class Person(object):    __count = 0    def __init__(self, name):        self.name = name        Person.__count += 1        print Person.__countp1 = Person('Bob')p2 = Person('Alice')try:    print Person.__countexcept AttributeError:    print 'attributeerror'为什么最后会抛出attributeerror，求大神指点，谢谢