[Python-ideas] An async facade? (was Re: [Python-Dev] Socket timeout and completion based sockets)

Steve Dower Steve.Dower at microsoft.com
Fri Nov 30 20:18:34 CET 2012

Guido van Rossum wrote:
> Futures or callbacks, that's the question...

I know the C++ standards committee is looking at the same thing right now, and they're probably going to provide both: futures for those who prefer them (which is basically how the code looks) and callbacks for when every cycle is critical or if the developer prefers them. C++ has the advantage that futures can often be optimized out, so implementing a Future-based wrapper around a callback-based function is very cheap, but the two-level API will probably happen.

> Richard and I have even been considering APIs like this:
> res = obj.some_call(<args>)
> if isinstance(res, Future):
>     res = yield res
> or
> res = obj.some_call(<args>)
> if res is None:
>     res = yield <magic>
> where <magic> is some call on the scheduler/eventloop/proactor that pulls the future out of a hat.
> The idea of the first version is simply to avoid the Future when the result happens to be immediately
> ready (e.g. when calling readline() on some buffering stream, most of the time the next line is
> already in the buffer); the point of the second version is that "res is None" is way faster than
> "isinstance(res, Future)" -- however the magic is a little awkward.
> The debate is still open.

How about:

value, future = obj.some_call(...)
if value is None:
    value = yield future


future = obj.some_call(...)
if future.done():
    value = future.result()
    value = yield future

I like the second one because it doesn't require the methods to do anything special to support always yielding vs. only yielding futures that aren't ready - the caller gets to decide how performant they want to be. (I would also like to see Future['s base class] be implemented in C and possibly even preallocated to reduce overhead. 'done()' could also be an attribute rather than a method, though that would break the existing Future class.)


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