实例的命名出问题，相当奇怪。

CODE:

#!/usr/bin/python
#Filename: objvar.py

class Person:
    '''Represents a person.'''
    population = 0

    def __init__(self,name):
        '''Initializes person's data.'''
        self.name=name
        print "(initializing %s)" %self.name
        #when a person is create , add it to population
        Person.population += 1

    def __del__(self):
        '''I am dying.'''
        print "%s say goodbye" %self.name
        Person.population -= 1

        if Person.population == 0:
            print "I am the last one"
        else:
            print "there're %d person left" %Person.population

    def sayHi(self):
        '''greeting by the people.'''
        print "Hi, My name is %s" %self.name

    def count_person(self):
        '''print the number of people'''
        if Person.population == 1:
            print "i am the only one person"
        else:
            print "there're %d of people now" %Person.population

aries = Person("aries")
aries.sayHi()

CODE:

Exception exceptions.AttributeError: "'NoneType' object has no attribute 'population'" in <bound method Person.__del__ of <__main__.Person instance at 0xb7d7998c>> ignored