Helper for :meth:`read`.

def _make_txn_selector(self):
        """Helper for :meth:`read`."""
        if self._transaction_id is not None:
            return TransactionSelector(id=self._transaction_id)

        if self._read_timestamp:
            key = "read_timestamp"
            value = _datetime_to_pb_timestamp(self._read_timestamp)
        elif self._min_read_timestamp:
            key = "min_read_timestamp"
            value = _datetime_to_pb_timestamp(self._min_read_timestamp)
        elif self._max_staleness:
            key = "max_staleness"
            value = _timedelta_to_duration_pb(self._max_staleness)
        elif self._exact_staleness:
            key = "exact_staleness"
            value = _timedelta_to_duration_pb(self._exact_staleness)
        else:
            key = "strong"
            value = True

        options = TransactionOptions(
            read_only=TransactionOptions.ReadOnly(**{key: value})
        )

        if self._multi_use:
            return TransactionSelector(begin=options)
        else:
            return TransactionSelector(single_use=options)