That this was not caught by the unit tests should be motivation for us to improve on that 😊

Noticed another issue: if self.optical is None (which is the default), then DELEGATED_ATTRIBUTES[name] is None and getattr(None, name) raises. Fixed that case too (well not "fixed" but the error message should be clear).

Can we add some tests for this? I will add some tests!

@frederikschubertflex oh sorry didnt see your edit, I added some tests now and force pushed! feel free to add more, I got a little bit confused. actually I still don't understand the root cause of the error I was seeing on my pydantic v2 branch. there, the _cached_properties lookup fails for MultiPhysicsMedium, but on develop it doesn't. I added a test to catch this but it currently passes on develop. still good to have probably. I did add tests for the other fix too though.

No, worries! I just figured that you have other things on your plate :) I will look into this in more depth later!

ok maybe just to conclude this discussion:
it looks like pydantic v1 handled PrivateAttr in __getattribute__, v2 does not, so it falls back to __getattr__ which now gets caught by MultiPhysicsMedium's override. another difference is that __dir__ in v1 includes private attrs, and v2 omits them..