Kivy checkbox (radiobutton). How to make radiobuttons persistent (not deselect if pressed on when active)

persistent_radio.py

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.checkbox import CheckBox


class CB(CheckBox):

    def on_touch_down(self, *args):
        if self.active:
            return
        super(CB, self).on_touch_down(*args)

KV = """


BoxLayout:
    orientation: "vertical"
    CB:
        group: "one"
    CB:
        group: "one"
    CB:
        group: "one"


"""


class MyApp(App):

    def build(self):
        return Builder.load_string(KV)


MyApp().run()