How to get nested Kivy widgets to interact between each other?

Question

I'm playing around with the examples provided in the Kivy tutorial. I was able to create nested widgets, but can't seem to get the buttons to modify objects in the other parts of the screen.

In this example, I've tried to modify the Kivy example script to turn it into a simple login window with a numpad drawn from buttons (let's ignore the vkeyboard for this case, I'm trying to get a hold of buttons...). In short, buttons on the left, login textinput on the right. I cannot seem to have the actions on the buttons affect the text input on the right. It sounds very simple, but I can't seem to figure it out. Should I use a global variable? Am I creating my objects incorrectly for this purpose?

from kivy.app import App
from kivy.uix.gridlayout import GridLayout
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.uix.textinput import TextInput


class AddNumpad(GridLayout):
    def __init__(self, **kwargs):
        super(AddNumpad, self).__init__(**kwargs)
        self.cols = 3
        self.padding = 50

        def callback(instance):
            print('The button <%s> is being pressed' % instance.text)
            # TODO: trying to populate the password textinput here

        for i in range(1,10):
            btn = Button(text=str(i))
            btn.bind(on_press=callback)
            self.add_widget(btn)
        self.add_widget(Label(text=''))
        btn = Button(text='0')
        btn.bind(on_press=callback)
        self.add_widget(btn)

class LoginScreen(GridLayout):

    def __init__(self, **kwargs):
        super(LoginScreen, self).__init__(**kwargs)
        self.password = TextInput(password=True, multiline=False)
        self.cols = 2
        self.numpad = AddNumpad()
        self.add_widget(self.numpad)
        self.entry = ''
        self.password.text = self.entry
        self.add_widget(self.password)


class MyApp(App):
    userInput = ''
    def build(self):
        return LoginScreen()


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

Answer 1

You can use a custom property and bind it to a callback in your LogginScreen class:

from kivy.app import App
from kivy.uix.gridlayout import GridLayout
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.uix.textinput import TextInput
from kivy.properties import StringProperty

class AddNumpad(GridLayout):
    passwIn = StringProperty()                            # <<<<<<<<<<<<<

    def __init__(self, **kwargs):
        super(AddNumpad, self).__init__(**kwargs)
        self.cols = 3
        self.padding = 50

        for i in range(1, 10):
            btn = Button(text=str(i))
            btn.bind(on_press=self.callback)
            self.add_widget(btn)

        self.add_widget(Label(text=''))
        btn = Button(text='0')
        btn.bind(on_press=self.callback)
        self.add_widget(btn)

    def callback(self, instance):
        self.passwIn += instance.text                     # <<<<<<<<<<<<<



class LoginScreen(GridLayout):
    def __init__(self, **kwargs):
        super(LoginScreen, self).__init__(**kwargs)
        self.password = TextInput(password=True, multiline=False)
        self.cols = 2
        self.numpad = AddNumpad()
        self.numpad.bind(passwIn=self.numpad_pressed)     # <<<<<<<<<<<<<
        self.add_widget(self.numpad)
        self.add_widget(self.password)

    def numpad_pressed(self, instance, value):
        self.password.text = value    


class MyApp(App):
    def build(self):
        return LoginScreen()


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