code: def append_manage_data_op(self, data_name, data_value, source=None):
          
          op = operation.ManageData(data_name, data_value, source)
          return self.append_op(op)
docs: Append a :class:`ManageData <stellar_base.operation.ManageData>`
          operation to the list of operations.
  
          :param str data_name: String up to 64 bytes long. If this is a new Name
              it will add the given name/value pair to the account. If this Name
              is already present then the associated value will be modified.
          :param data_value: If not present then the existing
              Name will be deleted. If present then this value will be set in the
              DataEntry. Up to 64 bytes long.
          :type data_value: str, bytes, None
          :param str source: The source account on which data is being managed.
              operation.
          :return: This builder instance.