def sign(self, signer: Signer):
        """ Sign message using signer. """
        message_data = self._data_to_sign()
        self.signature = signer.sign(data=message_data)
def sign ( self , signer : Signer ) : message_data = self . _data_to_sign ( ) self . signature = signer . sign ( data = message_data )
Sign message using signer.