Agreed. 'Empty' works, as does 'Blank' or 'Unassigned'.'Nil' is concise, but might be a little too easily confused with 'None' (especially so for someone coming from Ruby).

I'd love an approach that used the standard library, but there doesn't seem to be a type-agnostic approach. float('nan') won't make sense for other types, and using `types.NoneType` instead of `None` seems even less understandable.