One of the reasons I love using gevent (https://www.gevent.org) is that it's way of introducing concurrency that does preserve these atomicity guarantees! Broadly speaking, the only time your code will be interrupted by something else in the same process is if some function call yields control back to the event loop - so if your block is entirely made up of code you control that doesn't do any I/O, you can be sure it will run atomically. And you get it for free without needing to rewrite synchronous code into asyncio.

This does make me wonder, though, if gevent will survive PEP 703's massive changes to internal Python assumptions. That said, gevent does work on pypy, so there's some history with it being flexible enough to port. Hopefully it won't be left behind, otherwise codebases using gevent will see this as a Python 2 -> 3 level apocalypse!