Python类方法调用报错：缺少必要位置参数'cost'排查

调用MoneyMachine类的make_payment方法时触发TypeError，提示“MoneyMachine.make_payment() missing 1 required positional argument: 'cost'”，相关代码如下：

主文件代码 #

from menu import Menu, MenuItem
from coffee_maker import CoffeeMaker
from money_machine import MoneyMachine

"""1print report
2check resources sufficies
process coins 
check transaction successful
make coffee
"""
coffee_maker = CoffeeMaker()
menu = Menu()
money_machine = MoneyMachine  # 问题所在行

is_on = True
while is_on:

    print(menu.get_items())
    order = input("your order: ")
    if order == 'off':
        is_on = False
    elif order == "report":
        coffee_maker.report()
    else:
        drink = menu.find_drink(order)
        if coffee_maker.is_resource_sufficient(drink):
            if money_machine.make_payment(drink.cost):
                coffee_maker.make_coffee(drink)

MoneyMachine类代码 #

class MoneyMachine:

    CURRENCY = "$"

    COIN_VALUES = {
        "quarters": 0.25,
        "dimes": 0.10,
        "nickles": 0.05,
        "pennies": 0.01
    }

    def __init__(self):
        self.profit = 0
        self.money_received = 0

    def report(self):
        """Prints the current profit"""
        print(f"Money: {self.CURRENCY}{self.profit}")

    def process_coins(self):
        """Returns the total calculated from coins inserted."""
        print("Please insert coins.")
        for coin in self.COIN_VALUES:
            self.money_received += int(input(f"How many {coin}?: ") * self.COIN_VALUES[coin])
        return self.money_received

    def make_payment(self, cost):
        """Returns True when payment is accepted, or False if insufficient."""
        print(self.money_received)
        self.process_coins()
        if self.money_received >= cost:
            change = round(self.money_received - cost, 2)
            print(f"Here is {self.CURRENCY}{change} in change.")
            self.profit += cost
            self.money_received = 0
            return True
        else:
            print("Sorry that's not enough money. Money refunded.")
            self.money_received = 0
            return False

错误原因 #

• 核心问题：未实例化MoneyMachine类
  主文件中money_machine = MoneyMachine只是将类本身赋值给变量，而非创建类的实例。当直接通过类调用实例方法时，Python不会自动传入self参数，此时你只传入了drink.cost，相当于只给方法传了一个参数，但make_payment需要self和cost两个参数，因此报错提示缺少cost（实际是缺少self，但报错信息会显示下一个必填参数）。

修复方案 #

将主文件中的money_machine = MoneyMachine修改为：

money_machine = MoneyMachine()

这行代码会创建MoneyMachine类的实例，之后调用money_machine.make_payment(drink.cost)时，Python会自动将实例作为self参数传入方法，参数数量匹配后即可正常运行。

额外小问题修复 #

MoneyMachine类的process_coins方法中存在逻辑错误：

# 原错误代码
self.money_received += int(input(f"How many {coin}?: ") * self.COIN_VALUES[coin])

应该先将输入转换为整数，再乘以硬币面值，否则会先执行字符串乘浮点数的操作，导致错误或逻辑异常，修正为：

self.money_received += int(input(f"How many {coin}?: ")) * self.COIN_VALUES[coin]

内容的提问来源于stack exchange，提问作者TPAL