How can I learn how to implement a custom Python asyncio event loop?

Question

I’m looking into implementing a new event loop to plug into asyncio based on existing run loop implementations, such as Cocoa’s NSRunLoop and Qt’s QEventLoop. but find it difficult to to pick a place to start.

The documentation says that the system is designed to be pluggable, but nowhere does it say exactly how this can be done. Should I start with AbstractEventLoop, or BaseEventLoop? What method does what, and what components do I need to provide? The only alternative implementation I find useful is uvloop, but find it difficult to understand because it relies heavily on Cython and libuv, which I am not familiar with.

Is there some kind of a write-up on how the event loop implementation is done, and how a custom one can be made? Or a less involved implementation I can wrap my head around more quickly? Thanks for any pointers.

Answer 1

The documentation says to inherit from AbstractEventLoop.

For the rest of your question, I didn't find the documentation very clear, but the source code for the concrete event loop in asyncio was helpful. I've written up a pretty minimal example of inheriting from AbstractEventLoop to create an event driven simulator.

The main things that I'd have liked to be told are

• Implement create_task. The end-user schedules a coroutine using asyncio.ensure_future(coro()), but that just calls your loop's create_task method. It doesn't need to be anything more than def create_task(self, coro): return asyncio.Task(coro, loop=self).

• Implement call_soon, call_at and call_later. These are invoked by the end-user to schedule a plain callback function. They are also invoked by the async/await system automatically, whenever the end-user schedules a coroutine.

• If a regular callback raises an exception, it goes to your loop's call_exception_handler method. If a coroutine raises an exception, the exception lives in some asynchronous never-never land, and you have to catch it there.

• Look up the source code for AbstractEventLoop to see all the other methods that you should be overriding. Bonus: somewhat helpful comments.