code: def commit(self):
          
          num_mutations = len(self._rule_pb_list)
          if num_mutations == 0:
              return {}
          if num_mutations > MAX_MUTATIONS:
              raise ValueError(
                  "%d total append mutations exceed the maximum "
                  "allowable %d." % (num_mutations, MAX_MUTATIONS)
              )
  
          data_client = self._table._instance._client.table_data_client
          row_response = data_client.read_modify_write_row(
              table_name=self._table.name, row_key=self._row_key, rules=self._rule_pb_list
          )
  
          # Reset modifications after commit-ing request.
          self.clear()
  
          # NOTE: We expect row_response.key == self._row_key but don't check.
          return _parse_rmw_row_response(row_response)
docs: Makes a ``ReadModifyWriteRow`` API request.
  
          This commits modifications made by :meth:`append_cell_value` and
          :meth:`increment_cell_value`. If no modifications were made, makes
          no API request and just returns ``{}``.
  
          Modifies a row atomically, reading the latest existing
          timestamp / value from the specified columns and writing a new value by
          appending / incrementing. The new cell created uses either the current
          server time or the highest timestamp of a cell in that column (if it
          exceeds the server time).
  
          After committing the accumulated mutations, resets the local mutations.
  
          For example:
  
          .. literalinclude:: snippets_table.py
              :start-after: [START bigtable_row_commit]
              :end-before: [END bigtable_row_commit]
  
          :rtype: dict
          :returns: The new contents of all modified cells. Returned as a
                    dictionary of column families, each of which holds a
                    dictionary of columns. Each column contains a list of cells
                    modified. Each cell is represented with a two-tuple with the
                    value (in bytes) and the timestamp for the cell.
          :raises: :class:`ValueError <exceptions.ValueError>` if the number of
                   mutations exceeds the :data:`MAX_MUTATIONS`.