@Bar: I don't know about exposing _chain_future(). Certainly it's overkill for what the OP wants -- their PR only cares about chaining concurrent.future.Future.
@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) try: newf.set_result(f.result()) except CancelledError: newf.cancel() except Exception as err: newf.set_exception(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 <dancollin...@gmail.com> wrote: > 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 <gu...@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 <dancollin...@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 >> >> _______________________________________________ >> Python-ideas mailing list >> Python-ideas@python.org >> https://mail.python.org/mailman/listinfo/python-ideas >> Code of Conduct: http://python.org/psf/codeofconduct/ >> >> > > > -- > --Guido van Rossum (python.org/~guido) > > -- --Guido van Rossum (python.org/~guido)
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