Python/Kivy - Replace value of a label in another screen calling a function

Question

I have an app with 2 screens (ScreenManager).
I wrote a function in the second screen (SettingsScreen) and I want this function to update a label in the first screen.

Below the two classes:

class MenuScreen(Screen):
    count = NumericProperty(30)

class SettingsScreen(Screen):
    def set_20(self):
        self.count = 20

The classes are linked to a button and a label in the Kivy stylesheet with two different screens

<MenuScreen>:
    FloatLayout:
                orientation: 'horizontal'
                Label:
                    id: l_label
                    text: str(root.count)

<SettingsScreen>:
    BoxLayout: 
        orientation: 'vertical'
        padding: 50
        FloatLayout:
            Button:
                text: "20"
                on_release: root.set_20()

To be clearer, the user has to click on the Button in the SettingsScreen to set the value of the NumericProperty in the first screen to 20.

At the moment, if I click on the button nothing happens.

What you see above is an extract - The full code of the app is stored below if you want to see more.

https://github.com/marcogdepinto/LifeCounter

Thanks in advance for your help.

Answer 1

I think the problem is that when your button gets clicked and calls the set_20 method, that method is trying to set the property count of the SettingsScreen to 20.

In other words

def set_20(self):
        self.count = 20

would try to set the the count property inside the SettingsScreen (hence the word self) and it cannot find it. There is another count property inside MenuScreen which it knows nothing about.

To fix this I think you should give an id to MenuScreen

<MenuScreen>:
   id: menu

and in the on_release method of the Button do

on_release: menu.count = 20

EXAMPLE

Python file

 class MenuScreen(Screen):
        counter = NumericProperty(0)

 class SettingsScreen(Screen):
        pass

 class MyWidget(ScreenManager):
        pass


 class MyRandomApp(App):
        def build(self):
            return MyWidget()

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

kv file

<MyWidget>:
    id:manager
    MenuScreen:
        id: menu
        name: 'menuscreen'
        BoxLayout:
            orientation: 'vertical'
            Label:
                text: str(menu.counter)

            Button:
                text: 'Go to S2'
                on_press: manager.current = 'settingsscreen'
    SettingsScreen:
        id: settings 
        name: 'settingsscreen'
        BoxLayout:
            orientation: 'vertical'
            Button:
                text: 'Click to edit label of Prev screen'
                on_press: menu.counter = 20
            Button:
                text: 'Go to S1'
                on_press: manager.current = 'menuscreen'