Thanks, I've had a look at Marshmallow in the past, but it's still doing the extra bits of dealing with the extra bits of serialization, it never really felt all that dissimilar to Pydantic.

What I've really been looking for something that I can just use to check a data structure i'm passing in to a function call is valid. One use case is related to state machines, where I'm providing some data along with a transition name, and want to make sure the combination of transition + data is valid.