I was chatting with Guido about how there isn't an easy way to signal that something is a singleton. Concrete example: ```Python _NOTHING = object() class list(builtins.list): def __init__(self, iterable: Iterable | object = _NOTHING, /) -> None: if iterable is not _NOTHING: for item in iterable: ... # Fails to type check. ``` Even if I define a class just for this use-case, it still won't type check since the class that is instantiated once will still not be an iterable. ```Python class _NothingType: pass _NOTHING = _NothingType() ``` I'm not sure how best to handle this case. But while chatting with Guido he suggested one possibility might be to somehow use `Final` to signify that an object is acting as a singleton. Maybe something like: ```Python _NOTHING: Final[object] = object() ``` Then type checkers would know that `_NOTHING` is a singleton. Basically I want a way to write the following without a type failure: ```Python class list(builtins.list): def __init__(self, iterable: Iterable | _NOTHING = _NOTHING, /) -> None: if iterable is not _NOTHING: for item in iterable: ... ```