Python3.0下的一个练习题

程序要求：
1.用户启动程序时要求输入购物金额，并打印商品列表
2.允许用户不断的选择购买商品
3.购买时检测：余额是否足够--如果足够--直接扣款。如果不够--打印余额不足并退出
4.允许用户主动退出程序--退出时打印已购商品列表

小程序主要用到 列表、循环、判断、几个常用的方法 。

#商品原列表
shop_list = [
    ('电脑',5000),
    ('鼠标',50),
    ('手机',6000),
    ('显示器',2000),
    ('显卡',5888),
    ('香烟',100),
    ('眼镜',70)
]
shop_car = []
#用户输入购买金额，并判断是否为数字
cost = input("请输入圈入金额：")
if not cost.isdigit() :
    exit("输入错误，请输入数字")
else:
    cost = int(cost)
print("商品列表".center(50,'-'))
for shop_list_for in enumerate(shop_list):
    shop_num_base = shop_list_for[0]
    shop_num = shop_num_base +1     #商品编号
    shop_name = shop_list_for[1][0] #商品名
    shop_cost = shop_list_for[1][1] #商品金额
    print(shop_num,'.',shop_name,shop_cost)
for i in range(99):
    user_num = input("请选择商品编号进行购买，Q=QUIT,C=CHECK: \n")
    if user_num.isdigit():
        user_num = int(user_num)-1
        if user_num < len(shop_list):
            if cost >= int(shop_list[user_num][1]):
                cost -= shop_list[user_num][1]
                shop_car.append(shop_list[user_num])
                print('购买成功，您的余额还剩余',cost)
            else:
                print("当前余额",cost,"\n余额不足，本次购买商品如下\n",shop_car)
                exit()
        else:
            print('该商品编号不存在')
    if user_num=="q":
        print("欢迎您下次再来\n",shop_car)
        exit()
    if user_num=="c":
        print("购物车列表".center(50,'-'),"\n")
        for check_car in shop_car:
            check_name = check_car[0]
            check_cost = check_car[1]
            print(check_name,"\t",check_cost,"\n")
        print("----------当前余额：",cost,"￥----------")