gh-145850: Change some implementation details in struct.Struct

[serhiy-storchaka]

• calling it with non-ASCII string format will now raise a ValueError instead of UnicodeEncodeError
• calling it with non-ASCII bytes format will now raise a ValueError instead of struct.error
• getting the format attribute of uninitialized object will now raise an AttributeError instead of RuntimeError.

• Issue: Change some implementation details in struct.Struct #145850

[vstinner]

If we change exceptions, I would prefer a ValueError than an UnicodeDecodeError.

[serhiy-storchaka]

Thank you for your review, @vstinner.

[vstinner]

LGTM.

Raising ValueError if the format is invalid makes more sense than UnicodeEncodeError or struct.error.

Replacing RuntimeError with AttributeError is more surprising, but I'm fine with AttributeError.

If a project is impacted by this change, it's possible to become compatible with old and new Python versions by catching old and new exceptions. For example, except (UnicodeEncodeError, ValueError):. But I don't expect any project to catch exceptions since they're not supposed to pass invalid format string anyway :-)

[StanFromIreland]

CIFuzz found a bug, and it seems related to the changes! To download the testcase, see https://github.com/python/cpython/actions/runs/23007198595/artifacts/5892860339.

[StanFromIreland]

Oh, it is just incompatible with these changes, since they add a ValueError to the list of possible exceptions. Can you please update:

cpython/Modules/_xxtestfuzz/fuzzer.c

Line 136 in d4cc553

/* Ignore any struct.error exceptions, these can be caused by invalid