expression.py 文件源码

def columns(self):
        """A named-based collection of :class:`.ColumnElement` objects
        maintained by this :class:`.FromClause`.

        The :attr:`.columns`, or :attr:`.c` collection, is the gateway
        to the construction of SQL expressions using table-bound or
        other selectable-bound columns::

            select([mytable]).where(mytable.c.somecolumn == 5)

        """

        if '_columns' not in self.__dict__:
            self._init_collections()
            self._populate_column_collection()
        return self._columns.as_immutable()