def requestAvatarId(credentials):
        """
        @param credentials: something which implements one of the interfaces in
        self.credentialInterfaces.

        @return: a Deferred which will fire a string which identifies an
        avatar, an empty tuple to specify an authenticated anonymous user
        (provided as checkers.ANONYMOUS) or fire a Failure(UnauthorizedLogin).
        Alternatively, return the result itself.

        @see: L{twisted.cred.credentials}
        """

        @param credentials: something which implements one of the interfaces in
        self.credentialInterfaces.

        @return: a Deferred which will fire a string which identifies an
        avatar, an empty tuple to specify an authenticated anonymous user
        (provided as checkers.ANONYMOUS) or fire a Failure(UnauthorizedLogin).
        Alternatively, return the result itself.

        @see: L{twisted.cred.credentials}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/cred/checkers.py