python Exception AttributeError: "✀NoneType✀ object has no attribute ✀num_accounts✀"

class Account(object):
    num_accounts = 0
   派尘 def __init__(self, name, balance):
        self.name = name
        self.balance = balance
   没羡薯     Account.num_accounts += 1
        print '__init__:',Account.num_accounts
    def __del__(self):
        self.__class__.num_accounts -= 1
        print '__del__:',self.__class__.num_accounts
    枯者def deposit(self, amt):
        self.balance = self.balance + amt
        print self.balance
    def withdraw(self, amt):
        self.balance = self.balance - amt
        print self.balance
    def inquiry(self):
        return self.balance
        print self.balance

a = Account("Guido", 1000.00)
print 'a:',a
b = Account("Guiio", 10.00)
print 'b:',b

使用域变量时,可以通过self的__class__域来引用
与python的内存回收按照字典先后顺序有关
把__del__中的Account.num_accounts换成self.__class__.num_accounts，将不会出错！

运行结果如下：

__init__: 1

a: <__main__.Account object at 0x023EFBD0>

__init__: 2

b: <__main__.Account object at 0x023EFBB0>

__del__: 1

__del__: 0