second_screen.py

from kivy.lang import Builder
from kivy.uix.screenmanager import Screen
from kivy.properties import NumericProperty, StringProperty


Builder.load_file('second_screen.kv')  # Because this is not the main app, we must load the Kv explicitly.


class SecondScreen(Screen):
    message = StringProperty('Hello, World!')  # This property is changed by twoscreen.kv.
    roll = NumericProperty()

    def print_roll(self):
        print(f'The latest roll was a {self.roll}.')