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。