[Python-ideas] The async API of the future: yield-from

[Guido van Rossum]

On Sun, Oct 14, 2012 at 10:27 AM, Terry Reedy <tjreedy at udel.edu> wrote:
> On 10/14/2012 10:36 AM, Guido van Rossum wrote:
>
>> So, can par() be as simple as
>>
>> def par(*args):
>>    results = []
>>    for task in args:
>>      result = yield from task
>>      results.append(result)
>>    return results
>>
>> ???
>>
>> Or does it need to interact with the scheduler to ensure fairness?
>> (Not having built one of these, my intuition for how the primitives
>> fit together is still lacking, so excuse me for asking naive
>> questions.)
>>
>> Of course there's the question of what to do when one of the tasks
>> raises an error -- I haven't quite figured that out in NDB either, it
>> runs all the tasks to completion but the caller only sees the first
>> exception. I briefly considered having an "multi-exception" but it
>> felt too weird -- though I'm not married to that decision.
>
>
> One answer is to append the exception object to results and let the
> requesting code sort out what to do.
>
>
> def par(*args):
>    results = []
>    for task in args:
>       try:
>
>          result = yield from task
>          results.append(result)
>       except Exception as exc:
>          results.append(exc)
>    return results

But then the caller would have to sort through the results and check
for exceptions. I want the caller to be able to use try/except as
well.

So far the best I've come up with is to recommend that if you care
about distinguishing multiple exceptions, use separate yields
surrounded by separate try/except blocks. Note that the tasks can
still run concurrently, just create all the futures before doing the
first yield.

-- 
--Guido van Rossum (python.org/~guido)