Submit a job to an asyncio event loop

Question

I would like to submit jobs from a thread to an asyncio event loop (just like run_in_executor but the other way around).

Here's what the asyncio documentation says about concurrency and multithreading:

To schedule a callback from a different thread, the BaseEventLoop.call_soon_threadsafe() method should be used. Example to schedule a coroutine from a different thread: loop.call_soon_threadsafe(asyncio.async, coro_func())

That works fine but the result of the coroutine is lost.

Instead, it is possible to use a function that adds a done callback to the future returned by async (or ensure_future) so that the thread can access the result through a concurrent.futures.Future.

Is there a particular reason why such a feature is not implemented in the standard library? Or did I miss a simpler way to achieve that?

Answer 1

My request made its way and a run_coroutine_threadsafe function has been implemented here.

Example:

def target(loop, timeout=None):
    future = asyncio.run_coroutine_threadsafe(add(1, b=2), loop)
    return future.result(timeout)

async def add(a, b):
    await asyncio.sleep(1)
    return a + b

loop = asyncio.get_event_loop()
future = loop.run_in_executor(None, target, loop)
assert loop.run_until_complete(future) == 3

---

I originally posted a sub-class of concurrent.futures.Executor that can still be implemented as:

class LoopExecutor(concurrent.futures.Executor):
    """An Executor subclass that uses an event loop 
    to execute calls asynchronously."""

    def __init__(self, loop=None):
        """Initialize the executor with a given loop."""
        self.loop = loop or asyncio.get_event_loop()

    def submit(self, fn, *args, **kwargs):
        """Schedule the callable, fn, to be executed as fn(*args **kwargs).
        Return a Future object representing the execution of the callable."""
        coro = asyncio.coroutine(fn)(*args, **kwargs)
        return asyncio.run_coroutine_threadsafe(coro, self.loop)