[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