[Python][转]Exception AttributeError: "'NoneType' object has no attribute

原网页地址：

http://blog.sina.com.cn/s/blog_6b839701010118hg.html

今天学习Python类的类变量时，遇到下面的错误.

class Person:

   population = 0

    def__init__(self, name):

       self.name = name

       print '(Initializeing %s)' %self.name

       Person.population += 1

    def __del__(self):

       print '%ssays bye.' % self.name

      self.__class__.population -= 1

       ifself.__class__.population == 0:

          print 'I am the lastone'

      else:

          print 'There are still %dpeople left.' % self.__class__.population

    defsayHello(self):

       if Person.population == 1:

          print 'I am the only person here.'

       else:

          print 'We have %d persons here.' %Person.population

    defhowMany(self):

        if Person.population ==1:

           print 'I am the only person here.'

        else:

           print 'We have %d persons here.' %Person.population

p1 = Person('person1')

p1.sayHello()

xp = Person('xp')

xp.sayHello()

xp.howMany()

出现如下错误：

Exception AttributeError: "'NoneType' object has no attribute'population'" in <bound method Person.__del__ of<__main__.Person instance at0xb734040c>> ignored

xp says bye.

Exception AttributeError: "'NoneType' object has no attribute'population'" in <bound method Person.__del__ of<__main__.Person instance at0xb73403ec>> ignored

debian72:~/dw/pythonTest# Exception AttributeError: "'NoneType'object has no attribute 'population'" in <boundmethod Person.__del__ of <__main__.Person instanceat 0xb734040c

原因如下：

At interpreter shutdown, the module's global variables are setto None before the module itself is released.

__del__ methods may be called in those precaries circumstances, andshould not rely on any global state.

将__del__方法中对类变量的访问方式改为如下即可：

def __del__(self): 

self.__class__.population -=1