简明 Python 教程里面的一个例子的问题

sql

例子的链接：
http://www.woodpecker.org.cn:9081/doc/abyteofpython_cn/chinese/ch11s06.html

class Person:'''Represents a person.'''population=0def __init__(self,name):'''Initializes the person's data.'''self.name=nameprint '(Initializing %s)' %self.name#When this person is created, he/she adds to the populationPerson.population+=1def __del__(self):'''I am dying.'''print '%s says bye.' %self.namePerson.population-=1if Person.population==0:print 'I am the last one.'else:print 'There are still %d people left.' %Person.populationdef sayHi(self):'''Greeting by the person.Really, that's all it does.'''print 'Hi, my name is %s.' %self.namedef howMany(self):'''Prints the current population.'''if Person.population==1:print 'I am the only person here.'else:print 'We have %d persons here.' %Person.populationswaroop=Person('Swaroop')swaroop.sayHi()swaroop.howMany()kalam=Person('Abdul Kalam')kalam.sayHi()kalam.howMany()swaroop.sayHi()swaroop.howMany()
运行起来完全没有问题

但如果把测试那一段改为：
test=Person('Swaroop')test.sayHi()test.howMany()kalam=Person('Abdul Kalam')kalam.sayHi()kalam.howMany()test.sayHi()test.howMany()
运行的时候就会得到一个异常：
Exception exceptions.AttributeError: "'NoneType' object has no attribute 'popula
tion'" in <bound method Person.__del__ of <__main__.Person instance at 0x00BEC12
0>> ignored

然后改很多次那个变量都有这个异常，好像那个变量只有用swaroop才没有问题，非常怪异。。。。
有没有人知道这是怎么回事？

我用的python版本： Python 2.5.2 windows下面的。