Do we need a ValueAlias in addition to TypeAlias?

Typing
2

Even if a variable has a Final qualifier, it’s still a variable. The expression _Nothing.NOTHING is not a valid type expression, and NOTHING is not a valid type alias.

Here is how I recommend handling this pattern such that it is compatible with the type system.

import enum
from typing import Final, Literal

class _Nothing(enum.Enum):
    NOTHING = enum.auto()

type NothingType = Literal[_Nothing.NOTHING]
NOTHING: Final = _Nothing.NOTHING

def f(a: str | NothingType = NOTHING): ...

You might also be interested in this thread, which discusses the idea of adding a sentinel facility to the language.

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