gh-99553: add tests for ExceptionGroup wrapping

[Zac-HD]

Follows #99572 (comment) to clarify a potential point of confusion. CC @iritkatriel (thank you again!)

• Issue: Subclasses of ExceptionGroup can wrap BaseExceptions #99553

[sobolevn]

@Zac-HD I cannot do that, but if you change you PR title from gh-99572 to gh-99553 - the CI failure will go away :)

[iritkatriel]

I think this is a strange thing to say in a doc, because why would you expect it to not wrap any Exception?

Maybe instead of the second sentence we could just explain the motivation for the restriction: “This ensures that wrapped BaseExceptions are not caught by ‘except Exception’.”

[Zac-HD]

why would you expect it to not wrap any Exception?

Suppose we have class MyEG(BaseExceptionGroup, X):, where except X: will catch such a group because it extends X. We disallow wrapping BaseExceptions in an ExceptionGroup because they would not be caught by except Exception: if unwrapped.

However this logic is not applied consistently: if X is e.g. KeyError, then except KeyError: can indeed catch MyEG("", [ValueError()]). I find this inconsistency surprising, which is why I commented on #99572 and then opened this PR!

(I'm not trying to reopen the design discussion, just trying to explain my perspective)

[iritkatriel]

Ok, I see. Well yes, the Exception case is different - people use ‘expect Exception’ as ‘catch everything except fatal errors’.

[Zac-HD]

Ready to merge, I think?

[iritkatriel]

I don't think this comment is easy to understand without the justification. How about this?

Suggested change

	A subclass of :exc:`BaseExceptionGroup` that extends :exc:`Exception`
	cannot wrap :exc:`BaseException`\ s, as is the case with :exc:`ExceptionGroup`.
	However, there is no further restriction: a :exc:`BaseExceptionGroup` subclass
	which extends :exc:`KeyError` could wrap any :exc:`Exception`.
	Like :exc:`ExceptionGroup`, user-defined subclasses of :exc:`BaseExceptionGroup`
	that extend :exc:`Exception` can only wrap :exc:`Exception` subclasses.
	This is to prevent exceptions that represent fatal errors from being wrapped in an
	exception group which can be caught by ``except Exception``. See also :ref:`exceptions`.

[kumaraditya303]

Please address Irit's review.

[bedevere-bot]

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.

[Zac-HD]

I've taken a few attempts, and haven't found a way to explain this which reliably clarifies rather than confuses, so I'll just close the PR 🙂

[hauntsaninja]

If it's hard to figure out appropriate changes to the docs, I can at least merge the tests.

[miss-islington]

Thanks @Zac-HD for the PR, and @hauntsaninja for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11.
🐍🍒⛏🤖

[bedevere-bot]

GH-103435 is a backport of this pull request to the 3.11 branch.

[hauntsaninja]

Thank you!