Kivymd full app

from kivymd.app import MDApp
from kivymd.uix.screen import Screen
from kivymd.uix.button import MDRectangleFlatButton

class MainApp(MDApp):
  def build(self):
    screen = Screen()
    button = MDRectangleFlatButton(text='Hello',
                                   on_release=self.callback)
    screen.add_widget(button)
    return screen
  
  def callback(self, instance):
    print("The Button is pressed")

MainApp().run()