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
9176	NoReverseMatch for optional arguments	to.roma.from.djbug@…	Malcolm Tredinnick	"A number of URLs of my site have the form /<object_name>/something. The objects can have several representations, one of which is canonical, so I have a decorator that redirects requests with non-canonical names. The decorator (simplified) looks like this:
{{{
def url_from_view(view, **kwargs):
    return django.core.urlresolvers.reverse(view, kwargs=kwargs)

def decorator(view):
    def wrapper(request, obj_name, **kwargs):
        obj = load_object(obj_name)
        if canonical_name(obj) != obj_name:
            return HttpResponseRedirect(url_from_view(wrapper, obj_name=canonical_name(obj), **kwargs)
        return view(request, obj, obj_name, **kwargs)
    return wrapper
}}}
Everything worked excellently, until I added something like this to my urlpatterns:
{{{
(r""^(?P<parent>[a-zA-Z0-9_-])/something(?:/(?P<child>\d+))?/*$"", something),
}}}
For /parent_noncanonical/something/42 it calls something(parent=""parent_noncanonical"", child=""42"") which then reverses the URL and redirects correctly to /parent_canonical/something/42.
However, if the child part is omitted, because it’s optional in the regex, as in /parent_noncanonical/something, the view gets called as something(parent=""parent_noncanonical"", child=None) which then fails to reverse.

The problem is that the reverse operation turns out not to be defined for some of data that the regular expressions can produce: when I pass it the same view and the same data that have been chosen by the urlpatterns, it fails to produce a reverse match.

The reverse matching engine should handle None parameters by finding matches which do not contain the corresponding groups. For example, for r""(?P<first>[a-z])?(?P<second>[0-9])?(?P<third>[A-Z])?"" and first=""d"", second=None, third=""J"" the correct reverse match is ""dJ"".

Here’s the relevant Django code (core/urlresolvers.py, 227–241):
{{{
        for result, params in possibilities:
            if args:
                # skipped
            else:
                if set(kwargs.keys()) != set(params):
                    continue
                unicode_kwargs = dict([(k, force_unicode(v)) for (k, v) in kwargs.items()])
                candidate = result % unicode_kwargs
            if re.search(u'^%s' % pattern, candidate, re.UNICODE):
                return candidate
        raise NoReverseMatch(""Reverse for '%s' with arguments '%s' and keyword ""
                ""arguments '%s' not found."" % (lookup_view, args, kwargs))
}}}
For the URL pattern above, the possibilities are correctly enumerated:
[(u'%(parent)s/something', ['parent']), (u'%(parent)s/something/%(child)s', ['parent', 'child'])]
But then it fails to take into account the fact child is None, and chooses the wrong possibility.

Perhaps simply eliminating None by changing set(kwargs.keys()) != set(params) to set([k for (k, v) in kwargs.items() if v]) != set(params) could do the job."	Uncategorized	closed	Core (Other)	1.0	Normal	wontfix			Design decision needed	0	0	0	0	0	0