code: def append_cell_value(self, column_family_id, column, value):
          
          column = _to_bytes(column)
          value = _to_bytes(value)
          rule_pb = data_v2_pb2.ReadModifyWriteRule(
              family_name=column_family_id, column_qualifier=column, append_value=value
          )
          self._rule_pb_list.append(rule_pb)
docs: Appends a value to an existing cell.
  
          .. note::
  
              This method adds a read-modify rule protobuf to the accumulated
              read-modify rules on this row, but does not make an API
              request. To actually send an API request (with the rules) to the
              Google Cloud Bigtable API, call :meth:`commit`.
  
          For example:
  
          .. literalinclude:: snippets_table.py
              :start-after: [START bigtable_row_append_cell_value]
              :end-before: [END bigtable_row_append_cell_value]
  
          :type column_family_id: str
          :param column_family_id: The column family that contains the column.
                                   Must be of the form
                                   ``[_a-zA-Z0-9][-_.a-zA-Z0-9]*``.
  
          :type column: bytes
          :param column: The column within the column family where the cell
                         is located.
  
          :type value: bytes
          :param value: The value to append to the existing value in the cell. If
                        the targeted cell is unset, it will be treated as
                        containing the empty string.