debug error reporter should include traceback for nested exceptions

[John Firebaugh]

There are a number of place in the django codebase that trap Exception and then raise a new exception. This results in less-than-ideal debugging information being presented in the default error page; in particular, the traceback from the original exception is unavailable.

Most recently, I encountered this in the template renderer:

class DebugNodeList(NodeList):
    def render_node(self, node, context):
        try:
            result = node.render(context)
        except TemplateSyntaxError, e:
            if not hasattr(e, 'source'):
                e.source = node.source
            raise
        except Exception, e:
            from sys import exc_info
            wrapped = TemplateSyntaxError(u'Caught an exception while rendering: %s' % force_unicode(e, errors='replace'))
            wrapped.source = node.source
            wrapped.exc_info = exc_info()
            raise wrapped
        return result

When this wrapped TemplateSyntaxError is displayed in the default error page, the traceback ends at the 'raise wrapped' line; the only part of the original exception that is available is the message ("'RelatedManager' object is not iterable" in my current case). This makes the source of the problem more difficult to determine. The traceback in wrapped.exc_info should be displayed in addition to, or instead of, the traceback currently displayed.

This was briefly ​discussed (exactly 1 year ago) on django-developers. A ​partial patch was posted.

[Glenn Maynard]

This isn't the right fix. This would have to be implemented in every custom exception reporter, including third-party ones that don't know about Django exceptions at all.

The actual problem is that DebugNodeList is raising the new exception incorrectly. It needs to include the original traceback.

try:
    ...
catch Exception, e:
    raise NewException, None, sys.exc_info()[2]

See #11461.

[Chris Beaven]

I agree.

[Chris Beaven]

(actually, this is more of a duplicate than a wontfix)