Thanks a lot! That led me to find this November thread:

https://discuss.python.org/t/switch-pythons-parsing-tech-to-...

And I posted here about it:

https://www.reddit.com/r/ProgrammingLanguages/comments/brz2y...

It looks like the set and dict literals I noticed weren't so much the motivating use cases, but even more fundamentally assignments and keyword args!

Huh interesting. I've been using parsimonious for cases like this, didn't know python had a stdlib for it...