B042: don't treat a class whose only base is object as an exception

README.rst

@@ -613,6 +613,8 @@ UNRELEASED
   ``map(print, items)`` on its own line never calls ``print`` (#452)
 * B909: also check mutations in ``if`` conditions and ``elif``/``else``
   branches inside loops (#606)
+* B042: don't treat a class named like an exception as one when its only base is
+  ``object``, as in ``class ParsedException(object)`` (#538)
 
 26.9.30
 ~~~~~~~

bugbear.py

@@ -3011,8 +3011,14 @@ def is_exception(s: str):
         # A class must inherit from a super class to be an exception, and we also
         # require the class name or any of the base names to look like an exception name.
         # A parameterized generic base such as `ContextManager[T]` does not count as
-        # that super class unless its own name looks like an exception name.
-        concrete_bases = [b for b in node.bases if not isinstance(b, ast.Subscript)]
+        # that super class unless its own name looks like an exception name, and
+        # neither does `object`, as in `class ParsedException(object)`.
+        concrete_bases = [
+            b
+            for b in node.bases
+            if not isinstance(b, ast.Subscript)
+            and not (isinstance(b, ast.Name) and b.id == "object")
+        ]
         if not (is_exception(node.name) and concrete_bases):
             for base in node.bases:
                 if isinstance(base, ast.Subscript):

tests/eval_files/b042.py

@@ -96,6 +96,12 @@ def __init__(self, x): ...  # safe
 class FooException: # safe, doesn't inherit from anything
     def __init__(self, x): ...
 
+# `object` is not an exception base either
+class ParsedException(object):  # safe
+    def __init__(self, x): ...
+class ObjectError(Anything, object):
+    def __init__(self, x): ...  # B042: 4
+
 # a parameterized generic base is not an exception class unless its name says so
 class FailOnException(typing.ContextManager[int]):  # safe
     def __init__(self, x): ...