在全局声明之前使用名称“余额”答案

【问题标题】：name 'balance' is used prior to global declaration在全局声明之前使用名称“余额”
【发布时间】：2018-10-21 05:53:51
【问题描述】：

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

def service():
  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:
      print("You withdraw: " +str(amount) +" from your account and your 
             current balance is: " +str(balance-amount)+" $")
      global balance
      balance -= amount
      service()
  elif nextt == 2:
    print("Thank you for your visit // exiting...")
    return
  elif nextt == 3:
    print("Your current balance is: " +str(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 = wrong + 1
    if wrong > 3:
      print("\n You reached the max. amount to enter your PIN correctly, 
            exiting...")
      return
    else:
      card()


card()

我无法修复这个特殊错误：

on line 14: balance -= amount

我想在提取一些现金后更新余额，但它说：

  local variable 'balance' referenced before assignment

我加了

  global balance 
  balance -= amount

新错误：

name 'balance' is used prior to global declaration

我要做的就是：在那里提取一些现金后更新当前余额！

【问题讨论】：

为什么不将global balance 移动到函数定义的顶部（而不是将其埋在else 子句中）？
你说的对，我是说看卡片功能，全局错误，我做对了，搞砸了服务功能 OMG xD

标签： python python-3.x

【解决方案1】：

上面几行，你隐含地告诉 Python balance 是一个局部变量：

if amount > balance:

在这个命名空间（函数作用域）中之前没有见过它，所以它是一个局部变量。当你进入else 子句时，你突然给 Python 跳了一段“我撒谎”的小舞，宣布它是全球性的。 Python 不喜欢这种滑稽动作。

如果您希望 balance 成为全局变量（不好的做法），请按照编码指南中的建议在块的顶部声明它。更好的是，将其作为函数参数传入并在完成后返回。

【讨论】：

【解决方案2】：

有时准确列出正在运行的代码很方便：

nextt = int(input("\nWelcome! ..."))  # "nextt" is being set in the local scope
if nextt == 1:                        # "nextt" is being compared to 1
    amount = int(input("\nHow ..."))  # "amount" is being set in the local scope
    if amount > balance:              # "amount" is being compared to "balance"
                                      # Uh oh! We haven't defined "balance" yet!
        print("Sorry, ...")
        ...

要解决此问题，您可以使用global 关键字，它会在全局范围内查找变量。在找到解决方案之前，让我们看一个例子来说明它是如何工作的：

def f1():
    a = 2
    print("Value of a: {}; ID of a: {}".format(a, id(a)))

def f2():
    global a
    print("[Before] Value of a: {}; ID of a: {}".format(a, id(a)))
    a = 3
    print("[After] Value of a: {}; ID of a: {}".format(a, id(a)))

>>> a = 1  # we define "a" in the global scope

>>> a, id(a)  # id(variable) gets a unique value for this variable
(1, 1635934432)

>>> f1()
Value of a: 2; ID of a: 1635934464  # So you see that the ID is different, because this is f1's version of "a", not the global one

>>> a, id(a)
(1, 1635934432)  # the value and ID of the global variable remain unchanged

>>> f2()
[Before] Value of a: 1; ID of a: 1635934432  # see the value and ID of the local "a" are now the same as the global variable
[After] Value of a: 3; ID of a: 1635934496  # we replaced the value and ID here

>>> a, id(a)
(3, 1635934496)  # which has affected the global version now as well

最后，完整的解决方案：

global balance                        # "balance" is set to the global value
nextt = int(input("\nWelcome! ..."))  # "nextt" is being set
if nextt == 1:                        # "nextt" is being compared to 1
    amount = int(input("\nHow much ...: "))  # "amount" is being set
    if amount > balance:              # "amount" is being compared to "balance"
        print("Sorry, ...")
        ...

【讨论】：

【解决方案3】：

简单的解决方法（如果您坚持使用全局变量）是将全局语句重新定位到 def service 函数中的第一条语句：

def service():
   global balance

您的代码非常随意地使用全局变量，这是一个坏主意。已经提出了其他建议来纠正这个问题，所以我不会进一步扩展:)

【讨论】：