code: def _cache_key(self, attr_name):
          
          dn = self._ldap_user.dn
          return valid_cache_key(
              "auth_ldap.{}.{}.{}".format(self.__class__.__name__, attr_name, dn)
          )
docs: Memcache keys can't have spaces in them, so we'll remove them from the
          DN for maximum compatibility.