[Python-Dev] yield back-and-forth?

[Phillip J. Eby]

At 10:17 AM 01/20/2006 -0800, Guido van Rossum wrote:
>The discussion about PEP 343 reminds me of the following. Bram Cohen
>pointed out in private email that, before PEP 342, there wasn't a big
>need for a shortcut to pass control to a "sub-generator" because the
>following for-loop works well enough:
>
>  def main_generator():
>       ...
>       for value in sub_generator():
>           yield value
>
>but now that yield can return a value, that value might have to be
>passed into sub_generator(), not to mention of exceptions. I'm sure
>there's a way to write that (although I haven't found the time to
>figure it out) but I expect it to be cumbersome and subtle. I don't
>think a helper function can be created to solve this problem, because
>the yield syntax is essential.
>
>Bram suggested the following syntax:
>
>   def main_generator():
>       ...
>       yieldthrough sub_generator()
>
>I'm not keen on that particular keyword, but I do believe a syntactic
>solution is needed, if the problem is important enough to be solved.

What's the use case for this?  In the coroutine use case, the PEP 342 
sample trampoline takes care of this.  If you yield a sub-generator (i.e. 
'yield sub_generator()'), the trampoline effectively replaces the parent 
with the child until the child is complete.

So, that leaves only non-coroutine use cases, and I'm having a hard time 
thinking of any where there would be bidirectional communication.


>Thoughts?

If we have to have a syntax, "yield from sub_generator()" seems clearer 
than "yieldthrough", and doesn't require a new keyword.