Internal-only Variables?

I apologize in advance if this is covered somewhere, but I could not find it searching the repo and the docs.

Is there a way to have a variable be internal only so it's not exported, not included in to_dict, etc.? The use case is to subclass Box and have variables internal to the class, but not included in the data.

Start by locating the Box subclassing and serialization paths responsible for exports and to_dict. Check whether existing documentation or behavior already addresses subclass-owned variables. Done should mean a clearly defined way for internal variables to remain available to the subclass while being excluded from the named data outputs.

Written by the indexing model from the issue text.