Using GAsyncResult APIs with Python's asyncio

With a GLib implementation of the Python asyncio event loop, I can easily mix asyncio code with GLib/GTK code in the same thread. The next step is to see whether we can use this to make any APIs more convenient to use. A good candidate is APIs that make use of GAsyncResult.

These APIs generally consist of one function call that initiates the asynchronous job and takes a callback. The callback will be invoked sometime later with a GAsyncResult object, which can be passed to a "finish" function to convert this to the result type relevant to the original call. This sort of API is a good candidate to convert to an asyncio coroutine.

We can do this by writing a ready callback that simply stores the result in a future, and then have our coroutine await that future after initiating the job. For example, the following will asynchronously connect to the session bus:

import asyncio
from gi.repository import GLib, Gio

async def session_bus():
    loop = asyncio.get_running_loop()
    bus_ready = loop.create_future()
    def ready_callback(obj, result):
        try:
            bus = Gio.bus_get_finish(result)
        except GLib.Error as exc:
            loop.call_soon_threadsafe(bus_ready.set_exception, exc)
            return
        loop.call_soon_threadsafe(bus_ready.set_result, bus)

    Gio.bus_get(Gio.BusType.SESSION, None, ready_callback)
    return await bus_ready

We've now got an API that is conceptually as simple to use as the synchronous Gio.bus_get_sync call, but won't block other work the application might be performing.

Most of the code is fairly straight forward: the main wart is the two loop.call_soon_threadsafe calls. While everything is executing in the same thread, my asyncio-glib library does not currently wake the asyncio event loop when called from a GLib callback. The call_soon_threadsafe method does the trick by generating some dummy IO to cause a wake up.

Cancellation

One feature we've lost with this wrapper is the ability to cancel the asynchronous job. On the GLib side, this is handled with the GCancellable object. On the asyncio side, tasks are cancelled by injecting an asyncio.CancelledError exception into the coroutine. We can propagate this cancellation to the GLib side fairly seamlessly:

async def session_bus():
    ...
    cancellable = Gio.Cancellable()
    Gio.bus_get(Gio.BusType.SESSION, cancellable, ready_callback)
    try:
        return await bus_ready
    except asyncio.CancelledError:
        cancellable.cancel()
        raise

It's important to re-raise the CancelledError exception, so that it will propagate up to any calling coroutines and let them perform their own cleanup.

By following this pattern I was able to build enough wrappers to let me connect to the D-Bus daemon and issue asynchronous method calls without needing to chain together large sequences of callbacks. The wrappers were all similar enough that it shouldn't be too difficult to factor out the common code.

• asyncio
• glib
• Python