一个简单的购物车例子练习

#coding:utf-8

'''
购物车：
显示商品和购买价格。
购入商品，加入购物车。
付款，余额不足时提示，并可重新购买。
'''

shop_cart = []
amount = 0
wallet = 100
commodity = {"大米":60,"香蕉":8,"饼干":12,"榴莲":17}

print("商品名称:\t金额(元):")
for k,v in commodity.items():
    print("%s\t\t%s"%(k,v))

while True:
    print("\n结账请输入[B]，重新购买请输入[R]，退出请输入[Q]")
    shop = input("请输入购买商品名称:")
    if isinstance(shop,str):
        if shop == "Q":
            print("\nGoodbye Baby!!")
            break
        if shop == "R":
            print("\nI am back. baby!!")
            continue
        elif shop == "B":
            for i in shop_cart:
                amount += commodity[i]
            if wallet < amount:
                print("\n余额不足！！\n供需付款 %s (元)，\n剩余金额 %s (元)"%(amount,wallet))
            else:
                wallet -= amount
                for i in shop_cart:
                    print("%s %s (元)" % (i, commodity[i]))
                print("=================\n账单：%s (元)"%amount)
                print("余额 %s (元)"%wallet)
        elif shop in commodity.keys():
            shop_cart.append(shop)
            print("\n购物车：")
            for n in shop_cart:
                print("----------\n%s\t%s (元)"%(n,commodity[n]))
        else:
            print("\n输入有误，请输入需要购买商品的名称！！！\n")
    else:
        print("\n输入有误，请输入需要购买商品的名称！！！\n")