def signData(self, publicKey, signData):
        """
        Extend the base signing behavior by using an SSH agent to sign the
        data, if one is available.

        @type publicKey: L{Key}
        @type signData: L{str}
        """
        if self.agent is not None:
            return self.agent.signData(publicKey.blob(), signData)
        else:
            return SSHUserAuthClient.signData(self, publicKey, signData)

        Extend the base signing behavior by using an SSH agent to sign the
        data, if one is available.

        @type publicKey: L{Key}
        @type signData: L{str}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/endpoints.py