    def validate_refresh_token(self, refresh_token, client, request, *args, **kwargs):
        """Ensure the Bearer token is valid and authorized access to scopes.
        OBS! The request.user attribute should be set to the resource owner
        associated with this refresh token.
        :param refresh_token: Unicode refresh token
        :param client: Client object set by you, see authenticate_client.
        :param request: The HTTP Request (oauthlib.common.Request)
        :rtype: True or False
        Method is used by:
            - Authorization Code Grant (indirectly by issuing refresh tokens)
            - Resource Owner Password Credentials Grant (also indirectly)
            - Refresh Token Grant
        """
        return False
        raise NotImplementedError('Subclasses must implement this method.')
Ensure the Bearer token is valid and authorized access to scopes.
OBS! The request.user attribute should be set to the resource owner
associated with this refresh token.
:param refresh_token: Unicode refresh token
:param client: Client object set by you, see authenticate_client.
:param request: The HTTP Request (oauthlib.common.Request)
:rtype: True or False
Method is used by:
- Authorization Code Grant (indirectly by issuing refresh tokens)
- Resource Owner Password Credentials Grant (also indirectly)
- Refresh Token Grant
jupyterhub/oauth/provider.py