Python python __def__ Exception AttributeError: "'NoneType' object has no attribute

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

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

        Person.population +=1

    def __del__(self):
        ‘‘‘I am dying.‘‘‘
        print ‘%s says bye.‘ % self.name

        Person.population -=1

        if Person.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

jerry = Person(‘Jerry‘)
jerry.sayHi()
jerry.howMany()

qiu = Person(‘Qiu‘)
qiu.sayHi()
qiu.howMany()

jerry.sayHi()
jerry.howMany()

出现如下错误：

Exception AttributeError: "‘NoneType‘ object has no attribute ‘population‘" in <bound method Person.__del__ of <__main__.Person instance at 0x01AF97D8>> ignored

原因如下：
At interpreter shutdown, the module‘s global variables are set to None before the module itself is released.
__del__ methods may be called in those precaries circumstances, and should 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

Python python __def__ Exception AttributeError: "'NoneType' object has no attribute