On Thu, Jan 11, 2018 at 7:49 PM, Dima Tisnek <dim...@gmail.com> wrote: > Very nice read, Nathaniel. > > The post left me wondering how cancel tokens interact or should > logically interact with async composition, for example: > > with move_on_after(10): > await someio.gather(a(), b(), c()) > > or > > with move_on_after(10): > await someio.first/race(a(), b(), c()) > > or > > dataset = someio.Future(large_download(), move_on_after=9999) > > task a: > with move_on_after(10): > use((await dataset)["a"]) > > task b: > with move_on_after(10): > use((await dataset)["b"])
It's funny you say "async composition"... Trio's concurrency primitive (nurseries) is closely related to the core concurrency primitive in Communicating Sequential Processes, which they call "parallel composition". (Basically, if P and Q are processes, then "P || Q" is the process that runs both P and Q in parallel and then finishes when they've both finished.) If you were using that as your primitive, then tasks would form an orderly tree and this wouldn't be a problem :-). Given asyncio's actual primitives though, then yeah, this is clearly the big question, and I doubt there are any simple answers; so far my ambition has just been to articulate the problem well enough to start that conversation (see also the "asyncio" section in the blog post). One possibility might be a hybrid cancel token / cancel scope API: create a first class cancel token API like C# has, enhance make the low-level asyncio APIs to use them, and then on top of that add mechanisms to attach a stack of implicitly-applied cancel tokens to each task? That's just a vague handwave of an idea so far though. Note that last case is the one where asyncio cancellation semantics are already... well, surprising, anyway. If you cancel task a then task b will receive a CancelledError, even though task a was not cancelled. (I talked about this a bit in my "Some thoughts ..." blog post; search for "spooky-cancellation-at-a-distance.py".) -n -- Nathaniel J. Smith -- https://vorpus.org _______________________________________________ Async-sig mailing list Async-sig@python.org https://mail.python.org/mailman/listinfo/async-sig Code of Conduct: https://www.python.org/psf/codeofconduct/
