id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
25369	Wrong exception being used in official example creates very subtle bugs for the syndication feed framework.	Paul Graham	nobody	"
== '''The Setup''' ==

'''A. Example code from documentation'''

{{{
from django.contrib.syndication.views import Feed
from django.shortcuts import get_object_or_404

class BeatFeed(Feed):
    description_template = 'feeds/beat_description.html'

    def get_object(self, request, beat_id):
        return get_object_or_404(Beat, pk=beat_id)
}}}


'''B. Code from Feed(object)'''

{{{
def __call__(self, request, *args, **kwargs):
        try:
            obj = self.get_object(request, *args, **kwargs)
        except ObjectDoesNotExist:
            raise Http404('Feed object does not exist.')
}}}


In the '''A complex example''' ( [https://docs.djangoproject.com/en/1.8/ref/contrib/syndication/#a-complex-example] ) example code the method {{{get_object()}}} uses {{{get_object_or_404()}}} to raise an {{{Http404}}} exception (See A. above).  However the the Feed class reference on the same page indicates that {{{ObjectDoesNotExist}}} exception should be raised on error below looking at the code of the {{{__call__()}}} method of the Feed class shows that it calls the {{{get_object()}}} method it does indeed catch {{{ObjectDoesNotExist}}} exceptions (See B. above). However, the reason the example code runs without any obvious errors and the root of the problem at hand is that upon catching an ObjectDoesNotExist exception the Feed class then raises an {{{Http404}}} error. The {{{Http404}}} exception being raised in the example code for {{{get_object()}}} is not being caught where it should be, inside {{{Feed.__call__()}}}  but ultimately farther up the chain by the code that was meant to catch errors from {{{Feed.__call__()}}}


== '''The Problem''' ==

It should be noted that people have been depending upon the example in question as reference and that changing how the Feed class handles exceptions or changing code that handles from
{{{get_object()}}} could be hazardous as it would could break previously conformant code created by users. In fact, I discovered this bug while reading a third party Django RSS tutorial that was based on the official Django documentation.

There are at least two problematic scenarios that spring to mind:

The first problematic scenario is that the way the Feed class handles {{{ObjectDoesNotExist}}} exceptions changes and the new error handling code is skipped by an {{{Http404}}} error. Ex.

In class Feed:

{{{
def __call__(self, request, *args, **kwargs):
        try:
            obj = self.get_object(request, *args, **kwargs)
        except ObjectDoesNotExist:
            # Any new error handling code placed here will 
            # be skipped the Http404 exception client code
            # could be using. <-- Problem 
            raise Http404('Feed object does not exist.')
}}}

The second problematic scenario if users have been calling the {{{Feed.__call__()}}} method for any reason (subclassing Feed and overidding __call__ method perhaps) with the idea, backed by an example from the documentation, that they should only be catching {{{Http404}}} on error   Ex.

{{{
try:
        x = my_feed_instance = get_object(request, *args, **kwargs)
except Http404:
        # Code here won't be run if an ObjectDoesNotExist exception is raised.
}}}"	Bug	new	contrib.syndication	1.8	Normal		syndication feed future subtle documentation		Unreviewed	0	0	0	0	0	0