python中的generator, iterator, iterabel

先来看看如果遇到一个对象，如何判断其是否是这三种类型：

1 from types import GeneratorType
2 from collectiuons import Iterable, Iterator
3
4 isinstance( xx, GeneratorType )
5 isinstance( xx, Iterable )
6 isinstance( xx, Iterator )

生成器对象：

　　生成器是一个通过yield关键字构建的函数，其返回一个generator对象，同时其又是一个iterator对象，因为其实现了__iter__与next方法

In[4]: def gene(k):
...     for i in xrange(0, k):
...         yield i
...
In[5]: ge = gene(6)
In[6]: type(ge)
Out[6]: generator
In[8]: dir(ge)
Out[8]: [ ...,‘__iter__‘, ‘next‘, ... ]
In[9]: isinstance(ge, collections.Iterator)
Out[9]: True

　　生成器的一个特点是只能用一次

In[11]: for i in ge:
...     print i
...
0
1
2
3
4
5
In[12]: ge.next()
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/IPython/core/interactiveshell.py", line 2820, in run_code
    exec code_obj in self.user_global_ns, self.user_ns
  File "<ipython-input-12-b9172cd6050f>", line 1, in <module>
    ge.next()
StopIteration

迭代器对象：

　　实现了__iter__与__next__方法的对象。

　　__iter__返回一个实例作为最终使用的iterator对象--for...in...语句初始化时使用，next方法是后面每次迭代iterator对象时调用的方法

class Counter(object):
    def __init__(self, low, high):
        self.current = low
        self.high = high

    def __iter__(self):
        # ‘Returns itself as an iterator object‘
        return self

    def next(self):
        # ‘Returns the next value till current is lower than high‘
        if self.current > self.high:
            raise StopIteration
        else:
            self.current += 1
            return self.current - 1

>>> c = Counter(5,10)
>>> for i in c:
...   print(i, end=‘ ‘)
...
5 6 7 8 9 10>>> for i in c:...　　pritn i...# 无输出>>> c.next()

Traceback (most recent call last):
  File "<ipython-input-12-b9172cd6050f>"
    c.next()
StopIteration

可迭代对象：

　　可迭代对象就是用于for...in...循环的

　　不同于上面两种，其需要可以多次for...in...使用

　　通过generator写可迭代对象：

　　将__iter__写成生成器函数。注意for...in...时初始化时调用__iter__函数使得counter为low, 而后执行的都是yield所影响区域的了^【1】，除非下次再执行for...in...语句才会调用到counter初始化那句，这样也就实现了复用。但是有个问题是此时Counter的实例就既不是generator也不是iterator对象了。故其也没法调用next方法

class Counter(object):
    def __init__(self, low, high):
        self.low = low
        self.high = high

    def __iter__(self):
        counter = self.low
        while self.high >= counter:
            yield counter
            counter += 1

obj = Counter(5, 10)
print isinstance(obj, Iterator)
print isinstance(obj, GeneratorType)
print type(obj)
for i in obj:
    print iobj.next()

False
False
<class ‘__main__.Counter‘>
5
6
7
8
9
10

Traceback (most recent call last):
File "/home/pd/..."
obj.next()
AttributeError: ‘Counter‘ object has no attribute ‘next‘

注释【1】：

In [1]: def gener(k):
   ...:     print "====initial===="
   ...:     for i in range(0, k):
   ...:         print "before yield"
   ...:         yield i
   ...:         print "after yield"
   ...:     print "*****end******"
   ...:                                                                          

In [2]: g = gener(3)
In [3]: for i in g:
   ...:     print g
   ...:
====initial====
before yield
<generator object gener at 0x7fbc8d15e870>
after yield
before yield
<generator object gener at 0x7fbc8d15e870>
after yield
before yield
<generator object gener at 0x7fbc8d15e870>
after yield
*****end****** 

参考：

　　http://pymbook.readthedocs.io/en/latest/igd.html