Allow kwargs for BaseException.__init__

[gladhorn]

Exceptions take keyword args, let mypy accept those.

[gladhorn]

I'm at a loss for the failed test, can someone explain what it does? Is this something I broke?

[srittau]

You need to copy stdlib/2and3/builtins.pyi to stdlib/2/__builtin__.pyi. These two files need to remain synchronized.

[gladhorn]

Thanks :) I would have expected the build system to do such copying.

[JelleZijlstra]

Can you add : object as the type? We like to keep all arguments typed.

[gladhorn]

Makes sense, thanks.

[donvargax]

I recently updated mypy to the latest version and started facing an issue:

Too many arguments for "__init__" of "BaseException".

Since it used to work, I tried to figure out what went wrong and I found these two pull requests:

#1704
#2348

One to add kwargs, the second one to remove them. What's the correct behavior? Thanks!

[JelleZijlstra]

And I merged both of them, that's embarrassing. :)

I think removing them is right: BaseException really does not take kwargs

>>> BaseException(x=3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: exceptions.BaseException does not take keyword arguments

[srittau]

I agree with @JelleZijlstra. Can you give examples of code that is wrongly flagged by mypy without this patch? The example from python/mypy#4183 (referenced from #1704) is actually an example of why keyword arguments should not be allowed:

class ValidationError(Exception):
    def __init__(self, message: str, log_level: int=logging.ERROR, *args, **kwargs) -> None:
        ...
        super().__init__(message, log_level, *args, **kwargs)

Calling ValidationError with keyword arguments will fail at runtime:

ValidationError("", foo=123)  # TypeError: ValidationError does not take keyword arguments

[donvargax]

I didn't knew BaseException didn't accept kwargs, mypy wasn't showing any errors until I updated it yesterday. If no kwargs is the correct behavior then I'll update my code, I just wanted to point out the conflict between versions, and if you merge this one then the next version will work differently again.

[JelleZijlstra]

Well, if your code is showing errors for code that's working correctly, there might still be a problem somewhere. Can you share the problematic code, and did you confirm that it does work correctly at runtime?

[gladhorn]

At least in my case, I mis-interpreted the class and added the kwargs, which was clearly wrong, it hid a mistake in one exception definition we had, so removing them again is certainly correct.