elements.py 文件源码

def _create_nullsfirst(cls, column):
        """Produce the ``NULLS FIRST`` modifier for an ``ORDER BY`` expression.

        :func:`.nullsfirst` is intended to modify the expression produced
        by :func:`.asc` or :func:`.desc`, and indicates how NULL values
        should be handled when they are encountered during ordering::


            from sqlalchemy import desc, nullsfirst

            stmt = select([users_table]).\\
                        order_by(nullsfirst(desc(users_table.c.name)))

        The SQL expression from the above would resemble::

            SELECT id, name FROM user ORDER BY name DESC NULLS FIRST

        Like :func:`.asc` and :func:`.desc`, :func:`.nullsfirst` is typically
        invoked from the column expression itself using
        :meth:`.ColumnElement.nullsfirst`, rather than as its standalone
        function version, as in::

            stmt = (select([users_table]).
                    order_by(users_table.c.name.desc().nullsfirst())
                    )

        .. seealso::

            :func:`.asc`

            :func:`.desc`

            :func:`.nullslast`

            :meth:`.Select.order_by`

        """
        return UnaryExpression(
            _literal_as_label_reference(column),
            modifier=operators.nullsfirst_op,
            wraps_column_expression=False)