Mutations to the original exception sequence argument affects the exception group

[bswck]

Things to check first

• I have searched the existing issues and didn't find my bug already reported there

• I have checked that my bug is still present in the latest release

Exceptiongroup version

1.2.2

Python version

3.9, 3.11

What happened?

The CPython implementation of PEP 654 transforms the ExceptionGroup's exception sequence argument to a tuple during initialization and stores just that, effectively disallowing the original sequence mutations to affect the created group—contrary to the exceptiongroup implementation, which reflects all, even incorrect, mutations.

This is not a violation of PEP 654. It's an inconsistency with the upstream CPython implementation of PEP 654.

Therefore (despite I personally think that this is worth fixing) if it's not considered a bug in this package, please close this issue.

How can we reproduce the bug?

from exceptiongroup import ExceptionGroup
excs = [ValueError()]
group = ExceptionGroup("", excs)
print(group.exceptions)
excs.clear()
print(group.exceptions)

outputs:

• for Python 3.9.19 (main, Aug 14 2024, 05:11:09) [Clang 18.1.8 ] on linux:

  (ValueError(),)
  ()

• for Python 3.11.9 (main, Aug 14 2024, 05:07:28) [Clang 18.1.8 ] on linux:

  (ValueError(),)
  (ValueError(),)

Note that in the backport implementation exception groups, after clearing the stored sequence, it is still possible to raise such exception groups with (0 sub-exceptions)—despite the constructor disallowing that state initially.

Suggested resolution

Explicitly casting __exceptions to a tuple in

exceptiongroup/src/exceptiongroup/_exceptions.py

Line 97 in f821bc5

instance._exceptions = __exceptions

No longer casting self._exceptions to a tuple in the exceptions property

exceptiongroup/src/exceptiongroup/_exceptions.py

Lines 115 to 119 in f821bc5

	@property
	def exceptions(
	self,
	) -> tuple[_BaseExceptionT_co | BaseExceptionGroup[_BaseExceptionT_co], ...]:
	return tuple(self._exceptions)

and other necessary work for future maintenance.

[agronholm]

I get this, but I'm also somewhat wary of changes to this because the original sequence is passed also to __init__(). I'm wondering if I'll just be replacing one compatibility issue with another?

[agronholm]

I just checked, and CPython 3.13 and 3.14 also exhibit this behavior. So where exactly are you seeing a transformation into a tuple?

[agronholm]

I think I see the confusion: the exceptions attribute returns the original tuple, while repr(excgrp) reflects the original sequence and will change if it's modified.