How to reference objects in other screens using kivy screenmanager

Question

I am trying to update a field that exists in another screen but am not succeeding. I would be very very pleased when someone could tell me what I am doing wrong here.

myscreenskv.py:

style = r'''
# File: myscreenskv.py
#: import myscreens myscreens

<ScreenManagement>:
    MainScreen:
    Screen1:
    Screen2:

<MainScreen>:
    name: 'main'
    mainlog:mainlog
    id: scrmain
    BoxLayout:
        orientation: "vertical"
        Label:
            text: "Main"
        Label:
            id: mainlog
        Button:
            text: "go to screen 1"
            on_press:
                app.root.current = "screen1"
                root.action1()
        Button:
            text: "go to screen 2"
            on_press:
                app.root.current = "screen2"
                root.action2()

<Screen1>:
    name: 'screen1'
    sc1log:sc1log
    id: scr1
    BoxLayout:
        orientation: "vertical"
        Label:
            text: "Screen1"
        Label:
            id: sc1log
        Button:
            text: "go to main screen"
            on_press: app.root.current = "main"
        Button:
            text: "go to screen 2"
            on_press: app.root.current = "screen2"

<Screen2>:
    name: 'screen2'
    id: scr2
    sc2log:sc2log
    BoxLayout:
        orientation: "vertical"
        Label:
            text: "Screen2"
        Label:
            id: sc2log
        Button:
            text: "go to main screen"
            on_press: app.root.current = "main"
        Button:
            text: "go to screen 1"
            on_press: app.root.current = "screen1"

'''

.py:

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.lang import Builder
from myscreenskv import style

class MainScreen(Screen):
    def __init__(self, **kwargs):
        super(MainScreen, self).__init__(**kwargs)

    def action1(self):
        self.ids.scr1.sc1log.text = 'Coming from main'

    def action2(self):
        self.ids.scr2.sc2log.text = 'Coming from main'

class Screen1(Screen):
    def __init__(self, **kwargs):
        super(Screen1, self).__init__(**kwargs)

    def action1(self):
        self.ids.main.mainlog.text = 'Coming from screen1'

    def action2(self):
        self.ids.scr2.sc2log.text = 'Coming from screen1'


class Screen2(Screen):
    def __init__(self, **kwargs):
        super(Screen2, self).__init__(**kwargs)

    def action1(self):
        self.ids.main.mainlog.text = 'Coming from screen2'

    def action2(self):
        self.ids.scr1.sc1log.text = 'Coming from screen2'

class MyscreensApp(App):
    def build(self):
        Builder.load_string(style)
        sm = ScreenManager()
        sm.add_widget(MainScreen())
        sm.add_widget(Screen1())
        sm.add_widget(Screen2())
        sm.current = 'main'
        return sm


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

Answer 1

You are trying to access ids dictionary, that's nice, yet in a completely different instance, that's why this error:

AttributeError: 'super' object has no attribute '__getattr__'

You need to access the right instance, to be able to access its properties, which in your case you need to access the ScreenManager, to access its screens property (a list of instances), from which you can do the desired edits of for example text:

MainScreen.action1():

self.manager.screens[1].sc1log.text = 'Coming from main'
# no ids, because you put it into a variable before

To understand why it works let's look at the widget tree:

<MainScreen>:
    id: scrmain
    BoxLayout:
        Label:
        Label:
            id: mainlog
        Button:
        Button:

here the ids are a dictionary in MainScreen accessible from MainScreen().ids(an instance) and this is the output:

{'mainlog': <WeakProxy to <kivy.uix.label.Label object at 0x12345678>>}

which means you can't really assign the root widget to its own dictionary - at least not this way + makes no sense anyway, because you can just call root, which gives you the instance of the root widget.