B042 false positive

[henzef]

Hi all,
I was greeted by some fresh B042 errors in our CI this morning, which rightfully complained about some exception subclasses. In one case however, I am not sure what the correct solution would be and after reading some exception-best-practices guides, I think that its a false positive in the B042 code.
Consider the following example:

class MyException(Exception):
    def __init__(self, msg: str, additional_data: str = "", /, *args: object) -> None:
        super().__init__(msg, *args)
        self.additional_data = additional_data

This code works just fine and also seems to support copy.copy() and pickling, but flake8-bugbear reports:

myexception.py:2:5: B042 Exception class with `__init__` should pass all args to `super().__init__()` in order to work with `copy.copy()`. It should also not take any kwargs.

[henzef]

Hi @jakkdl, can you maybe have a look?

[cooperlees]

It seems to work just passing the extra arg. But interesting you say everything works, so I am subscribing to see what others think. I don't have context on the bugs that can cause here not doing this ...

MacBookPro:~ cooper$ python3
Python 3.14.0 (main, Oct  7 2025, 09:34:52) [Clang 17.0.0 (clang-1700.3.19.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> class MyException(Exception):
...     def __init__(self, msg: str, additional_data: str = "", /, *args: object) -> None:
...         super().__init__(msg, *args)
...         self.additional_data = additional_data
...
>>>
>>>
>>> class MyException(Exception):
...     def __init__(self, msg: str, additional_data: str = "", /, *args: object) -> None:
...         super().__init__(msg, additional_data, *args)
...         self.additional_data = additional_data
...
>>> MyException("foo")
MyException('foo', '')

Some more context here: #421

[LincolnPuzey]

For another example of a potential false positive, we have the following exception:

class FooError(Exception):
    """Base class for all errors."""

    def __init__(self, message):
        self.message = message

    def __str__(self) -> str:
        return str(self.message) or repr(self.message)

which is now getting reported as a B042 error.

As far as I can tell from testing in my shell, instances of this work fine with copy.copy() and pickle.
All instances (original and copied) have .message and .args attributes as I would expect.