TypeError: Exception does not take keyword arguments

[euresti]

I wrote some bad code which mypy didn't catch. It boils down to:

class FooError(Exception):
    pass
raise FooError("Couldn't foo {username}", username="username")

Running the above yields: TypeError: FooError does not take keyword arguments but mypy doesn't complain. Probably because:

class BaseException:
    def __init__(self, *args: object, **kwargs: object) -> None: ...

I'd just make a change but part of me wonders if this was done on purpose.

[JelleZijlstra]

I think this is just a bad annotation. BaseException doesn't take kwargs in any Python version I tried (2.7, 3.5, 3.6).