Python类的定义与使用

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

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

    def __init__(self, name):
        ''' Initializes the person's data. '''
        self.name = name
        print '(Initializeing %s)' % self.name

        # When this person is created, he/she adds to the population
        Person.population += 1
    
    def __del__(self):
        ''' I am dying. '''
        print '%s says bye.' % self.name

        self.__class__.population -= 1

        if self.__class__.population == 0:
            print 'I am the last one.'
        else:
            print 'There are still %d people left.' % Person.population
    
    def sayHi(self):
        ''' Greeting by the person.
        Really, that's all it does. '''
        print 'Hi, my name is %s.' % self.name

    def howMany(self):
        ''' Prints the current population. '''
        if Person.population == 1:
            print 'I am the only person here.'
        else:
            print 'We have %d persons here.' % Person.population

# The definion of class is end

zjw = Person('zjw')
zjw.sayHi()
zjw.howMany()

lbz = Person('lbz')
lbz.sayHi()
lbz.howMany()

zjw.sayHi()
zjw.howMany()

　　程序刚开始是有错误的，就是在__del__函数中如果使用Person.population来调用全局变量的话，会出现下面这个错误

Exception AttributeError: "'NoneType' object has no attribute 'population'" in <bound method Person.__del__ of <__main__.Person instance at 0xb72e9aec>> ignored

在网上搜了一下，应该是说在del函数中不能访问全局变量，因此就修改成了现在这个这样，使用对象的一个方法间接的访问到了全局变量。

发现现在网上到处都是转载的文章，很少用真正原创的，我想说的是，就算是转载的，也请你看懂了之后再转，否则还是半知半解，只是在网络上图增垃圾而已。这个问题我还没搞明白到底的原因，如果哪位知道是什么原因，请留言告诉我，谢谢。