python 入门级问题，关于__del__
初学者，看a byte of python上的一个例子，出了个异常看不出来原因，大家帮忙看看：

Python code

#!/usr/bin/python# Filename: objvar.pyclass Person:    ''' Represents a person '''    population = 0    def __init__ (self, name):        '''Initializes the person's data.'''        self.name = name        print '(Initializeing %s)' %self.name        Person.population += 1    def __del__ (self):        '''I am dying'''        print '%s says bye.' %self.name        Person.population -= 1        if Person.population == 0:            print 'I am the last one.'        else:            print 'There are still %d people left' %Person.population                def say_hi (self):        '''Greeting by the person.'''        print 'Hi, my name is %s.' %self.name    def how_many (self):        if Person.population == 1:            print 'I am the only person here.'        else:            print 'We have %d persons here.' %Person.populationljia = Person ('ljia')ljia.say_hi ()ljia.how_many ()zhanzhao = Person ('zhanzhao')zhanzhao.say_hi ()zhanzhao.how_many ()ljia.say_hi ()ljia.how_many ()

运行结果：

Python code

(Initializeing ljia)Hi, my name is ljia.I am the only person here.(Initializeing zhanzhao)Hi, my name is zhanzhao.We have 2 persons here.Hi, my name is ljia.We have 2 persons here.ljia says bye.There are still 1 people leftzhanzhao says bye.Exception AttributeError: "'NoneType' object has no attribute 'population'" in <bound method Person.__del__ of <__main__.Person instance at 0xb76ad8ec>> ignored

前面析构可以啊，后面析构怎么就出问题了？

[解决办法]
因为执行的时候__main__里的Person已经被删了。
Person和zhanzhao一样，都是__main__ module里的两个属性。之所以程序会调用zhanzhao的__del__，就是因为这个__main__模块退出的时候已经把zhanzhao的引用从模块里删掉了。那么Person自然也要删掉，而且可能删掉的更早。
一个解决方案是把Person换成self.__class__。
不过你最好别在这种地方浪费时间。使用__del__的代码基本都是错误的。Python不是C++。