def authenticate_redirect(
        self, callback_uri=None, ax_attrs=["name", "email", "language",
                                           "username"]):

        """Returns the authentication URL for this service.

        After authentication, the service will redirect back to the given
        callback URI.

        We request the given attributes for the authenticated user by
        default (name, email, language, and username). If you don't need
        all those attributes for your app, you can request fewer with
        the ax_attrs keyword argument.
        """
        callback_uri = callback_uri or self.request.uri
        args = self._openid_args(callback_uri, ax_attrs=ax_attrs)
        self.redirect(self._OPENID_ENDPOINT + "?" + urllib.urlencode(args))
Returns the authentication URL for this service.

        After authentication, the service will redirect back to the given
        callback URI.

        We request the given attributes for the authenticated user by
        default (name, email, language, and username). If you don't need
        all those attributes for your app, you can request fewer with
        the ax_attrs keyword argument.