迭代器协议：__iter__和__next__

class Foo:
    def __init__(self,x):
        self.x = x
    def __iter__(self):
        return self
    def __next__(self):
        if self.x == 15:
            raise StopIteration('数字过大')     #抛出异常
        self.x += 1
        return self.x

f1 = Foo(10)
print(f1.__next__())        #11
print(f1.__next__())        #12
print(f1.__next__())        #13
print(next(f1))             #14
print(next(f1))             #15
print(next(f1))             #StopIteration: 数字过大
for i in f1:
    print(i)
# 11
# 12
# 13
# 14
# 15

## 迭代器实现斐波那契数列
class Foo:
    def __init__(self):
        self._a = 1
        self._b = 1
    def __iter__(self):
        return self
    def __next__(self):
        if self._a > 100:
            raise StopIteration('程序终止')
        self._a,self._b = self._b,self._a + self._b
        return self._a
f1 = Foo()
print(f1.__next__())
print(f1.__next__())
print(f1.__next__())
print(f1.__next__())
print(f1.__next__())
print(f1.__next__())
print('-------------')
for i in f1:            #for循环遍历f1是接着上面的内容继续，不会从头开始也不会往回返
    print(i)