简明 Python 教程里面的一个例子的问题

hongblue

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

class Person:
'''Represents a person.'''
population=0
def __init__(self,name):
'''Initializes the person's data.'''
self.name=name
print '(Initializing %s)' %self.name
#When this person is created, he/she adds to the population
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 sayHi(self):
'''Greeting by the person.
Really, that's all it does.'''
print 'Hi, my name is %s.' %self.name
def 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.population
swaroop=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。