Issues when showing broken Exception.__notes__

Bug report

When notes on an exception cannot be shown because the repr and str raise exceptions, a <note str() failed> message is shown instead. In the case where exception.__notes__ is not a sequence and cannot be shown, a <__notes__ repr() failed> is shown. In the second case, the message does not include a trailing newline.

__notes__ is a list containing a broken "note" - output includes a newline:

class A:
    def __repr__(self):
        raise Exception()

e = Exception()
e.__notes__ = [A()]  # !!!
raise e

user@host $ ./repro.py
Traceback (most recent call last):
  File "./repro.py", line 8, in <module>
    raise e
Exception
<note str() failed>
user@host $

__notes__ is just a single broken "note" - output does not include a newline:

class A:
    def __repr__(self):
        raise Exception()

e = Exception()
e.__notes__ = A()  # !!!
raise e

user@host $ ./repro.py
Traceback (most recent call last):
  File "./repro.py", line 8, in <module>
    raise e
Exception
<__notes__ repr() failed>user@host $

Additionally, when __notes__ is a string/bytes, the contents are expoded over multiple lines because of an isinstance(__notes__, Sequence) check.

String:

e = Exception()
e.__notes__ = "a note"
raise e

user@host $ ./repro.py
Traceback (most recent call last):
  File "./repro.py", line 8, in <module>
    raise e
Exception
a

n
o
t
e
user@host $

Bytes:

e = Exception()
e.__notes__ = b"a note"
raise e

user@host $ ./repro.py
Traceback (most recent call last):
  File "./repro.py", line 8, in <module>
    raise e
Exception
97
32
110
111
116
101
user@host $

Even though the above are all edge cases, since there are some checks that handle these cases already, it makes sense to handle them more gracefully.

Your environment

CPython versions tested on: 3.12.0a7+
Operating system and architecture: macOS Mojave (x86)

Linked PRs

gh-103895: Improve how invalid Exception.__notes__ are displayed #103897

Is Exception.__notes__ supposed to be directly written? add_note() is definitely the recommended interface to use the feature. The docs mentioned __notes__ as a "list of the notes", and is breaking that a "corner case", or simply the wrong way to use it?

It is not supposed to be directly written (as far as I can tell), but while I was experimenting with some unrelated exception handling I was digging into this and noticed the inconsistency with the newlines so I figured I'd make a quick patch.

The only reason I bothered at all is because there's already some amount of handling of invalid notes so it seems it's at least been considered that someone may write Exception.__notes__ directly.

gh-103895: Improve how invalid `Exception.__notes__` are displayed (#…

Merged into 3.12. Leaving open pending decision about backporting to 3.11.

Merge branch 'main' into supermon

Merge branch 'main' into inlinecomp2

@ambv

Assigned to @ambv for a backport decision.

@ambv

@ambv Shall we backport or close this issue?

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Issues when showing broken `Exception.notes` #103895

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Issues when showing broken Exception.__notes__ #103895

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Bug report

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gaogaotiantian commented on Apr 26, 2023

pR0Ps commented on Apr 26, 2023

iritkatriel commented on May 1, 2023

iritkatriel commented on Jun 27, 2023

hugovk commented on Nov 10, 2023

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Issues when showing broken `Exception.notes` #103895