python 类的析构问题

julley

在练习python的时候遇到一个问题，请教大家。
   先贴代码：
class Person:
    population = 0

    def __init__(self, name):
        self.name = name
        print "Add name", self.name
        Person.population += 1

    def __del__(self):
        print "Remove name", self.name
        Person.population -= 1
        if Person.population == 0:
            print "I'm the only one."
        else:
            print "There are", Person.population, "person left."


    def sayHi(self):
        print "Hello, my name is", self.name
        if Person.population == 1:
            print "I'm the last one."
        else:
            print "There are", Person.population, "person left."

    def howMany(self):
        if Person.population == 1:
            print "There is only one person here."
        else:
            print "There is %d person here" % Person.population

    b = Person("brian")
    b.sayHi()
    b.howMany()

    t = Person("Tom")
    t.sayHi()
    t.howMany()

    b.sayHi()
    b.howMany()


打印的结果为：

Add name brian
Hello, my name is brian
I'm the last one.
There is only one person here.
Add name Tom
Hello, my name is Tom
There are 2 person left.
There is 2 person here
Hello, my name is brian
There are 2 person left.
There is 2 person here
Remove name brian
There are 1 person left.
Remove name Tom
Exception AttributeError: "'NoneType' object has no attribute 'population'" in <bound method Person.__del__ of <__
main__.Person instance at 0xb738380c>> ignored

我觉得可能是：
Person类已经被从内存中清除，所以也就找不到该类对应的变量population。


请求大牛解答，谢谢。