Python全局变量balance报错：赋值前引用/全局声明前置问题

嘿，我来帮你搞定这个全局变量的问题！你遇到的报错根源很明确：在函数里修改全局变量时，必须在第一次使用该变量之前就声明global。

你的service()函数里，先在if amount > balance:这行使用了balance变量，之后才在取款成功的分支里声明global balance，这就触发了"name 'balance' is used prior to global declaration"的错误——Python会认为你先想用局部变量balance，之后又突然说它是全局的，逻辑冲突了。

修复方案 #

只需要在service()函数的最开头就声明global balance，这样函数内所有对balance的读取和修改都会指向全局定义的那个变量。

修正后的完整代码 #

pin = input("Set your PIN: ")
balance = 1000
wrong = 1

def service():
    global balance  # 提前声明，确保函数内所有balance操作都指向全局变量
    nextt = int(input("\nWelcome! \n Press 1 - to withdraw some money and \n Press 2 - to exit the service \n Press 3 - to show current balance \n "))
    if nextt == 1:
        amount = int(input("\nHow much money do you want: "))
        if amount > balance:
            print("Sorry, but your current balance is only: " + str(balance) + " $")
            service()
        else:
            new_balance = balance - amount
            print(f"You withdraw: {amount} from your account and your current balance is: {new_balance} $")
            balance = new_balance
            service()
    elif nextt == 2:
        print("Thank you for your visit // exiting...")
        return
    elif nextt == 3:
        print(f"Your current balance is: {balance} $")
        service()
    else:
        print("\n Failure: Press 1 to withdraw some money, 2 to exit the service and 3 to show current balance!")
        service()

def card():
    global wrong
    choose = input("\nEnter your card PIN: ")
    if choose == pin:
        service()
    else:
        print("\n You entered the wrong PIN!")
        wrong += 1
        if wrong > 3:
            print("\n You reached the max. amount to enter your PIN correctly, exiting...")
            return
        else:
            card()

card()

关键修改点说明 #

1. 提前声明全局变量：在service()函数第一行就加上global balance，这样函数内所有地方用到的balance都是全局变量，不管是读取（比如判断余额是否足够）还是修改（取款后更新余额）都不会有问题。
2. （可选优化）我把字符串拼接改成了f-string，让代码更简洁易读，功能和原来完全一致。
3. 把wrong = wrong + 1改成了wrong += 1，这是Python里更简洁的写法。

现在你运行代码试试：设置PIN后，输入正确PIN进入服务，取款后再查询余额，就能看到余额已经正确更新啦！

内容的提问来源于stack exchange，提问作者lorde