urlresolvers.RegexURLPattern._get_callback doesn't handle ValueError

[django@…]

It seems import raises a ValueError if the module name is empty. This can occur in _get_callback and isn't handled properly:

Using a named URL pattern "blog-post" in a permalink-decorated get_absolute_url method causes the exception detailed below.

Switching the name to "blog.post" instead fools the reverse functions into allowing it because _get_callback receives a non-empty module name.

Traceback (most recent call last):
File "d:\python25\Lib\site-packages\django\template\__init__.py" in render_node
  741. result = node.render(context)
File "d:\python25\Lib\site-packages\django\template\defaulttags.py" in render
  125. nodelist.append(node.render(context))
File "d:\python25\Lib\site-packages\django\template\__init__.py" in render
  791. output = self.filter_expression.resolve(context)
File "d:\python25\Lib\site-packages\django\template\__init__.py" in resolve
  576. obj = resolve_variable(self.var, context)
File "d:\python25\Lib\site-packages\django\template\__init__.py" in resolve_variable
  674. current = current()
File "d:\python25\Lib\site-packages\django\utils\functional.py" in _curried
  3. return _curried_func(*(args+moreargs), **dict(kwargs, **morekwargs))
File "d:\python25\Lib\site-packages\django\db\models\base.py" in get_absolute_url
  453. return settings.ABSOLUTE_URL_OVERRIDES.get('%s.%s' % (opts.app_label, opts.module_name), func)(self)
File "d:\python25\Lib\site-packages\django\db\models\__init__.py" in inner
  27. return reverse(bits[0], None, *bits[1:3])
File "d:\python25\Lib\site-packages\django\core\urlresolvers.py" in reverse
  251. return '/' + resolver.reverse(viewname, *args, **kwargs)
File "d:\python25\Lib\site-packages\django\core\urlresolvers.py" in reverse
  222. return pattern.reverse_helper(lookup_view, *args, **kwargs)
File "d:\python25\Lib\site-packages\django\core\urlresolvers.py" in reverse_helper
  233. sub_match = self.reverse(lookup_view, *args, **kwargs)
File "d:\python25\Lib\site-packages\django\core\urlresolvers.py" in reverse
  225. elif pattern.callback == lookup_view or pattern.name == lookup_view:
File "d:\python25\Lib\site-packages\django\core\urlresolvers.py" in _get_callback
  134. self._callback = getattr(__import__(mod_name, {}, {}, ['']), func_name)

  ValueError at /blog/articles/
  Empty module name

[django@…]

Can't get this to work at all in fact.

Switching to "blog.post" (an existing named urlpattern) causes NoReverseMatch exceptions.
Using the generic view name "django.views.generic.date_based.object_detail" causes ValueErrors because of empty module name in import.

[Malcolm Tredinnick]

Could you give an example of a url pattern and the associated get_absolute_url() method that causes this problem, please?

[django@…]

Sorry, false alarm. I had an URL pattern with a blank view function in there:

  (r'articles/category/(?P<category>.+)/page/(?P<page>.+)', ''),

which is what was causing the problem. I'd assumed that this was safe enough during development - I was filling in the views bit by bit, but I guess the reverse resolution checks each URL pattern's view function so problems showed up when I used the permalink decorator.

Further down, I had the named pattern:

                        url(r'articles/(?P<year>\d+)/(?P<month>\d+)/(?P<day>\d+)/(?P<slug>.+)/$',
                            'object_detail',
                            dict(posts_dict, slug_field = 'slug', month_format = '%m'),
                            'blog-post'),

which I had thought was causing the problems.

(However I'm still not convinced that the empty view functions shouldn't just be disregarded by the reverse URL processing. My feeling is that it's either valid or invalid to have an empty string in there. If it's valid then it should just be silently passed over by the reverse URL processing. If it's invalid you shouldn't be unaware of it until you start using reverse lookups.)

[Malcolm Tredinnick]

(In [5360]) Report an error if urlpatterns contain a string view name and it is empty.
Refs #4319.