Too many arguments for "__setattr__" of "object" on valid code

[tavianator]

This code is a reasonable pattern for initializing "immutable" objects:

class Foo:
    def __new__(cls, bar: int):
        ret = super().__new__(cls)
        super().__setattr__(ret, "bar", bar)
        return ret

But mypy complains

foo.py:4: error: Too many arguments for "__setattr__" of "object"

[gvanrossum]

This is another special case for __new__ that mypy misses -- in __new__, super() is a class object (its type is something like Type[B] where B is the base class -- if there are multiple base classes the type becomes more problematic).

In the grand scheme of things it doesn't feel like it's a big priority, though if you come up with a fix we'll take it!

[tavianator]

This is actually broken for things other than __new__ too:

class Foo:
    @classmethod
    def create(cls, bar: int) -> "Foo":
        ret = cls.__new__(cls)
        super().__setattr__(ret, "bar", bar)
        return ret

I'll try to find a fix.