> from collections import Iterable, Iterator
 > a = [1,2,3]  # 众所周知,list是一个iterable
 > b = iter(a)  # 通过iter()方法,得到iterator,iter()实际上调用了__iter__(),
 > isinstance(a, Iterable)
 True
 > isinstance(a, Iterator)
 False
 > isinstance(b, Iterable)
 True
 > isinstance(b, Iterator)
 True

 > dir(a)
 ['__add__','__class__','__contains__','__delattr__','__delitem__','__dir__','__doc__','__eq__','__format__','__ge__','__getattribute__','__getitem__','__gt__','__hash__','__iadd__','__imul__','__init__','__iter__','__le__','__len__','__lt__','__mul__','__ne__','__new__','__reduce__','__reduce_ex__','__repr__', '__reversed__','__rmul__', '__setattr__','__setitem__','__sizeof__','__str__', '__subclasshook__','append','clear' 'copy','count','extend','index','insert', 'pop','remove', 'reverse','sort']
 
 >dir(b)
 ['__class__','__delattr__', '__dir__', '__doc__','__eq__', '__format__','__ge__' ,'__getattribute__', '__gt__','__hash__','__init__','__iter__','__le__','__length_hint__',
 '__lt__','__ne__','__new__','__next__','__reduce__','__reduce_ex__','__repr__','__setattr__', '__setstate__','__sizeof__','__str__','__subclasshook__']

 > c = list(b)
 > c
 [1, 2, 3]
 > d = list(b)
 > d
 []
 
 
 # 空的iterator并不等于None.
 > if b:
 ...  print(1)
 ...
 1
 > if b == None:
 ...  print(1)
 ...

 In [73]: e = iter(a)
 
 In [74]: next(e)
 Out[74]: 1
 
 In [75]: e.__next__
 Out[75]: <method-wrapper '__next__' of list_iterator object at 0x7f05571c8518>
 
 In [76]: e.__next__()
 Out[76]: 2
 
 In [77]: e.__next__()
 Out[77]: 3
 
 In [78]: e.__next__()
 ---------------------------------------------------------------------------
 StopIteration               Traceback (most recent call last)
 <ipython-input-78-6024b5bd9bd2> in <module>()
 ----> 1 e.__next__()
 StopIteration: