doctest fails to compare tracebacks with notes

[sobolevn]

Bug report

Minimal reproducer:

"""
Module doctest

>>> raise_with_note()
Traceback (most recent call last):
  ...
ValueError: Text
Note
"""

def raise_with_note():
    err = ValueError('Text')
    err.add_note('Note')
    raise err

Now, run it, you will get a very strange error:

» ./python.exe -m doctest ex.py
**********************************************************************
File "/Users/sobolev/Desktop/cpython/ex.py", line 4, in ex
Failed example:
    raise_with_note()
Expected:
    Traceback (most recent call last):
      ...
    ValueError: Text
    Note
Got:
    Traceback (most recent call last):
      File "/Users/sobolev/Desktop/cpython/Lib/doctest.py", line 1374, in __run
        exec(compile(example.source, filename, "single",
      File "<doctest ex[0]>", line 1, in <module>
        raise_with_note()
      File "/Users/sobolev/Desktop/cpython/ex.py", line 14, in raise_with_note
        raise err
    ValueError: Text
    Note
**********************************************************************
1 items had failures:
   1 of   1 in ex
***Test Failed*** 1 failures.

Things that do not help:

• IGNORE_EXCEPTION_DETAIL
• ELLIPSIS
• Coping the whole traceback

Related to how exceptions with notes are represented in

cpython/Lib/doctest.py

Line 1396 in 7237fb5

exc_msg = traceback.format_exception_only(*exception[:2])[-1]

I am working on this issue right now :)

Refs #111157

Linked PRs

• gh-111159: Fix doctest output comparison for exceptions with notes #111160
• [3.12] gh-111159: Fix doctest output comparison for exceptions with notes (GH-111160) #111169
• [3.11] gh-111159: Fix doctest output comparison for exceptions with notes (GH-111160) #111170
• gh-111159: Fix SyntaxError doctests for non-builtin exception classes #111541
• [3.12] gh-111159: Fix SyntaxError doctests for non-builtin exception classes (GH-111541) #111732
• [3.11] gh-111159: Fix SyntaxError doctests for non-builtin exception classes (GH-111541) #111733

[scoder]

With this change in place, I now get a (reproducible) doctest failure in lxml that seems rather random:
https://github.com/lxml/lxml/blob/7356acba91bcf37b2c1c6ab5b7c9e37a4f1145b9/doc/xpathxslt.txt

Doctest: xpathxslt.txt ... 
======================================================================
ERROR: txt (xpathxslt)
Doctest: xpathxslt.txt
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/opt/python3.13/lib/python3.13/unittest/case.py", line 58, in testPartExecutor
    yield
  File "/opt/python3.13/lib/python3.13/unittest/case.py", line 636, in run
    self._callTestMethod(testMethod)
  File "/opt/python3.13/lib/python3.13/unittest/case.py", line 589, in _callTestMethod
    if method() is not None:
       ^^^^^^^^
  File "/opt/python3.13/lib/python3.13/doctest.py", line 2257, in runTest
    failures, tries = runner.run(
                      ^^^^^^^^^^^
  File "/opt/python3.13/lib/python3.13/doctest.py", line 1537, in run
    return self.__run(test, compileflags, out)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/python3.13/lib/python3.13/doctest.py", line 1402, in __run
    exc_msg_index = next(
                    ^^^^^
StopIteration

I'm not using "notes" anywhere in the file (and wouldn't know what I'd use them for), so this breaks existing tests.

[sobolevn]

Thanks, I will have a look!

[scoder]

I think the problem is with subclasses of SyntaxError. If they live in packages, the startswith() comparison in the newly added code can fail.

[hugovk]

Anything left for this issue or can it be closed? Thanks!