[Kivy] GUI properties defined (and changed) by variables in .py file

[j.crater]

Hello all,
I am trying to make a basis for a MVC-sytle program, where GUI properties (in .kv file) would be stored and accessed from a separate .py file. I tried a couple of things, but as I am not yet too familiar with Kivy, I had no success so far.
Value of label_text does change with pressing buttons (increment or decrement), but label text remains same ("0"), though I would like it to change dynamically.
The code in files is:

main.py:

from kivy.app import App

import values


class MainApp(App):
    def increment(self):
        values.label_text = str(int(values.label_text) + 1)

    def decrement(self):
        values.label_text = str(int(values.label_text) - 1)


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

values.py:

from kivy.properties import StringProperty


label_text = "0"
# label_text = StringProperty("0") # gives: ValueError: Label.text accept only str

main.kv:

#:import val values


BoxLayout:
    orientation: "vertical"

    Label:
        text: val.label_text

    Button:
        text: "increment"
        on_press: app.increment()

    Button:
        text: "decrement"
        on_press: app.decrement()

Any tips are most appreciated, thanks.
JC