Passing asyncio loop by argument or using default asyncio loop

Question

I'm using asyncio in my application and i'm a litte bit confused about passing the event loop as argument.

You've three possibilities when writing a function/method using the event loop:

Pass the asyncio event loop as argument
Don't use an argument for the event loop and use asyncio.get_event_loop()
Make it optional to pass the event loop as argument. If it is not passed, use asyncio.get_event_loop()

It seems that the last case is used most of the time but even in the asyncio api the usage is inconsistent. As I don't indent to use two seperated event loops what speaks against just using asyncio.get_event_loop() where needed?

What's the best way to go?

Can you please elaborate on "passing the event loop as argument"? Passing the event loop from where to where? This is a bit application specific rather than a generic question (depending on how the application is using the event loop the answer might vary). — farzad
– farzad, Commented Oct 31, 2016 at 14:04
asyncio.get_event_loop() returns an event loop, you can change it with asyncio.set_event_loop(). This will get/set the event loop on the _event_loop_policy, a global variable in the asyncio module. This is an easy way to share the eventloop in the application without a need to pass it around. So if you want to avoid having 2 separate event loops around, you may register the one event loop you've got in this global variable by set_event_loop(). — farzad
– farzad, Commented Oct 31, 2016 at 14:10
@farzad: Typically my application starts with loop=asyncio.get_event_loop(). Then I'm setting up the asynchronous stuff (coroutines, async servers, ...) and my own functions where I've got this question about passing the loop argument. When everything is set up, I run loop.run_forever(). — Günther Jena
– Günther Jena, Commented Oct 31, 2016 at 14:25
This is a great question, hope some more light can be shed on this by someone in the know... — songololo
– songololo, Commented Nov 5, 2016 at 12:32

ttyridal · Accepted Answer · 2017-05-09 08:30:53Z

12

A good (as in praised by Guido van Rossum) blog post discussing this is Some thoughts on asynchronous API design in a post-async/await world. With a follow up discussion from python core developers here.

TLDR;

If you're only using one event loop, it doesn't matter.

If you're managing multiple loops, and have python >= 3.6 it mostly doesn't matter: Do not use argument and use asyncio.get_event_loop() where needed, it will give you the correct loop.

answered May 9, 2017 at 8:30

ttyridal

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2 Comments

Bryce Drennan Over a year ago

It took me a while to verify that Guido was onboard with this. This message was useful for that: groups.google.com/d/msg/python-tulip/yF9C-rFpiKk/tk5oA3GLHAAJ

Chris Bouchard Dec 3, 2024 at 17:28

@JHS It looks like it's only deprecated to call get_event_loop to get a new event loop when there isn't already one running, bringing it in line with get_running_loop.

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