Finish deprecation in asyncio.get_event_loop()

[serhiy-storchaka]

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 make asyncio.get_event_loop() an alias of asyncio.get_running_loop().

But maybe we should first deprecate set_event_loop()? It will be a no-op now.

Linked PRs

• gh-93453: No longer create an event loop in get_event_loop() #98440
• [3.11] gh-93453: Only emit deprecation warning in asyncio.get_event_loop when a new event loop is created #99949
• [3.10] gh-93453: Only emit deprecation warning in asyncio.get_event_loop when a new event loop is created #100059

[lschoe]

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 -Werror flag), any existing code calling aysncio.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 calling asyncio.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+?

[dantownsend]

I'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?

[serhiy-storchaka]

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 predates asyncio.get_running_loop(), it was decided to make it just an alias of asyncio.get_running_loop().

[dantownsend]

Thanks @serhiy-storchaka

[lschoe]

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 of asyncio'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 using asyncio.run() etc., not handling e.g. any bare asyncio.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?

[graingert]

@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 your async def amain(): seems to be the best choice

[lschoe]

Thanks, these are interesting options, indeed for Python 3.11+ which will have this new Runner class.

Using a loop_factory() does not seem preferable because asyncio.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.

[graingert]

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