code: def fetch_by_refresh_token(self, refresh_token):
          
          if refresh_token not in self.refresh_tokens:
              raise AccessTokenNotFound
  
          return self.refresh_tokens[refresh_token]
docs: Find an access token by its refresh token.
  
          :param refresh_token: The refresh token that was assigned to an
                                ``AccessToken``.
          :return: The :class:`oauth2.datatype.AccessToken`.
          :raises: :class:`oauth2.error.AccessTokenNotFound`