Memcache keys can't have spaces in them, so we'll remove them from the
        DN for maximum compatibility.

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