class LazyUser(object):

    def __get__(self, request, obj_type=None):

        if not hasattr(request, '_cached_user'):

            from django.contrib.auth import get_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