如何自定义python迭代器的类

class example(object):
    def __init__(self,num):
        self.num=num
    def __iter__(self):
        return self
    def __next__(self):
        if self.num <= 0:
            raise StopIteration
        tmp = self.num
        self.num -= 1
        return tmp
    
a = example(3)
print(a.__next__())
print(a.__next__())
print(a.__next__())
print(a.__next__())