A filterspec to attach to your models for showing __isnull fields

NoneFilterSpec.py

class NoneFilterSpec(ChoicesFilterSpec):
    """Filters based on a field being either None or populated """

    def __init__(self, f, request, params, model, model_admin):
        super(NoneFilterSpec, self).__init__(f, request, params, model, model_admin)
        self.lookup_kwarg = '%s__isnull' % f.name
        self.lookup_val = request.GET.get(self.lookup_kwarg, None)
        self.lookup_choices = [True, False]
        self.lookup_choices.sort()

    def choices(self, clist):
        yield {
            'selected': self.lookup_val is None,
            'query_string': cl.get_query_string({}, [self.lookup_kwarg]),
            'display': _('All')
        }
        for val in self.lookup_choices:
            yield {
                'selected': smart_unicode(val) == self.lookup_val,
                'query_string': clist.get_query_string({self.lookup_kwarg: val}),
                'display': repr(val)
            }

    def title(self):
        return _('%(field_name)s that is empty') %  {'field_name': self.field.verbose_name}