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Finish deprecation in asyncio.get_event_loop() #93453
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I wonder a bit how much existing code will be broken by this change in Python 3.12?
If the deprecation warning in Python 3.10-11 remains unnoticed (like for me, until I ran Python with the
-Werrorflag), any existing code callingaysncio.get_event_loop()outside a running event loop will break.A simple workaround that worked in my case is to call
asyncio.get_event_loop_policy().get_event_loop()instead. This will return the loop already attached to the current thread, or create a new one if not existing yet. (This seems better than just callingasyncio.new_event_loop()because that creates an extra loop next to already existing loops.)Will this workaround of using
asyncio.get_event_loop_policy().get_event_loop()remain available in Python 3.12+?Reacted by Daniel TownsendI'm interesting in knowing the answer to @lschoe's question too.
A user of one of my libraries reported this deprecation warning. Is using
asyncio.get_event_loop_policy().get_event_loop()a valid workaround?Ask @asvetlov. I do not know plans about
asyncio.get_event_loop_policy().get_event_loop(). It may be deprecated too: you either use the existing running loop, or explicitly create a new one.asyncio.get_event_loop()was initially proposed to remove, but since it is use so much in the code that predatesasyncio.get_running_loop(), it was decided to make it just an alias ofasyncio.get_running_loop().Reacted by Daniel TownsendThanks @serhiy-storchaka
Thanks as well. It's good to have something next to
asyncio.get_running_loop()because that can be only be called from a running loop;) For lower-level usage ofasyncio's event loops, accessing a loop even when it's not running seems still very useful to me. (For higher-level usage stricter, disciplined calls usingasyncio.run()etc., not handling e.g. any bareasyncio.Futures is indeed advised.)For example, I want to call
loop.set_exception_handler(handler)to set my own exception handler for the event loop. To do this once and for all, it's convenient to do this before the loop is running. Not sure what the intended way to do this would be, if one can only access the loop after it started running?@lschoe I think the intended pattern is
def loop_factory(): loop = asyncio.new_event_loop() loop.set_exception_handler(...) with asyncio.Runner(loop_factory=loop_factory) as runner: runner.run(amain())
or:
with asyncio.Runner() as runner: runner.get_loop().set_exception_handler(...) runner.run(amain())
But calling
asyncio.get_running_loop().set_exception_handler(...)as the first thing in yourasync def amain():seems to be the best choiceThanks, these are interesting options, indeed for Python 3.11+ which will have this new Runner class.
Using a
loop_factory()does not seem preferable becauseasyncio.new_event_loop()is still called, which may cause problems.
If a loop is already attached to the current thread, this will create a new loop next to the existing one. However, only one of these two loops can run at the same time, so this gives a Runtime error if one tries to run the other one.With
asyncio.get_event_loop()you can join the existing loop, if any. Whether that loop is running or not doesn't matter either.For example, one can run some code in Python's asyncio REPL (via
python -m asyncio) or in Jupyter notebooks, where there is already a loop running providing top-level await, or one can run the same code from a Python script where there is no loop running yet.I've also tried the second way you propose, calling
runner.get_loop(), but this also gives a Runtime error when called from the asyncio REPL, for example.And your third option requires a running loop again (which will not be there yet, if this code is executed upon importing and initializing a module).
To deal with such varying circumstances, global access to the event loop attached to the current thread via
asyncio.get_event_loop()is very useful.But maybe we should first deprecate set_event_loop()? It will be a no-op now.
@serhiy-storchaka set_event_loop is not a no-op it sets up the child watcher system #93896
This is a subtle issue and I would like to go slowly here (but not so slowly to miss the 3.12 feature cut-off!).
I wasn't aware of this issue and need some time to think about the consequences.
The growing asymmetry between get_event_loop() and set_event_loop() is definitely bothering me.
Should we perhaps also deprecate the behavior of Policy.get_event_loop() to sometimes create a new event loop?
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on Dec 16, 2022 The documentation of asyncio.get_event_loop() says:
If there is no running event loop set, the function will return the result of calling
get_event_loop_policy().get_event_loop().Which by default raises a RuntimeError, so it is rather confusing. Should the documentation be updated to explicitly say something like:
If there is no running event loop set, the function will return the result of calling
get_event_loop_policy().get_event_loop()which raises aRuntimErrorwithDefaultEventLoopPolicy.?
Oooh, the decumentation is indeed a mess. There's no place that I can find where the behavior of
DefaultEventLoopPolicy.get_event_loop()is currently documented.AbstractEventLoopPolicyis vague (just says it returns a loop and never None),DefaultEventLoopPolicydoesn't say anything specific, andBaseDefaultEventLoopPolicyis undocumented (and private, apparently, though in the light of the PEP 387 discussions it might be implicitly public). The documentation section linked to should be hyperlinked toget_event_loop_policy()and some policy class'sget_event_loop()method, which should be documented.Is it fair to say this is now a documentation issue, or is there more to be done? Should this hold up 3.12.0a4?
Is it fair to say this is now a documentation issue, or is there more to be done? Should this hold up 3.12.0a4?
Hm, #100410 isn't merged yet, even though I approved it two weeks ago. @serhiy-storchaka do you have any hesitations?
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on Jan 13, 2023 - moved this from In Progress to Done in Release and Deferred blockers 🚫
on Jan 13, 2023 - added a commit that references this issue
on Mar 23, 2025
Since 3.10
asyncio.get_event_loop()emits a deprecation warning if used outside of the event loop (see #83710). It is a time to turn a warning into error and makeasyncio.get_event_loop()an alias ofasyncio.get_running_loop().But maybe we should first deprecate
set_event_loop()? It will be a no-op now.Linked PRs