Returns the column names.

        Returns
        -------
        out : list[string]
            Column names of the SFrame.

def column_names(self):
        """
        Returns the column names.

        Returns
        -------
        out : list[string]
            Column names of the SFrame.
        """
        if self._is_vertex_frame():
            return self.__graph__.__proxy__.get_vertex_fields()
        elif self._is_edge_frame():
            return self.__graph__.__proxy__.get_edge_fields()