Python/Kivy - How to “connect” a function and a label in kv/py files

Question

I created a counter inside a class to increment the value of count

count = 0

class LifeCounterApp(App):
    def incr(n):
        global count 
        count += 1
        return count

I have a kv file in which I am creating the structure of my app.

What I want to do: the button "+" inside my app has to update the value of the Label.

Example: the label has default value 0 . If I click on the button the label has to change its value to 1 and so on.

My questions:

1) How to take the value of the label from the .py file?

2) The way in which I am calling the function incr is correct? Because actually clicking on the button nothing happens.

Button:
    text: "+"
    on_release:
        app.incr()
Label:
    text: app.count (?)

Hope that my question is clear and well formulated.

Answer 1

You should use NumericProperty for that and not a Python global variable.

Example:

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import NumericProperty
from kivy.lang import Builder


Builder.load_string('''
<LifeCounter>:
    BoxLayout:
        orientation: "vertical"

        Button:
            text: "+"
            on_release: root.count += 1
        Label:
            id: l_label
            text: str(root.count)

''')


class LifeCounter(BoxLayout):
    count = NumericProperty(0)
    def __init__(self, **kwargs):
        super(LifeCounter, self).__init__(**kwargs)

class DemoApp(App):
    def build(self):
        return LifeCounter()

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

If you want to use incr method to increase the value of count you can do it:

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import NumericProperty
from kivy.lang import Builder


Builder.load_string('''
<LifeCounter>:
    BoxLayout:
        orientation: "vertical"

        Button:
            text: "+"
            on_release: root.incr()
        Label:
            id: l_label
            text: str(root.count)

''')


class LifeCounter(BoxLayout):
    count = NumericProperty(0)
    def __init__(self, **kwargs):
        super(LifeCounter, self).__init__(**kwargs)

    def incr(self):
        self.count += 1

class DemoApp(App):
    def build(self):
        return LifeCounter()

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