class MyEG(ExceptionGroup):
    """Holds BaseExceptions without itself being a BaseException."""

oops = MyEG("oops", [KeyboardInterrupt()])
assert isinstance(oops, Exception)
assert not isinstance(oops.exceptions[0], Exception)

I believe that this is a bug tracing to the period when PEP-654 did not intend (Base)ExceptionGroup to be usable as a parent class; and I think a sufficient fix would be to replace the type-equality check with an isinstance check in:

cc @iritkatriel; raised via agronholm/exceptiongroup#40 (comment)

Linked PRs

• gh-99553: fix bug where an ExceptionGroup subclass can wrap a BaseException #99572
• [3.11] gh-99553: fix bug where an ExceptionGroup subclass can wrap a BaseException (GH-99572) #99580
• gh-99553: add tests for ExceptionGroup wrapping #99615
• [3.11] gh-99553: add tests for ExceptionGroup wrapping (GH-99615) #103435