def credentials(self):
        """Returns credentials from the OAuth 2.0 session.

        :meth:`fetch_token` must be called before accessing this. This method
        constructs a :class:`google.oauth2.credentials.Credentials` class using
        the session's token and the client config.

        Returns:
            google.oauth2.credentials.Credentials: The constructed credentials.

        Raises:
            ValueError: If there is no access token in the session.
        """
        return google.oauth2.oauthlib.credentials_from_session(
            self.oauth2session, self.client_config)