gh-99553: add tests for ExceptionGroup wrapping by Zac-HD · Pull Request #99615 · python/cpython

Zac-HD · 2022-11-20T08:13:48Z

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

Issue: Subclasses of ExceptionGroup can wrap BaseExceptions #99553

sobolevn · 2022-11-20T09:05:39Z

@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 :)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

iritkatriel · 2022-11-20T10:58:43Z

+ 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`.


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’.”

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 ExceptionGroupbecause 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)

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

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

Zac-HD · 2022-11-29T00:02:16Z

Ready to merge, I think?

iritkatriel · 2022-11-29T10:12:54Z

+ 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`.


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 · 2023-01-01T15:53:33Z

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 · 2023-04-11T02:50:44Z

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 · 2023-04-11T03:40:03Z

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

miss-islington · 2023-04-11T06:44:57Z

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

(cherry picked from commit 4cd1cc8) Co-authored-by: Zac Hatfield-Dodds <zac.hatfield.dodds@gmail.com>

bedevere-bot · 2023-04-11T06:45:10Z

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

hauntsaninja · 2023-04-11T06:45:34Z

Thank you!

(cherry picked from commit 4cd1cc8) Co-authored-by: Zac Hatfield-Dodds <zac.hatfield.dodds@gmail.com>

Zac-HD requested a review from iritkatriel as a code owner November 20, 2022 08:13

bedevere-bot added awaiting review tests Tests in the Lib/test dir labels Nov 20, 2022

pythongh-99553: clarify doc and add tests
4b9e01a

Zac-HD force-pushed the zac/custom-exceptiongroup-docs-tests branch from 8158778 to 4b9e01aCompare November 20, 2022 08:26

sobolevn added the skip news label Nov 20, 2022

AlexWaygood changed the title ~~gh-99572: clarify doc and add tests~~gh-99553: clarify doc and add testsNov 20, 2022

bedevere-bot mentioned this pull request Nov 20, 2022
Subclasses of ExceptionGroup can wrap BaseExceptions #99553
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hauntsaninja closed this Nov 20, 2022

hauntsaninja reopened this Nov 20, 2022

hauntsaninja reviewed Nov 20, 2022
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Comment threadDoc/library/exceptions.rst Outdated

Update Doc/library/exceptions.rst
29b9f27
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

iritkatriel reviewed Nov 20, 2022
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AlexWaygood changed the title ~~gh-99553: clarify doc and add tests~~gh-99553: ExceptionGroups: clarify doc and add testsNov 20, 2022

hauntsaninja reviewed Nov 21, 2022
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Comment threadDoc/library/exceptions.rst Outdated

Update Doc/library/exceptions.rst
07bdfea
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

iritkatriel reviewed Nov 29, 2022
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kumaraditya303 requested changes Jan 1, 2023
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Zac-HD closed this Apr 11, 2023

hauntsaninja reopened this Apr 11, 2023

undo docs changes
9bdb9c6

hauntsaninja added the needs backport to 3.11 only security fixes label Apr 11, 2023

Merge branch 'main' into zac/custom-exceptiongroup-docs-tests
774b0b8

Merge branch 'main' into zac/custom-exceptiongroup-docs-tests
043d002

hauntsaninja changed the title ~~gh-99553: ExceptionGroups: clarify doc and add tests~~gh-99553: add tests for ExceptionGroup wrappingApr 11, 2023

hauntsaninja merged commit 4cd1cc8 into python:mainApr 11, 2023

bedevere-bot removed the awaiting changes label Apr 11, 2023

miss-islington pushed a commit to miss-islington/cpython that referenced this pull request Apr 11, 2023
pythongh-99553: add tests for ExceptionGroup wrapping (pythonGH-99615)
0fd85aa
(cherry picked from commit 4cd1cc8) Co-authored-by: Zac Hatfield-Dodds <zac.hatfield.dodds@gmail.com>

bedevere-bot removed the needs backport to 3.11 only security fixes label Apr 11, 2023

miss-islington added a commit that referenced this pull request Apr 11, 2023
gh-99553: add tests for ExceptionGroup wrapping (GH-99615)
254494c
(cherry picked from commit 4cd1cc8) Co-authored-by: Zac Hatfield-Dodds <zac.hatfield.dodds@gmail.com>

Zac-HD deleted the zac/custom-exceptiongroup-docs-tests branch April 11, 2023 07:58

warsaw pushed a commit to warsaw/cpython that referenced this pull request Apr 11, 2023
pythongh-99553: add tests for ExceptionGroup wrapping (python#99615)
18df988

aisk pushed a commit to aisk/cpython that referenced this pull request Apr 18, 2023
pythongh-99553: add tests for ExceptionGroup wrapping (python#99615)
dd9946d

-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`.

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