gh-141732: Fix ExceptionGroup repr changing when original exception sequence is mutated

[dr-carlos]

• BaseExceptionGroup.__repr__ now overrides BaseException.__repr__, using the group's exception tuple instead of the mutable argument passed in.
• ComplexExtendsException (a private macro in Objects/exceptions.c) now has a parameter to modify the exception's __repr__ and not just its __str__.

• Issue: ExceptionGroup repr is misleading if original exception sequence is mutated #141732

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📚 Documentation preview 📚: https://cpython-previews--141736.org.readthedocs.build/

[iritkatriel]

Thanks.

We don't make user-visible changes to builtin types without a PEP, so this PR cannot be merged as is.

We could document current behaviour (i.e., don't mutate the list) if it's not clear from the docs and then consider if we want to change it.

If we do want to change it, maybe it should be to make a shallow copy of the list rather than make it a tuple (so the repr/str won't change and break existing code that relies on current behaviour).

[dr-carlos]

Thanks.

We don't make user-visible changes to builtin types without a PEP, so this PR cannot be merged as is.

We could document current behaviour (i.e., don't mutate the list) if it's not clear from the docs and then consider if we want to change it.

If we do want to change it, maybe it should be to make a shallow copy of the list rather than make it a tuple (so the repr/str won't change and break existing code that relies on current behaviour).

Thanks for the quick response! That all makes sense.

Just to clarify - is c87b66b not a very similar change in this release without a PEP? But maybe I'm missing something :)

[iritkatriel]

If you implement BaseExceptionGroup_repr so that the output doesn't change, then this change makes sense and can be merged and backported.

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[picnixz]

PyObject_Repr and PySequence_List can raise so we cannot continue calling the C API unless the calls succeeeded.

[dr-carlos]

Currently we're using PyList_Check and PyTuple_Check to determine whether we can use the list or tuple repr - is it worth doing exact checks here instead, in case list or tuple are subclassed whilst overriding the __repr__?

[dr-carlos]

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@picnixz, @iritkatriel: please review the changes made to this pull request.