Just to be clear: by initializing nest_asyncio in notebook it has resolved the issue this PR originally set out to solve. So if you want to try you can upgrade notebook==6.4.6 and get the latest jupyter_client and see if you still have the issue.

I was able to reproduce this issue easily enough using notebook==6.4.4 and any jupyter_client > 7. It requires you to be quick, but if you launch the notebook server, create a new notebook from the tree view, wait for the new tab to open, and as quickly as possible execute some code e.g. 1 + 1 then in the notebook server log you should see an Exception being thrown for the event loop.

If you aren't using notebook then you can investigate adding:

to your application code to initialize it as early as possible: before launching any tasks.

Even better would be migrating your application to only use the asynchronous methods of jupyter_client and not relying on the sync ones.

But that's a question for you too -- is it possible in real applications / practical to use Jupyter w/o nest_asyncio?

I'm not an expert on this either, but my understanding is that this nest_asyncio stuff is only needed to maintain compatibility with the older sync code. If you can rely only on the async ones then in theory you won't ever hit the code that needs nested loops.

You don't consider this to be an option?

It would require a long deprecation cycle I believe: