Iterable—可迭代 可迭代的序列类型包括，list,str,tuple,非序列类型包括dict,file,任何类只是你定义了iter方法就会被看作为可迭代对象。
In [3]: from collections import Iterable, Iterator
In [4]: class A:
...: def __init__(self):
...: self.n = 0
...: def __iter__(self):
...: return self
...:
In [5]: a = A()
In [6]: isinstance(a, Iterable)
Out[6]: True
Iterator—迭代器 Iterator必须实现iter()和next()(python3)或next()(python2)两个方法，组成了迭代器协议。
In [13]: class yrange:
...: def __init__(self, n):
...: self.i = 0
...: self.n = n
...:
...: def __iter__(self):
...: return self
...:
...: def __next__(self):
...: if self.i < self.n:
...: i = self.i
...: self.i += 1
...: return i
...: else:
...: raise StopIteration()
...:
In [14]: it = yrange(3)
In [15]: it.__next__()
Out[15]: 0
In [16]: it.__next__()
Out[16]: 1
In [17]: it.__next__()
Out[17]: 2
In [18]: it.__next__()
---------------------------------------------------------------------------
StopIteration Traceback (most recent call last)
<ipython-input-18-a1cc91b7bc73> in <module>()
----> 1 it.__next__()
<ipython-input-13-55955e9c2cc7> in __next__(self)
13 return i
14 else:
---> 15 raise StopIteration()
16
StopIteration:
反向迭代一个序列，使用内置的reversed()函数，可以通过在自定义类上实现reversed () 方法来实现反向 迭代。
In [19]: class yrange:
...: def __init__(self, n):
...: self.i = 0
...: self.n = n
...:
...: def __iter__(self):
...: return self
...:
...: def __next__(self):
...: if self.i < self.n:
...: i = self.i
...: self.i += 1
...: return i
...: else:
...: raise StopIteration()
...: def __reversed__(self):
...: index = self.n - 1
...: while index >= 0:
...: yield index
...: index -= 1
...:
In [20]: it = reversed(yrange(3))
In [21]: it.__next__()
Out[21]: 2
In [22]: it.__next__()
Out[22]: 1
In [23]: it.__next__()
Out[23]: 0
generator—生成器 生成器是一个返回迭代器对象的函数。它和普通函数看起来一样，除了它包含了yield语句，产生可以通过for循环访问的值，或者可以通过调用next方法一次访问一个值。 生成的iter()和next()方法是自动创建的。。每次next()调用时，生成器再恢复它离开的位置（它记忆语句最 后一次执行的位置和所有的数据值）
In [25]: def integer():
...: i = 1
...: while True:
...: yield i
...: i += 1
...:
In [26]: i = integer()
In [27]: i
Out[27]: <generator object integer at 0x7fb940696fc0>
In [29]: i.__next__()
Out[29]: 1
生成器—send方法,调用send()方法前，必须先调用next()方法（pep342)
In [37]: def double_input():
...: while True:
...: x = yield
...: yield x * 2
In [59]: gen = double_input()
In [60]: gen.__next__()
In [61]: gen.send(4)
Out[61]: 8
In [62]: gen.send(4)
In [63]: gen.send(4)
Out[63]: 8
In [64]: gen.__next__()
In [65]: gen.send(5)
Out[65]: 10
Ref： 1.http://anandology.com/python-practice-book/iterators.html 2.官方文档