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.

Personally I would use "assigned" 1) to better reflect the meaning and 2) to follow PEP-8 and not use an uppercase.

Uppercase was chosen to match Python's existing singletons, `True`, `False`, and `None`. Lowercase might look a little too much like a variable, and all-upper might look too much like a module-level const, IMHO.

I think I'd've gone for Unset. Empty's a bit close to None.

How about 'Omitted'?

foo=Omitted

if foo is not Omitted:

To my ear, this does sound the best. Thanks for the suggestion!