python - 使用生成器发送方法。仍在尝试理解发送方法和古怪的行为

Question

这是我为理解发送方法而写的一个小函数:

>>> def test():
...     for x in xrange(10):
...         res = yield
...         yield res
>>> a = test()
>>> next(a)
>>> next(a)
>>> next(a)
>>> next(a)
>>> a.send(0)
Traceback (most recent call last):
   <ipython-input-220-4abef3782000> in <module>()
StopIteration
>>> a = test()
>>> a.send(0)
Traceback (most recent call last):
   <ipython-input-222-4abef3782000> in <module>()    
TypeError: can't send non-None value to a just-started generator
>>> a.send(None)
>>> a.send(0)
0
>>> a.send(0)
>>> a.send(0)
0
>>> a.send(0)
>>> a.send(0)
0
>>> a.send(0)

为什么第一次报错？

>>> a.send(0)
StopIteration 

为什么要求第一个 send() 为 None？与此错误一样:

>>> a.send(0)
Traceback (most recent call last):
    <ipython-input-222-4abef3782000> in <module>()
TypeError: can't send non-None value to a just-started generator

然后第一个发送启动生成器(我不知道为什么)我发送一个“0”并打印它但是第二个 0 再次没有并恢复我发送的任何内容(此处为 0)

>>> a.send(None)
>>> a.send(0)
0
>>> a.send(0)
>>> a.send(0)
0
>>> a.send(0)
>>> a.send(0)
0

此链接帮助不大 Python 3: send method of generators

Answer 1

为什么要求第一个 send() 为 None？

您不能第一次发送()一个值，因为生成器直到您有 yield 语句的那一点才执行，所以与该值无关。

以下是 pep 中的相关段落，介绍了与生成器协同例程的特性 (http://www.python.org/dev/peps/pep-0342/):

Because generator-iterators begin execution at the top of thegenerator's function body, there is no yield expression to receivea value when the generator has just been created. Therefore,calling send() with a non-None argument is prohibited when thegenerator iterator has just started, and a TypeError is raised ifthis occurs (presumably due to a logic error of some kind). Thus,before you can communicate with a coroutine you must first callnext() or send(None) to advance its execution to the first yieldexpression

一个小的演练:

def coro():
   print 'before yield'
   a = yield 'the yield value'
   b = yield a
   print 'done!'
 c=coro() # this does not execute the generator, only creates it

 # If you use c.send('a value') here it could _not_ do anything with the value
 # so it raises an TypeError! Remember, the generator was not executed yet,
 # only created, it is like the execution is before the `print 'before yield'`

 # This line could be `c.send(None)` too, the `None` needs to be explicit with
 # the first use of `send()` to show that you know it is the first iteration
 print next(c) # will print 'before yield' then 'the yield value' that was yield

 print c.send('first value sent') # will print 'first value sent'

 # will print 'done!'
 # the string 'the second value sent' is sent but not used and StopIterating will be raised     
 print c.send('the second value sent') 
 
 print c.send('oops') # raises StopIterating