Bug report

While testing pybind11 with Python 3.12alpha6 I ran into what looks like aPyErr_Fetch() behavior change. I also tried with the main branch @c6858d1.

I think the issue reduces to:

• PyErr_SetObject() as e.g. here:https://github.com/pybind/pybind11/blob/442261da585536521ff459b1457b2904895f23b4/tests/test_exceptions.cpp#L309

• Followed more-or-less immediately by aPyErr_Fetch() as e.g. here:https://github.com/pybind/pybind11/blob/442261da585536521ff459b1457b2904895f23b4/include/pybind11/pytypes.h#L482

The special twist is that the exception type involved raises aValueError in its__init__:

• https://github.com/pybind/pybind11/blob/442261da585536521ff459b1457b2904895f23b4/tests/test_exceptions.py#L295

For easy reference the same code copy-pasted here:

class FlakyException(Exception):    def __init__(self, failure_point):        if failure_point == "failure_point_init":            raise ValueError("triggered_failure_point_init")        self.failure_point = failure_point

• Up to and including 3.12alpha3:PyErr_Fetch() produces theFlakyException type (this here).

• With 3.12alpha6:PyErr_Fetch() produces theValueError type instead.

Additional detail:

Up to and including 3.12alpha3: onlyPyErr_NormalizeException() hits theValueError inFlakyException._init__, but notPyErr_Fetch().

Your environment

• CPython versions tested on: 3.12alpha6, main branch @c6858d1
• Operating system and architecture: Debian 5.19.11 clang14, ubuntu22 gcc11

Linked PRs

• gh-102594: test_fetch_exception_with_broken_init @ 3.12.0a3 #102606
• gh-102594: test_fetch_exception_with_broken_init @ main HEAD #102607
• gh-102594: PyErr_SetObject adds note to exception raised on normalization error #102675