class Screen_Position_Keeper:

    # functions to add stuff

    def get_px_row( self, item ):
        try:
            return self.item_to_row[ item ]
        except KeyError:
            raise KeyError( 'You never registered that item' )

class Screen_Position_Keeper:

    # functions to add stuff

    def get_px_row( self, item ):
        return self.item_to_row[ item ]