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 GeneratorType2 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 i
obj.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******
补充：
关于generator的yield与send：
总的来说：yield返回的是个generator对象，send是向generator输送值
1 In [35]: def gen():2 ...: while True:
3 ...: print("before x_yield")
4 ...: x=yield
5 ...: print("after x_yield:", x)
6 ...: yield x*2
7 ...: print("after yield")
8 ...:
9 ...:
10
11 In [36]: g = gen()
12
13 In [37]: g.send(10)
14 ---------------------------------------------------------------------------
15 TypeError Traceback (most recent call last)
16 <ipython-input-37-98b613a2ec82> in <module>()
17 ----> 1 g.send(10)
18
19 TypeError: can't send non-None value to a just-started generator
20
21 In [38]: g.__next__(10)
22 ---------------------------------------------------------------------------
23 TypeError Traceback (most recent call last)
24 <ipython-input-38-bc5eb41f1cb6> in <module>()
25 ----> 1 g.__next__(10)
26
27 TypeError: expected 0 arguments, got 1
28
29 In [39]: g.__next__()
30 before x_yield
31 # 由上可知， “x=yield”中的yield也会成为__next__()调用的终点，也就是说如果上面要两个__next__()才能走完。
31 # 通过实验发现，其实x=yield的执行并不是同时的，比如send（10），此时相当于到达了x=yield中的yield，而此时再执行send（11）才会执行x=yield这个赋值操作
31 # 所以输出是22而不是第一次就能传入的20【见43，44的输出】32
33 In [40]: g.send(10)
34 after x_yield: 10
35 Out[40]: 20 # 进行到yield x*2
36
37 In [41]: g.__next__()
38 after yield
39 before x_yield
40
41 In [42]: g.send(10)
42 after x_yield: 10
43 Out[42]: 20
44
45 In [43]: g.send(10) #
46 after yield
47 before x_yield
48
49 In [44]: g.send(11)
50 after x_yield: 11
51 Out[44]: 22
补充：关于send与__next__()
1 In [71]: def gen():2 ...: print("enhen")
3 ...: while True:
4 ...: print("before yield")
5 ...: yield "pd the handsome"
6 ...: print("afer yield")
7 ...:
8
9 In [72]: g=gen()
10
11 In [73]: g.send(10)
12 ---------------------------------------------------------------------------
13 TypeError Traceback (most recent call last)
14 <ipython-input-73-98b613a2ec82> in <module>()
15 ----> 1 g.send(10)
16
17 TypeError: can't send non-None value to a just-started generator
18
19 In [74]: g.__next__()
20 enhen
21 before yield
22 Out[74]: 'pd the handsome'
23 # 由上可见，send(xx)并不能用于初始化调用generator, 但是通过 send(None)其实可以，纯粹的相当于74的调用__next__（）
24 # 可见，send方法实际也调用了__next__()从而执行到了下一次的yield
25 In [75]: g.send(10)
26 afer yield
27 before yield
28 Out[75]: 'pd the handsome'
29
30 In [76]: g.send(20)
31 afer yield
32 before yield
33 Out[76]: 'pd the handsome'
补充：用send实现协程【来源：寥雪峰】
1 def consumer():2 r = ''
3 while True:
4 n = yield r
5 if not n:
6 return
7 print('[CONSUMER] Consuming %s...' % n)
8 r = '200 OK'
9
10 def produce(c):
11 c.send(None)
12 n = 0
13 while n < 5:
14 n = n + 1
15 print('[PRODUCER] Producing %s...' % n)
16 r = c.send(n) // 执行一次c
17 print('[PRODUCER] Consumer return: %s' % r)
18 c.close()
19
20 c = consumer()
21 produce(c)
22
23 执行结果：
24
25 [PRODUCER] Producing 1...
26 [CONSUMER] Consuming 1...
27 [PRODUCER] Consumer return: 200 OK
28 [PRODUCER] Producing 2...
29 [CONSUMER] Consuming 2...
30 [PRODUCER] Consumer return: 200 OK
31 [PRODUCER] Producing 3...
32 [CONSUMER] Consuming 3...
33 [PRODUCER] Consumer return: 200 OK
34 [PRODUCER] Producing 4...
35 [CONSUMER] Consuming 4...
36 [PRODUCER] Consumer return: 200 OK
37 [PRODUCER] Producing 5...
38 [CONSUMER] Consuming 5...
39 [PRODUCER] Consumer return: 200 OK
挺叼，传入个generator做操作！
补充：关于yield from :http://simeonvisser.com/posts/python-3-using-yield-from-in-generators-part-1.html
参考：
http://pymbook.readthedocs.io/en/latest/igd.html
一篇关于yield的好文：
http://stackoverflow.com/questions/231767/what-does-the-yield-keyword-do?rq=1
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