Bug report

Bug description:

lst = []

class C:
    def __repr__(self):
        raise RuntimeError("Oh no!")

try:
    lst.index(C())
except ValueError as e:
    print("This is good")
    print(repr(e))

On Python 3.10 this prints

This is good
ValueError()

On Python 3.11 this prints

Traceback (most recent call last):
  File "<path>", line 8, in <module>
    lst.index(C())
  File "<path>", line 5, in __repr__
    raise RuntimeError("Oh no!")
RuntimeError: Oh no!

When PyErr_Format fails in current versions of Python, it doesn't set the exception and instead raises the new one:

In earlier versions of Python it used to raise the requested exception unconditionally

This looks to have been a deliberate change but it can affect the behaviour of real code (cython/cython#5894). Interestingly PyPy got there before you with this behaviour (https://foss.heptapod.net/pypy/pypy/-/issues/3978)

CPython versions tested on:

3.11

Operating systems tested on:

Linux