BaseException.__init__ accepts **kwargs

[anentropic]

This is to fix python/mypy#4183

[JelleZijlstra]

Can you apply the same change in Python 3, and use object instead of Any?

[anentropic]

certainly!

why does Python 3 BaseException need object rather than Any?

I don't question it, I'd just like to understand.

I see it already has *args: object ... but a bunch of the other builtins have **kwargs: Any

[JelleZijlstra]

I think they should be object in both Python 2 and 3. Any generally means "the type system is not powerful enough to describe this type", and object means "all instances of object are accepted". In this case, we want the latter, not the former. (I think it's purely a semantic difference here; it doesn't actually accept the code type checkers will accept right now.)

[anentropic]

wouldn't that mean in Python 2 that old-style classes were not acceptable args?

So it's maybe correct for Python 2 but in Python 3 more of the Anys in the typeshed should be object?

[JelleZijlstra]

object includes old-style classes in Python 2 (isinstance(o, object) returns True for them). I don't think Python 2 and 3 should be different here.

[anentropic]

then I am curious what, if anything, would pass a type-check for Any but fail for object or vice-versa

I appreciate there's a semantic difference but it is pretty subtle, what makes it apply here?

[JelleZijlstra]

I don't think in this case the difference will directly affect type checker output; it's more about cleanliness, because we try to minimize usage of Any where possible.

In some cases it can make a difference though. If you have an API that takes a callback of type Callable[[Any], None], then a user can pass a callback that actually takes int as an argument. But if you take a callback of type Callable[[object], None], then the callback with an int argument will be rejected by the type checker.

Would it be easier to discuss this further in the typing chatroom at https://gitter.im/python/typing?