[Python-ideas] More general "for" loop handling

[Guido van Rossum]

On Fri, May 1, 2015 at 8:52 AM, Joseph Martinot-Lagarde <
joseph.martinot-lagarde at m4x.org> wrote:

> Le 01/05/2015 02:35, Steven D'Aprano a écrit :
>
>>
>> If you still wish to argue for this, one thing which may help your case
>> is if you can identify other programming languages that have already
>> done something similar.
>>
>>
>>  Cython has prange. It replaces range() in the for loop but runs the loop
> body in parallel using openmp:
>
> from cython.parallel import prange
>
> cdef int func(Py_ssize_t n):
>     cdef Py_ssize_t i
>
>     for i in prange(n, nogil=True):
>         if i == 8:
>             with gil:
>                 raise Exception()
>         elif i == 4:
>             break
>         elif i == 2:
>             return i
>
> This is an example from the cython documentation:
> http://docs.cython.org/src/userguide/parallelism.html
>

Interesting. I'm trying to imagine how this could be implemented in CPython
by turning the for-loop body into a coroutine. It would be a complicated
transformation because of the interaction with local variables in the code
surrounding the for-loop. Perhaps the compiler could mark all such
variables as implicitly nonlocal. The Cython example also shows other
interesting issues -- what should return or break do?

In any case, I don't want this idea to distract the PEP 492 discussion --
it's a much thornier problem, and maybe coroutine concurrency isn't what we
should be after here -- the use cases here seem to be true (GIL-free)
parallelism. I'm imagining that pyparallel has already solved this (if it
has solved anything :-).

-- 
--Guido van Rossum (python.org/~guido)
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20150501/47b8762d/attachment.html>