[请教]映射list为何会影响不相关的类

初学python，写了如下代码，但是运行过程中抛出异常：

import sys,os import random class Person:     population = 0     def __init__(self, name):         self.name = name         erson.population += 1     def __del__(self):         print "%s says goodbye" %self.name         erson.population -= 1              def print_name(self):         print "population=", Person.population, ", name=", self.name          p = Person("aaronpan") p.print_name() p2 = Person("yangyan") p2.print_name() randnum = [] for index in range(0,100):     randnum.append(random.randint(0, 99))   print "len(randnum)= ", len(randnum) randnum.sort() num_count = [0 for i in range(0, 100)]     #注释1 #for item in randnum: #    num_count[item] += 1      #print num_count #alias_person = Person p3 = Person("new_s") #print type(Person) #print type(alias_person) p3.print_name() p4 = Person("delete") p4.print_name()

异常如下：
population= 1 , name= aaronpan
population= 2 , name= yangyan
len(randnum)=  100
population= 3 , name= new_s
population= 4 , name= delete
yangyan says goodbye
delete says goodbye
aaronpan says goodbye
new_s says goodbye
Exception exceptions.AttributeError: "'NoneType' object has no attribute 'population'" in <bound method Person.__del__ of <__main__.Person instance at 0x00B538C8>> ignored
如果将“注释1”行代码注释掉，或将p3，p4两个对象放在“注释1”行的前面，则不抛异常

哪位能帮我解释一下啊？

我这儿跑的没有异常

可能是解释器的原因，这个__del__是解释器调的，在程序要退出的时候，C++这样写没啥问题，python就不一定了，如果你__del__里只是输出名字，没有变量的引用的话，是没问题的。