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B042 has naive definition of exception #538
Description
B042 (introduced in #421 / #512) has a naive way of determining whether a class is an exception:
Lines 1798 to 1802 in 915edea
It is based solely on the name of the class, which means it returns True even if the class is not an exception.
For example, if I have a context manager class called FailOnException, it will trigger, even though that class is not an exception at all:
from typing import (ContextManager, TypeVar)
T = TypeVar("T")
class FailOnException(ContextManager[T]): pass
print(FailOnException.__mro__)(<class '__main__.FailOnException'>, <class 'contextlib.AbstractContextManager'>, <class 'abc.ABC'>, <class 'typing.Generic'>, <class 'object'>)
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cooperlees commentedon Feb 13, 2026
I get the "buggy" nature, but calling something an exception that isn't an exception is misleading right? Probably don't want to do that in Python ...
But open to maybe using inheritance from BaseException etc. to be smarter here if others agree.
GPHemsley-RELX commentedon Feb 13, 2026
FailOnExceptionis a context manager that catches exceptions and then fails gracefully; it's not calling itself an exception.I think checking for inheritance from BaseException is the only right way to do this, and indeed is a more Pythonic way.
B042: don't count a parameterized generic base as an exception super …
Merge main: resolve UNRELEASED changelog conflict
DG-Elom commentedon Oct 14, 2026
Checked on
main: since #578,class FailOnException(ContextManager[T])with an__init__that doesn't callsuper().__init__()is no longer flagged by B042 (it was on the commit before). #578 only excludes parameterized generic bases rather than resolving real inheritance fromBaseException, so I'll leave it to you whether that's enough to close this.GPHemsley-RELX commentedon Oct 14, 2026
It sounds like it addresses my usecase (I haven't tested it yet), so thanks for that, but it sounds like it is still technically "naive", so I'll leave it open. 🙂