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As an evening project, I’ve been working on a small library that integrates the GLib main loop with Python’s asyncio. I think I’ve gotten to the point where it might be useful to other people, so have pushed it up here:

https://github.com/jhenstridge/asyncio-glib

This isn’t the only attempt to integrate the two event loops, but the other I found (Gbulb) is unmaintained and seems to reimplement a fair bit of the asyncio (e.g. it has its own transport classes). So I thought I’d see if I could write something smaller and more maintainable, reusing as much code from the standard library as possible.

My first step was writing an implementation of the selectors.BaseSelector interface in terms of the GLib main loop. The select() method just runs a GMainLoop with a custom source that will quit the loop if any of the file descriptors are ready, or the timeout is reached.

For the asyncio event loop, I was able to reuse the standard library asyncio.SelectorEventLoop with my new selector. In action, it looks something like this:

1. Let the GMainLoop spin until any asyncio events come in.
2. Return control to the asyncio event loop to process those events.
3. Repeat

As far as testing goes, the Python standard library comes with a suite of tests parameterised on an event loop implementation. So I’ve just reused that as the bulk of my test suite, and done the same with the selector tests. There are a handful of test failures I still need to diagnose, but for the most part things just work.

Making an asyncio application use this event loop is simple:

import asyncio
import asyncio_glib
asyncio.set_event_loop_policy(asyncio_glib.GLibEventLoopPolicy())

The main limitation of this code is that it relies on asyncio running the GLib main loop. If some other piece of code runs the main loop, asyncio callbacks will not be triggered and will probably lead to busy looping. This isn’t a problem my project (an asyncio server making use of GStreamer), but would be a problem for e.g. a graphical application calling gtk_dialog_run().