code: def sign(self, data):
          
          if self.signing_key is None:
              raise MissingSigningKeyError("KeyPair does not contain secret key. "
                                           "Use Keypair.from_seed method to create a new keypair with a secret key.")
          return self.signing_key.sign(data)
docs: Sign a bytes-like object using the signing (private) key.
  
          :param bytes data: The data to sign
          :return: The signed data
          :rtype: bytes