Support special `object.__new__` and `object.__init__` in `super`.

[o11c]

Currently, __new__ is not special at all. While full support is complicated, it is particularly irritating that object.__new__ cannot be added to stubs at all, since subclasses need to override it with different signatures and that is an error.

object.__new__ and object.__init__ are special in another way. For the object class itself, their signatures are (Subclass[T]) -> T and (T) -> None. However, when called from subclass constructors via super(), the signatures are (Subclass[T], *args, **kwargs) -> T and (T, *args, **kwargs) -> None.

In the above, T is a TypeVar that is bounded by the current class, and Subclass[T] is basically Intersection[type, Callable[..., T]] (i.e. supports duck typing once the Protocol stuff works).

(In my notes I am using use the term Class[T] to refer to Intersection[type, Callable[[signature of __init__], T]])

As an icky workaround, I tried using cast(Any, super()), but that says that super is not defined, before settling on # type: ignore

[ddfisher]

This is probably partially out of date -- we should investigate what the current situation is.

[Syeberman]

I believe this issue still exists. The following code:

class IntSubclass(int):
    def __new__(cls, v: int) -> IntSubclass:
        return super().__new__(cls, v)

Produces these errors:

[C:\Projects]python -m mypy --version
mypy 0.501

[C:\Projects]python -m mypy --strict syetest.py
syetest.py:3: warning: Returning Any from function with declared return type "syetest.IntSubclass"
syetest.py:3: error: Too many arguments for "__new__" of "object"

[frankcarey]

Yes, also getting an error..

class VersionID(bytes):
    """A byte string of len 32 that only allows a-z, 0-9, and underscores."""
    vid: bytes

    # Must use __new__ since bytes is immutable.
    def __new__(cls, version_id: Union[bytes, str]) -> 'VersionID':
        # Allow passing as a string and then convert to ascii bytes.
        if isinstance(version_id, str):
            vid = version_id.encode('ascii')
        else:
            vid = version_id
        assert isinstance(vid, bytes), \
            "version_id should be bytes-like, but found {}".format(
                type(vid))
        assert len(vid) > 0, \
            "version_id should be at least one character."
        assert len(vid) <= 32, \
            "version_id should be less than 32 characters."
        assert re.match(b'^[a-z0-9_]*$', vid), \
            "version_id should only contain lowercase letters, numbers, " \
            "and underscores."
        ## noinspection PyArgumentList
        return super().__new__(cls, vid)

    def __str__(self) -> str:
        return self.decode('ascii')

Produces: error: Too many arguments for "__new__" of "object"

mypy version: 0.560

[Syeberman]

I believe that is a separate issue, @frankcarey. It looks like mypy is interpreting super().__new__(cls, vid) as object.__new__(cls, vid), when really it's bytes.__new__(cls, vid). You should log a separate issue for this (or add to an existing issue if it has already been reported).

Incidentally, I would remove the line vid: bytes, which suggests that VersionID().vid is a valid attribute containing the ID, when actually VersionID() itself is the ID.