def next_mode(self):
        """Change the add mode for the Plasma layout."""
        if self._layout is None:
            return

        index = self.modes.index(self._mode)
        index = (index + 1) % len(self.modes)

        self._layout.add_mode = self.modes[index]
Change the add mode for the Plasma layout.
libqtile/widget/plasma.py