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

Greg Ewing greg.ewing at canterbury.ac.nz
Wed Oct 17 12:16:17 CEST 2012

Guido van Rossum wrote:
> On Tue, Oct 16, 2012 at 12:20 AM, Greg Ewing
> <greg.ewing at canterbury.ac.nz> wrote:
>>it blurs the distinction
>>between invoking a subtask synchronously and waiting for the
>>result of a previously spawned independent task.
> Are you sure you really want to distinguish between those though?

I think I do. Partly because I feel that not doing so would
make code harder to reason about. Async stuff is difficult
enough as it is without hiding the boundaries between one
thread of control and another.

There are technical reasons as well. If you use 'yield from'
to wait for completion of an independent task, then it would
seem like you should be able to do this:

    t1 = task1()
    t2 = task2()
    r1 = yield from t1
    r2 = yield from t2

But that can't work -- the object that you wait on has to be
different from the generator instance passed to spawn(). The
reason is that if the task finishes before anyone waits on it,
the return value needs to be stored somewhere.

Having spawn() return an object that deliberately does *not*
have the interface of a generator, and having to explicitly wait
for it, makes it much less likely that anyone will make that kind
of mistake. If you wrote

    t1 = task1()
    t2 = task2()
    r1 = yield from t1.wait()
    r2 = yield from t2.wait()

you would quickly get an exception, because generators don't
have a wait() method.


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