How to interface blocking and non-blocking code with asyncio

Question

I'm trying to use a coroutine function outside of the event loop. (In this case, I want to call a function in Django that could also be used inside the event loop too)

There doesn't seem to be a way to do this without making the calling function a coroutine.

I realize that Django is built to be blocking and a therefore incompatible with asyncio. Though I think that this question might help people who are making the transition or using legacy code.

For that matter, it might help to understand async programming and why it doesn't work with blocking code.

Even if you push Django WSGI request handling into asyncio event loop it's synchronous call by definition -- and bad idea in general. — Andrew Svetlov
– Andrew Svetlov, Commented May 29, 2014 at 9:05
It's hard to let go of Django, where it provides you so much — leech
– leech, Commented May 29, 2014 at 15:49

leech · Accepted Answer · 2014-06-05 20:40:49Z

2

After a lot of research I think these solutions could be helpful:

Update your legacy code to use asyncio:

Yes i know that it can be hard and painful, but it might be the sanest choice. If you are wanting to use Django like I was... Well, you've got a lot of work to do to make Django async'd. I'm not sure it is possible, but I found at least one attempt: https://github.com/aaugustin/django-c10k-demo (Though, in a youtube video the author explained all the shortcomings of this).
use asyncio.async or asyncio.Task:

These items will enable you to run something async inside of blocking code, but the downfall here is that you will not be able to wait for them to finish without doing something ugly like a while loop that checks if the future has completed... ugh, but if you don't need the result, that might work for you.

answered Jun 5, 2014 at 20:40

leech

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leech Over a year ago

There is a django-channels project that could help anyone using django to start getting async

MadeR · Accepted Answer · 2017-11-08 14:10:33Z

2

About case #2: Blocking code should be at least wrapped with .run_in_executor.

answered Nov 8, 2017 at 14:10

MadeR

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2 Comments

Timothy Schoonover Over a year ago

Could you provide a code sample and brief explanation?

MadeR Over a year ago

Suppose you have a "fn" function doing blocking I/O. If you run it from a coroutine, the coroutine will block at least until the IO is completed. Threads in python are great for this and asyncio uses the loop.run_in_executor method to suspend a thread until it finished. loop = asyncio.get_event_loop(); result = await loop.run_in_executor(None, fn, *args)

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