Python 名称'；余额'；在全局声明之前使用

我无法纠正这个错误：

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()

我想在提取一些现金后更新余额，但上面写着：

on line 14: balance -= amount 

我补充说

  local variable 'balance' referenced before assignment

新错误：

  global balance 
  balance -= amount

---

我想做的就是：在那里提取一些现金后更新当前余额

在上面的几行中，您隐式地通知Python，

balance

是一个局部变量：

name 'balance' is used prior to global declaration

以前在这个名称空间（函数范围）中没有见过它，所以它是一个局部变量。当你进入

else

子句时，你突然让Python跳了一段“我撒谎”的舞蹈，宣称它是全球性的。巨蟒对这种滑稽动作不感兴趣

---

如果希望

balance

成为一个全局变量（错误做法），则按照编码指南中的建议，在块的顶部声明它。更好的方法是，将其作为函数参数传入，并在完成后返回。

有时可以方便地准确列出正在运行的代码：

if amount > balance:

要解决这个问题，可以使用

global

关键字，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，说明这是如何工作的：

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, ...")
        ...

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：

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, ...")
        ...

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

简单的修复方法（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：

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, ...")
        ...

---

您的代码非常自由地使用globals，这是一个坏主意。有人提出了其他建议来纠正这一点，因此我不再进一步详述：）

为什么不将

全局平衡

移到函数定义的顶部（而不是将其埋在

else

子句中）？你完全正确，我的意思是看一下卡函数，全局错误，我做得对，把服务功能OMG xD搞砸了