[Typing-sig] Using Final to signify a singleton?

Sept. 27, 2021

      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: ...
```

[Typing-sig] Using Final to signify a singleton?

Brett Cannon