python __def__ Exception AttributeError: "'NoneType' object has no attribute

 14         def __del__(self):
 15                 '''delete a person.'''
 16                 print 'delete person %s' % self.name
 17                 self.__class__.population -= 1
 18                 if self.__class__.population == 0:
 19                         print 'there is no one in here'
 20                 else:
 21                         print 'there are %d in here' % self.__class__.popula    tion

开始的时候我是用的Object.population。所以，出现了上面的错误。后来网上查了下资料，要将其改为：

self.__class__.population才行。