Yeah, great, my hello world program is deterministic.

What happens when you introduce I/O? Is every network call deterministic? Can you depend on reading a file taking the same amount of time and being woken up by the scheduler in the same order every time?

Yes, the stdlib asyncio event loop does have deterministic scheduling, but that's an implementation detail and I would not rely on it for anything critical. Other event loops - for instance trio [1] - explicitly randomize startup order so that you won't accidentally write code that relies on it.

[1] https://github.com/python-trio/trio/issues/32

Don't you need something like a network clock to get deterministic replay?

It can't use immediate return on replay, or else the order will change.

This makes me twitchy. The dependencies should be better modelled, and idempotency used instead of logging and caching.