Add limit_group option to traceback.format_exception_only

[sobolevn]

Feature or enhancement

Right now there's no limit in both width and depth that format_exception_only prints exception groups.

I propose adding limit_group=True parameter that will respect max_group_depth and max_group_width values and add ... (max_group_depth is {self.max_group_depth})\n and and {remaining} more exception{plural}\n, just like .format() does.

There are two reasons why I propose adding this option.

The first reason is that it will be consistent with .format(). One of the use-cases for format_exception_only is to compare parsed traceback produced by .format() with the one returned from format_exception_only. Right now (in my patch) it fails to compare these two corner-cases: too deep group and too wide group.

The second reason is that this code simply fails:

import sys, traceback
>>> exc = TypeError('bad type')
>>> for i in range(sys.getrecursionlimit() + 1):
...   exc = ExceptionGroup(f'eg{i}', [ValueError(i), exc, ValueError(-i)])
... 
>>> traceback.format_exception_only(exc, show_group=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    traceback.format_exception_only(exc, show_group=True)
  File "/Users/sobolev/Desktop/cpython2/Lib/traceback.py", line 169, in format_exception_only
    return list(te.format_exception_only(show_group=show_group))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/sobolev/Desktop/cpython2/Lib/traceback.py", line 942, in format_exception_only
    yield from ex.format_exception_only(show_group=show_group, _depth=_depth+1)
  File "/Users/sobolev/Desktop/cpython2/Lib/traceback.py", line 942, in format_exception_only
    yield from ex.format_exception_only(show_group=show_group, _depth=_depth+1)
  File "/Users/sobolev/Desktop/cpython2/Lib/traceback.py", line 942, in format_exception_only
    yield from ex.format_exception_only(show_group=show_group, _depth=_depth+1)
  [Previous line repeated 993 more times]
  File "/Users/sobolev/Desktop/cpython2/Lib/traceback.py", line 918, in format_exception_only
    formatted = _format_final_exc_line(
                ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/sobolev/Desktop/cpython2/Lib/traceback.py", line 175, in _format_final_exc_line
    valuestr = _safe_string(value, 'exception')
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RecursionError: maximum recursion depth exceeded

This is not very realistic, but it is totally possible.
This option will prevent this case from happening.

Questions:

• I am open to param name options, I choose limit_group, because it is in sync with show_group
• Default value: I use True, because it is safer (RecursionError case won't happen)

I have a PR ready :)
This is a part of #104150

Linked PRs

• gh-111448: Make traceback.format_exception_only respect max_group_width and max_group_depth #111952

[iritkatriel]

TracebackException constructor has max_group_width and max_group_depth.

[sobolevn]

Yes :)
But, format_exception_only did not respect these values until #111952

[iritkatriel]

Well, if format_exception_only should respect them then it should give them the same names.

But it's not the intention that the convenience wrappers will support everything that TracebackException supports. The wrappers should do something reasonable, and users who need to fine control over the traceback should use TracebackException directly.

[sobolevn]

Sorry, I am a bit confused about which names you are talking about. Maybe we should move this discussion to the PR? So we can have the same context.

[iritkatriel]

My point is that we don't need to complicate the API with another parameter (limit_group). Just make format_exception_only respect the existing limits on groups, and if anyone wants to turn that off they can do it by controlling those args.

[sobolevn]

Oh, I see 👍