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？