Python 3.8 asyncio is going to make the "proactor" event loop the default, instead of the current "selector" event loop. This is a problem for Tornado because the proactor event loop doesn't support the unix-style add_reader APIs that Tornado uses.

Anyone using Tornado 5+ on windows with python 3.8 will need to configure asyncio to use the selector event loop; we'll have to document this. We should also try to detect the use of a proactor event loop and give a clear error message (the current NotImplementError message can be seen in this SO post).

I don't think it would be appropriate for Tornado itself to configure the selector event loop automatically, since it has drawbacks (less scalability than the proactor loop, the user may want to use another event loop entirely like uvloop). Applications using Tornado (like jupyter notebook) may wish to do so, though. Maybe there should even be some more magical way to choose the selector loop (an environment variable? a special package to install?) to avoid the need to build this in to each application, although i don't really like the idea of that much magic.