如何修复代码中的“NoneType”错误？

Question

我刚刚开始学习python并编写了一个包含三个函数的脚本。 在 Q 输入通过后，我收到一个 Traceback 错误。

这只是一个学习练习，所以我可以发展我的 Python 技能。 学习模块源代码和我的代码看起来完全一样，但由于某种原因我的输出返回了一个错误。

 File "C:\Users\Desktop\DearWorld\new.py", line 36, in <module>
    main()
  File "C:\Users\Desktop\DearWorld\new.py", line 33, in main
    total = bill_total(orders, menu)
  File "C:\Users\Desktop\DearWorld\new.py", line 24, in bill_total
    for order in orders:
TypeError: 'NoneType' object is not iterable

menu = {'Angel Dust Blunt': 6.66, 'The OG Blunt': 4.20, 'Caviar Blunt': 7.10, 'The Chronic Blunt' : 4.20}

def print_menu(menu):
    for name, price in menu.items():
        print(name, ': $', format(price, '.2f'), sep = '')

def get_order(menu):        
    orders = []
    order = input("What would you like to order (Q to quit)")

    while (order.upper()  != 'Q'):
    #Find the order and add it to the list if it exists
        found = menu.get(order)
        if found:
            orders.append(order)
        else:
            print("Menu item doesn't exist")

        order = input("Anything else? (Q to Quit)")

def bill_total(orders, menu):
    total = 0

    for order in orders:
        total += menu[order]

    return total

def main():
    menu = {'Angel Dust Blunt': 6.66, 'The OG Blunt': 4.20, 'Caviar Blunt': 7.10, 'The Chronic Blunt' : 4.20}
    print_menu(menu)
    orders = get_order(menu)
    total = bill_total(orders, menu)
    print("You ordered:" ,order, "Your total is: $", format(total, '.2f'), sep='')

main()

The script is supposed to return the bill_total and the items ordered as the output. What is returned instead when the user enters 'Q' is a 'TypeError'

Answer 1

您的get_order函数没有返回任何内容，因此该行中的orders值：

orders = get_order(menu)

将None 。 然后你试图迭代变量，它会失败。

您需要在get_order函数的末尾添加这一行：

return orders

所以函数应该是这样的：

def get_order(menu):        
    orders = []
    order = input("What would you like to order (Q to quit)")

    while (order.upper()  != 'Q'):
    #Find the order and add it to the list if it exists
        found = menu.get(order)
        if found:
            orders.append(order)
        else:
            print("Menu item doesn't exist")

        order = input("Anything else? (Q to Quit)")

    return orders  # Added this line

Answer 2

原因是orders是None ，这是因为函数get_order返回任何内容。

看看这一行：

orders = get_order(menu)

你应该添加：

return orders

在get_order函数结束时。 像这样：

def get_order(menu):        
    orders = []
    order = input("What would you like to order (Q to quit)")

    while (order.upper()  != 'Q'):
    #Find the order and add it to the list if it exists
        found = menu.get(order)
        if found:
            orders.append(order)
        else:
            print("Menu item doesn't exist")

        order = input("Anything else? (Q to Quit)")

    return orders

Answer 3

您的get_order()函数没有return语句。 因此，它将始终返回None 。 因此，在main() ：

orders = get_order(menu)
total = bill_total(orders, menu)

orders将获得None值，然后将其传递给bill_total() 。 当您尝试在 for 循环中迭代它时，结果是您看到的异常

Answer 4

迟到的答案，但您可以使用：

menu = {
        "1": {"name":"Angel Dust Blunt", "price":6.66},
        "2": {"name":"The OG Blunt", "price":6.66},
        "3": {"name":"Caviar Blunt", "price":7.10},
        "4": {"name":"The Chronic Blunt", "price":4.20}
        }
orders = []

def print_menu():
    for k, v in menu.items():
        print(f"[{k}] - ", v['name'], ': $', format(v['price'], '.2f'), sep = '')

def bill_total():
    total = 0
    for id in orders:
        total += menu[id]['price']
    return total

def get_order():
    while 1:
        order = input(f"What would you like to order (Q to quit)\n")
        if order.strip().upper()  == 'Q':
            return
        else:
            found = menu.get(order)
            if found:
                # avoid dup orders, can be improved to increase the count of product items.
                if order in orders:
                    print("Product already in cart")
                    continue
                orders.append(order)
                print(f"Item {found['name']} added to cart")
            else:
                print("Menu item doesn't exist")

print_menu(), get_order(), print("You ordered:\n")
for id in orders:
    print(menu[id]['name'], f"${menu[id]['price']}")
print(f"\nYour total is: ${bill_total()}" )

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1. Python 演示
2. Asciinema （只是为了好玩）