Helper for :meth:`set_cell`

        Adds a mutation to set the value in a specific cell.

        ``state`` is unused by :class:`DirectRow` but is used by
        subclasses.

        :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 or :class:`int`
        :param value: The value to set in the cell. If an integer is used,
                      will be interpreted as a 64-bit big-endian signed
                      integer (8 bytes).

        :type timestamp: :class:`datetime.datetime`
        :param timestamp: (Optional) The timestamp of the operation.

        :type state: bool
        :param state: (Optional) The state that is passed along to
                      :meth:`_get_mutations`.

def _set_cell(self, column_family_id, column, value, timestamp=None, state=None):
        """Helper for :meth:`set_cell`

        Adds a mutation to set the value in a specific cell.

        ``state`` is unused by :class:`DirectRow` but is used by
        subclasses.

        :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 or :class:`int`
        :param value: The value to set in the cell. If an integer is used,
                      will be interpreted as a 64-bit big-endian signed
                      integer (8 bytes).

        :type timestamp: :class:`datetime.datetime`
        :param timestamp: (Optional) The timestamp of the operation.

        :type state: bool
        :param state: (Optional) The state that is passed along to
                      :meth:`_get_mutations`.
        """
        column = _to_bytes(column)
        if isinstance(value, six.integer_types):
            value = _PACK_I64(value)
        value = _to_bytes(value)
        if timestamp is None:
            # Use -1 for current Bigtable server time.
            timestamp_micros = -1
        else:
            timestamp_micros = _microseconds_from_datetime(timestamp)
            # Truncate to millisecond granularity.
            timestamp_micros -= timestamp_micros % 1000

        mutation_val = data_v2_pb2.Mutation.SetCell(
            family_name=column_family_id,
            column_qualifier=column,
            timestamp_micros=timestamp_micros,
            value=value,
        )
        mutation_pb = data_v2_pb2.Mutation(set_cell=mutation_val)
        self._get_mutations(state).append(mutation_pb)