why greenlet, gevent or the stackless are needed?

[Devin Jeanpierre]

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