> Typing Python that wasn't written with type hints in the first place is always going to be a complete nightmare though

Couldn’t agree more. I joined a new team and recently finished converting a 20K line repo. It was a grueling grind to get mypy running in strict mode.

I’m continually finding that a huge benefit of typed python code isn’t autocomplete or catching bad argument passing, but preventing developers from creating patterns that are unmaintainable.

If your code is too complex to type correctly, its probably too complex in general :)