python 入门级问题，关于__del__

[jialejiahi]

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：

#!/usr/bin/python
# Filename: objvar.py

class 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.population
ljia = 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：

(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 left
zhanzhao 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？

[iambic]

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++。

[iambic]

C++用户吗？折腾del干什么。

[jialejiahi]

[Quote=引用 1 楼 iambic 的回复:]

C++用户吗？折腾del干什么。
[/Quote]
嗯，刚看python，不是想折腾它，就是这个异常不明白原因呢。
程序很短，劳烦帮忙看下。