1.需求：
    1：啟動程式後，讓使用者輸入工資，然後列印商品列表
    2：允許使用者根據商品編號購買商品
    3：使用者選擇商品後，檢測餘額是否夠，夠就直接付款，不夠就提醒
    4：可隨時推出，推出時，列印已購買商品和餘額

2.程式碼如下：

__author__ = "B J"


product_list = [
    ('Iphone',5800),
    ('Mac Pro',9800),
    ('Bike',800),
    ('Watch',10600),
    ('Coffee',31),
]
shopping_list = []
salary = input("Input your salary:
 
")
if salary.isdigit():
    salary = int(salary)
    while True:
        for index,item in enumerate(product_list):
            #print(product_list.index(item),item)
            print(index,item)
        user_choice = input("選擇要買嘛？>>>:")
        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: #買的起
                    shopping_list.append(p_item)
                    salary -= p_item[1]
                    
 
print("Added %s into shopping cart,your current balance is \033[31;1m%s\033[0m" %(p_item,salary) )
                else:
                    print("\033[41;1m你的餘額只剩[%s]啦，還買個毛線\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: