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`ExceptionGroup` repr is misleading if original exception sequence is mutated

gschaffner

Bug report

Bug description:

The exception listing in repr(exc_grp) diverges from exc_grp.exceptions when the initial sequence of exceptions passed to the constructor has been mutated.

Expected behavior: The exceptions listed in repr(exc_grp) match exc_grp.exceptions, which is immutable.

Reproducer:

exceptions = list[Exception]()
exceptions.append(ValueError())
exc_grp = ExceptionGroup("", exceptions)
print(f"{exc_grp.exceptions=!r}")
print(f"{exc_grp=!r}")
print()
exceptions.clear()
print(f"{exc_grp.exceptions=!r}")
print(f"{exc_grp=!r}")

Output:

exc_grp.exceptions=(ValueError(),)
exc_grp=ExceptionGroup('', [ValueError()])

exc_grp.exceptions=(ValueError(),)
exc_grp=ExceptionGroup('', [])

Expected output:

exc_grp.exceptions=(ValueError(),)
exc_grp=ExceptionGroup('', [ValueError()])

exc_grp.exceptions=(ValueError(),)
exc_grp=ExceptionGroup('', [ValueError()])

CPython versions tested on:

3.11, 3.12, 3.13, 3.14

Operating systems tested on:

Linux

Linked PRs