Custom auth backend with an extended AnonymousUser

auth_backend.py

from django.contrib.auth import SESSION_KEY, BACKEND_SESSION_KEY, load_backend
from users.models import AnonymousUser

def get_user(request):
    try:
        user_id = request.session[SESSION_KEY]
        backend_path = request.session[BACKEND_SESSION_KEY]
        backend = load_backend(backend_path)
        user = backend.get_user(user_id) or AnonymousUser()
    except KeyError:
        user = AnonymousUser()
    return user


class LazyUser(object):
    def __get__(self, request, obj_type=None):
        if not hasattr(request, '_cached_user'):
            request._cached_user = get_user(request)
        return request._cached_user


class AuthenticationMiddleware(object):
    def process_request(self, request):
        assert hasattr(request, 'session'), "The Django authentication middleware requires session middleware to be installed. Edit your MIDDLEWARE_CLASSES setting to insert 'django.contrib.sessions.middleware.SessionMiddleware'."
        request.__class__.user = LazyUser()
        return None