why greenlet, gevent or the stackless are needed?

Sat Jul 7 04:29:11 EDT 2012

r
2012년 7월 7일 토요일 오후 4시 33분 26초 UTC+9, Devin Jeanpierre 님의 말:
> On Sat, Jul 7, 2012 at 3:09 AM, self.python <howmuchistoday at gmail.com> wrote:
> > it there somthing that "yield" can't do
> > or just it is easier or powerful?
>> couroutine-like generators can't give up control flow unless they are
> the top level function handled by the coroutine controller thing. For
> example, we can do this:
>> def foo():
> while True:
> next_value = (yield)
> print next_value
>> But we can't do this:
>> def yap():
> next_value = (yield)
> print next_value
>> def foo():
> while True:
> yap()
>> If we explicitly say that "yap" can control us, via "yield from" (new
> in Python 3.3), then we can do something like the above, but this
> still requires explicit markup. In all other releases of Python, this
> is impossible.
>> On the other hand, coroutines in greenlet et al can do a coroutine
> context switch at any point. The upside is that this is more flexible
> (and does something generators pre-3.3 cannot). The downside is that
> you now need locking structures to guarantee atomic interactions with
> a shared resource, whereas with generators you know that you always
> are the sole thing running, until you do a yield (and unless real
> threads or greenlet or whatever are involved, of course.)
>> -- Devin

first, thanks for good answer:)
but I don't understand why the code
 def yap(): 
 next_value = (yield) 
 print next_value 
 def foo(): 
 while True: 
 yap() 
really do.
what is the purpose of that code?