Have debug page show "During handling of this exception, another exception occurred.."

[Ram Rachum]

Python 3 has this thing where exceptions have optional __cause__ and __context__, which is useful when an exception causes another exception. When you get a Python 3 traceback, you see tracebacks for all the exceptions in the tree. Currently it seems that the Django debug page ignores this information. It would be nice if it would display a separate traceback for each such exception.

[Baptiste Mispelon]

I like this idea.

I find that it really helps with debugging (especially when Django has a bad habit of masking exceptions).

[Tomáš Ehrlich]

The easiest way to review, hook up this view into your urlconf:

def chained_exception(request):
    try:
        raise AttributeError('Top level')
    except AttributeError as explicit:
        try:
            raise ValueError('Second exception') from explicit
        except ValueError:
            raise IndexError('Final exception')

[Tim Graham <timograham@…>]

In 8414fcf1:

Fixes #23643 -- Added chained exception details to debug view.

[Tim Graham]

Ran into a crash (Python 2 only) when a view raised an IntegrityError:

Traceback (most recent call last):
  File "/usr/lib/python2.7/wsgiref/handlers.py", line 85, in run
    self.result = application(self.environ, self.start_response)
  File "/home/tim/code/django/django/contrib/staticfiles/handlers.py", line 63, in __call__
    return self.application(environ, start_response)
  File "/home/tim/code/django/django/core/handlers/wsgi.py", line 177, in __call__
    response = self.get_response(request)
  File "/home/tim/code/django/django/core/handlers/base.py", line 218, in get_response
    response = self.handle_uncaught_exception(request, resolver, sys.exc_info())
  File "/home/tim/code/django/django/core/handlers/base.py", line 261, in handle_uncaught_exception
    return debug.technical_500_response(request, *exc_info)
  File "/home/tim/code/django/django/views/debug.py", line 97, in technical_500_response
    html = reporter.get_traceback_html()
  File "/home/tim/code/django/django/views/debug.py", line 387, in get_traceback_html
    c = Context(self.get_traceback_data(), use_l10n=False)
  File "/home/tim/code/django/django/views/debug.py", line 332, in get_traceback_data
    frames = self.get_traceback_frames()
  File "/home/tim/code/django/django/views/debug.py", line 503, in get_traceback_frames
    tb = self.tb if not exceptions else exc_value.__traceback__
AttributeError: 'IntegrityError' object has no attribute '__traceback__'

[Tomáš Ehrlich]

1. I've submitted new PR with updated plain-text output ​https://github.com/django/django/pull/4412

2. I couldn't reproduce the AttributeError. I tried this simple view:

def view(request):
    raise IntegrityError('Error raised')

We could simple replace this line:

      tb = self.tb if not exceptions else exc_value.__traceback__

with safer, but longer:

        if not exceptions or not hasattr(exc_value, '__traceback__'):
            tb = self.tb
        else:
            tb = exc_value.__traceback__

The point was that exceptions should have been already empty in Python 2, but somewhat it wasn't. Would be great if we could reproduce your problem.

[Tim Graham]

Here's a view to reproduce AttributeError: 'IntegrityError' object has no attribute '__traceback__':

def vote(request, poll_id):
    Question.objects.create(question_text='foobar')

[Florian Apolloner]

See the PR ​https://github.com/django/django/pull/4530 for more details.

[fero]

And the PR ​https://github.com/django/django/pull/4568

the cause attribute is set in db/utils.py in wrapping DatabaseException

[Tim Graham <timograham@…>]

In 38eacbd:

Refs #23643 -- Fixed debug view regression on Python 2.

Thanks Tomáš Ehrlich for help with the patch.

[Tim Graham]

The one item remaining is adding a test for the ​plain text exception chain report. Tomas said he will do that soon.

[Tomáš Ehrlich]

Tests added, thank you for patience!

[Tim Graham]

In 59383f1e3a8ab5a6477dbd5bb1d7c32366a9d8f8:

Ref #23643 -- Added plain text report of exception chain.