python求助

#!usr/bin/python

class Person:
population = 0

def __init__(self,name):
self.name=name
print 'Initializing %s' % self.name
Person.population += 1

def __del__(self):
Person.population -= 1

if Person.population == 0:
print 'I am the last one.'
else:
print "There are still %d people left." %Person.population
print '%s says bye.' % self.name
wkm = Person("wang")
ztt = Person("ztt") #第20行

程序执行结果：
Initializing wang
Initializing ztt
There are still 1 people left.
wang says bye.
Exception exceptions.AttributeError: "'NoneType' object has no attribute 'population'" in <bound method Person.__del__ of <__main__.Person instance at 0x7f6eac025368>> ignored

但是如果将第20行的对象ztt改名之后，也不一定好使，比如，改成zhaoyu之后程序正常运行，
执行结果如下：
Initializing wang
Initializing zhaoyu
There are still 1 people left.
zhaoyu says bye.
I am the last one.
wang says bye.
但如果改成z之后，执行结果就又跟ztt的时候一样了。

也就是说这段程序，出错还是不出错 跟对象名有关，这是为什么呢？

python版本：Python 2.5.2
系统信息:Ubuntu 10.04