3-1介绍可迭代对象和迭代器对象

>>> l = [1,2,3,4,5]
>>> s = '12345'
>>> iter(l)
<listiterator object at 0x02A82E50>
>>> iter(s)
<iterator object at 0x02BB7A90>
>>> iter(5)

Traceback (most recent call last):
  File "<pyshell#4>", line 1, in <module>
    iter(5)
TypeError: 'int' object is not iterable
>>> 

L列表对象，s字符串对象，都是可迭代的对象。Iter(l)或iter(s)为迭代器对象。Iter（5）会抛出异常，因为5是不可迭代的对象。

>>> l.__iter__
<method-wrapper '__iter__' of list object at 0x02BBB0F8>

__iter__()接口就是迭代的标准接口如对象没有__iter__()接口有__getitem__()序列接口也可以。iter(l)实际调用的是 l.__iter__()。

>>> help(iter)
Help on built-in function iter in module __builtin__:

iter(...)
    iter(collection) -> iterator
    iter(callable, sentinel) -> iterator
    
    Get an iterator from an object.  In the first form, the argument must
    supply its own iterator, or be a sequence.
In the second form, the callable is called until it returns the sentinel.

使用此方法，支付自身的迭代器或是一个序列。

#t为迭代器对象，只有一个接口next()接口，列表都迭代完时，会抛出一个异常。

>>> t = iter(l)  
>>> t.next()
1
>>> t.next()
2
>>> t.next()
3
>>> t.next()
4
>>> t.next()
5
>>> t.next()

Traceback (most recent call last):
  File "<pyshell#22>", line 1, in <module>
    t.next()
StopIteration

>>>for x in l : print x

for循环的机制，就是从列表l中得到迭代器，不断执行next()，直到捕获到StopIteration异常后，跳出循环。