 允许用户根据商品编号购买商品

 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

 可随时退出，退出时，打印已购买商品和余额'''

 # -*- coding: utf-8 -*-

 print("-------shopping_cart-------")

 print("There is the goods and its price:")

 flag = [1, 2, 3, 4, 5]

 GoodsList = ["A", "B", "C", "D", "E"]

 value = [50, 100, 150, 200, 250]//这里把这俩列表嵌套会更好一些！！

 for i in flag:

     a = GoodsList[i - 1]

     b = value[i - 1]

     print(str(i) + ". " + a + ", " + str(b))

 Salary = input("please input your salary:")

 LeftSalary = int(Salary)

 AlreadyBuy = []

 while True:

     Number = input("please input your choice(input the number):")

     if Number.isdigit() and int(Number) in range(1, 6):

         if LeftSalary < value[int(Number) - 1]:

             print("sorry, your money isn't enough")

         else:

             AlreadyBuy.append(GoodsList[int(Number) - 1])

             LeftSalary = LeftSalary - value[int(Number) - 1]

             print("you have %d left" % LeftSalary)

     elif Number == "quit":

         print("you have end the process,you have %d left" % LeftSalary)

         print("you have bought:", AlreadyBuy)

         break

     else:

         print("you have input the wrong commend!")