def clear_rect(self, x=0, y=0, width=0, height=0):
        """
        Erases the background area specified by parameters. By default,
        the whole Drawer is cleared.

        The ability to clear a smaller area may be useful when you want to
        erase a smaller area of the drawer (e.g. drawing widget decorations).
        """
        if width <= 0:
            width = self.width
        if height <= 0:
            height = self.height

        self.ctx.save()
        self.ctx.set_operator(cairocffi.OPERATOR_CLEAR)
        self.ctx.rectangle(x, y, width, height)
        self.ctx.fill()
        self.ctx.restore()

        Erases the background area specified by parameters. By default,
        the whole Drawer is cleared.

        The ability to clear a smaller area may be useful when you want to
        erase a smaller area of the drawer (e.g. drawing widget decorations).
        
libqtile/backend/base/drawer.py