Kivy - update numerical property in KV file on_touch_down

Question

I have a python variable val which is updated based on when a location is pressed in the canvas, and I'm having trouble understanding how to update the value in the KV file.

I think I need to use some kind of binding event, but unsure how to go about doing this. Can someone suggest a solution?

Minimal working example (if you touch near the tip of the slider on the left, I want to it to jump to the opposite side)

---

main.py

import kivy
from kivy.config import Config
kivy.require('1.9.1')
Config.set('graphics', 'resizable', 1)
Config.set('graphics', 'width', '400')
Config.set('graphics', 'height', '400')

from kivy.app import App
from test import TestWidget

class TestApp(App):

    def build(self):
        return TestWidget()   

if __name__ == '__main__':
    TestApp().run()

test.py

from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.relativelayout import RelativeLayout
from kivy.properties import NumericProperty

class TestWidget(RelativeLayout):

    def __init__(self, **kwargs):
        super(TestWidget, self).__init__(**kwargs)

        Builder.load_file('test.kv')

        sm = ScreenManager()
        sm.add_widget(MainScreen(name='MainScreen'))
        self.add_widget(sm)

class MainScreen(Screen):
    lineX = 395 / 2
    lineY = 405 / 2
    circleRad = 400 / 2
    val = NumericProperty(2500)

    def on_touch_down(self, touch):
        # left
        if 50 <= touch.x <= 75 and 195 <= touch.y <= 210:
            val = 2500
            print val

        # right
        elif 320 <= touch.x <= 350 and 200 <= touch.y <= 215:
            val = 7500
            print val

test.kv

#:import math math

<MainScreen>:
    FloatLayout:
        canvas:
            Line:
                points: [root.lineX, root.lineY, root.lineX +.75 *(root.circleRad *math.sin((math.pi /5000 *(root.val)) +math.pi)), root.lineY +.75 *(root.circleRad *math.cos((math.pi /5000 *(root.val)) +math.pi))]
                width: 2

Answer 1

You are not changing val property of MainScreen , You are declaring a local variable named val in your if clause. Simply use self.val instead of val

    if 50 <= touch.x <= 75 and 195 <= touch.y <= 210:
        self.val = 2500
        print self.val

    # right
    elif 320 <= touch.x <= 350 and 200 <= touch.y <= 215:
        self.val = 7500
        print self.val