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.populationjerry = Person(‘Jerry‘)jerry.sayHi()jerry.howMany()qiu = Person(‘Qiu‘)qiu.sayHi()qiu.howMany()jerry.sayHi()jerry.howMany()

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