Issues when showing broken Exception.__notes__

[pR0Ps]

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

[gaogaotiantian]

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?

[pR0Ps]

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.

[iritkatriel]

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

[iritkatriel]

Assigned to @ambv for a backport decision.

[hugovk]

@ambv Shall we backport or close this issue?