Pyramid route factory for a blog post.

gistfile1.py

class Post(object):

    #...

    @classmethod
    def factory(cls, request):
        if 'post_id' in request.matchdict:
            post = cls.from_identifier(request.matchdict['post_id'])
            if not post or not post.is_published():
                raise NotFound()
            if 'post_slug' in request.matchdict and \
                    post.slug != request.matchdict['post_slug']:
                raise HTTPMovedPermanently(
                        location=request.route_url('blog-post',
                        post=post))
            return post
        else:
            return None