Make `test --pdb` pass exception to `pdb.post_mortem()` on Python 3.13

[Adam Johnson]

Python 3.13 added support for navigating between chained exceptions in pdb with ​the new 'exceptions' command.

Currently, using test --pdb does not allow this navigation, failing with this message:

(Pdb) exceptions
Did not find chained exceptions. To move between exceptions, pdb/post_mortem must be given an exception object rather than a traceback.

PDBDebugResult currently passes pdb.post_mortem() the traceback object. But it turns out that the pdb changes in 3.13 also allow passing an exception here, which is currently undocumented but I ​submitted a PR.

If we switch to passing the exception, it enables switching between chained exceptions:

> /.../django/core/management/commands/dumpdata.py(285)handle()
-> raise CommandError("Unable to serialize database: %s" % e)
(Pdb) exceptions
    0 TypeError("string argument expected, got 'bytes'")
>   1 CommandError("Unable to serialize database: string argument expected, got 'by...
(Pdb) exceptions 0
> /.../django/core/management/base.py(181)write()
-> self._out.write(style_func(msg))
(Pdb)

(I actually made this change to debug a chained exception failure whilst working on #36056.)

[Adam Johnson]

pytest is already taking advantage of this “secret feature“: ​https://github.com/pytest-dev/pytest/issues/12707

[Natalia Bidart]

Thank you Adam! After some investigation following your links, I think this makes sense. I haven't checked myself yet, but do you think there is a sensible test to add to ensure we don't regress in this code? Also, a nitpick, the commit message should "Refs #34900" (same with the other Python 3.13 related ticket).

[Adam Johnson]

I haven't checked myself yet, but do you think there is a sensible test to add to ensure we don't regress in this code?

We don’t seem to have any test coverage of the --pdb option right now, so it’s not super easy to add. Even if we did, I don’t think we can easily do it without mocking pdb, which wouldn't cover the change here.

Also, a nitpick, the commit message should "Refs #34900" (same with the other Python 3.13 related ticket).

Should it? This is to take advantage of a new feature, not to fix something for the new version. Similarly, my other ticket is a bug that existed before 3.13, just got better exposed by it.

[GitHub <noreply@…>]

In 51df0dff:

Fixed #36057 -- Enabled test runner to debug chained exceptions with --pdb on Python 3.13+.