[Python-ideas] .then execution of actions following a future's completion

Guido van Rossum guido at python.org
Fri Jan 26 11:59:22 EST 2018

@Bar: I don't know about exposing _chain_future(). Certainly it's overkill
for what the OP wants -- their PR only cares about chaining

@Daniel: I present the following simpler solution -- it requires you to
explicitly pass the executor, but since 'fn' is being submitted to an
executor, EIBTI.

def then(executor, future, fn):
    newf = concurrent.futures.Future()
    def callback(fut):
        f = executor.submit(fn, fut)
        except CancelledError:
        except Exception as err:
    return executor.submit(callback)

I can't quite follow your reasoning about worker threads (and did you
realize that because of the GIL, Python doesn't actually use multiple
cores?). But I suppose it doesn't matter whether I understand that -- your
point is that you want the 'fn' function submitted to the executor, not run
as a "done callback". And that's reasonable. But modifying so much code
just so the Future can know which to executor it belongs so you can make
then() a method seems overkill.

On Fri, Jan 26, 2018 at 8:54 AM, Daniel Collins <dancollins34 at gmail.com>

> So, just going point by point:
> Yes, absolutely put this off for 3.8. I didn’t know the freeze was so
> close or I would have put the 3.8 tag on originally.
> Yes, absolutely it is only meant for concurrent.futures futures, it only
> changes async where async uses concurrent.futures futures.
> Here’s a more fleshed out description of the use case:
> Assume you have two functions. Function a(x: str)->AResult fetches an
> AResult object from a web resource, function b(y: AResult) performs some
> computationally heavy work on AResult.
> Assume you’re calling a 10 times with a threadpoolexecutor with 2 worker
> theads.  If you were to schedule a as future using submit, and b as a
> callback, the executions would look like this:
> ExecutorThread: b*10
> Worker1: a*5
> Worker2: a*5
> This only gets worse as more work (b) is scheduled as a callback for the
> result from a.
> Now you could resolve this by, instead of submitting b as a callback,
> submitting the following lambda:
> lambda x: executor.submit(b, x)
> But then you wouldn’t have easy access to this new future. You would have
> to build a lot of boilerplate code to collect that future into some
> external collection, and this would only get worse the deeper the nesting
> goes.
> With this syntax on the other hand, if you run a 10 times using submit,
> but then run a_fut.then(b) for each future, execution instead looks like
> this:
> ExecutorThread:
> Worker1: a*5 b*5
> Worker2: a*5 b*5
> You can also do additional depth easily. Suppose you want to run 3 c
> operations (processes the output of b) for each b operation. Then you could
> call this like
> b_fut = a_fut.then(b)
> for i in range(3):
>     b_fut.then(c)
> And the execution would look like this:
> ExecutorThread:
> Worker1: a*5 b*5 c*15
> Worker2: a*5 b*5 c*15
> Which would be very difficult to do otherwise, and distributes the load
> across the workers, while having direct access to the outputs of the calls
> to c.
> -dancollins34
> Sent from my iPhone
> On Jan 26, 2018, at 1:07 AM, Guido van Rossum <guido at python.org> wrote:
> I really don't want to distract Yury with this. Let's consider this (or
> something that addresses the same need) for 3.8.
> To be clear this is meant as a feature for concurrent.futures.Future, not
> for asyncio.Future. (It's a bit confusing since you also change asyncio.)
> Also to be honest I don't understand the use case *or* the semantics very
> well. You have some explaining to do...
> (Also, full links: https://bugs.python.org/issue32672;
> https://github.com/python/cpython/pull/5335)
> On Thu, Jan 25, 2018 at 8:38 PM, Daniel Collins <dancollins34 at gmail.com>
> wrote:
>> Hello all,
>> So, first time posting here. I’ve been bothered for a while about the
>> lack of the ability to chain futures in python, such that the next future
>> will execute upon the first’s completion.  So I submitted a pr to do this.
>> This would add the .then(self, fn) method to concurrent.futures.Future.
>> Thoughts?
>> -dancollins34
>> Github PR #5335
>> bugs.python.org issue #32672
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> --
> --Guido van Rossum (python.org/~guido)

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