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, May 29 '14 at 09:05
It's hard to let go of Django, where it provides you so much — leech, May 29 '14 at 15:49

score 2 · Accepted Answer · answered Jun 05 '14 at 20:40

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.

There is a django-channels project that could help anyone using django to start getting async — leech, Dec 18 '15 at 17:25

score 2 · Answer 2 · answered Nov 08 '17 at 14:10

2

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

answered Nov 08 '17 at 14:10

MadeR

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Could you provide a code sample and brief explanation? – Timothy Schoonover Nov 08 '17 at 14:36
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) – MadeR Nov 08 '17 at 16:44

How to interface blocking and non-blocking code with asyncio

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