如何实现迭代对象和迭代器对象？

代码：

>>> from collections import Iterator,Iterable

>>> 

>>> 

>>> from collections import Iterator,Iterable

>>> l = [1,2,3,4,5]

>>> for x in l:
...     print(x)
... 
1
2
3
4
5

>>> isinstance(l,Iterable)
True

>>> issubclass(list,Iterable)  # 判断List是否是iterable的子类
True

>>> issubclass(str,Iterable)
True

>>> issubclass(dict,Iterable)
True

>>> issubclass(int,Iterable)
False

>>> iter(l)  # 由可迭代对象生成一个迭代器对象
<list_iterator at 0x7f7947313d68>

>>> l.__iter__()  # iter实际调用的是__iter__()方法
<list_iterator at 0x7f7946d8e668>

>>> Iterable.__abstractmethods__   # 实际调用的是抽象基类的抽象方法
frozenset({'__iter__'})

>>> it = iter(l)

>>> next(it)  # 迭代器对象的next方法
1

>>> next(it)
2

>>> next(it)
3

>>> next(it)
4

>>> next(it)
5

>>> next(it)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-131-bc1ab118995a> in <module>
----> 1 next(it)

StopIteration: 

>>> it.__next__()  # 实际调用的是迭代器对象的__next__()方法
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-132-74e64ed6c80d> in <module>
----> 1 it.__next__()

StopIteration: 

>>> it = iter(l)

>>> next(it)
1

>>> next(it)
2

>>> list(it)   # 迭代器对象是一次性消费的
[3, 4, 5]

>>> list(it)
[]

>>> it = iter(l)

>>> list(it)
[1, 2, 3, 4, 5]

>>> it = iter(l)

>>> it2 = iter(l)

>>> next(it)
1

>>> next(it)
2

>>> next(it2)   # 两个迭代器对象消费各不干扰
1

>>> it = iter(l)

>>> for x in it:  # 迭代器本身也是可以迭代的，它也是可迭代对象
...     print(x)
... 
1
2
3
4
5

>>> isinstance(it,Iterable)
True

>>> isinstance(it,Iterator)
True

>>> it.__iter__()
<list_iterator at 0x7f7946deb2e8>

>>> it.__iter__() is it  # 迭代器对象的__iter__()方法返回的是自身
True

>>>