Python | Button Action in Kivy

# import kivy module 
import kivy 
  
# this restrict the kivy version i.e 
# below this kivy version you cannot 
# use the app or software 
kivy.require("1.9.1") 
  
# base Class of your App inherits from the App class. 
# app:always refers to the instance of your application 
from kivy.app import App

# BoxLayout arranges children 
# in a vertical or horizontal box.
from kivy.uix.boxlayout import BoxLayout

# class in which we are defining action on click
class RootWidget(BoxLayout):

    def btn_clk(self):
        self.lbl.text = "You have been pressed"


# creating action class and calling 
# Rootwidget by returning it
class ActionApp(App):

    def build(self):
        return RootWidget()

# creating the myApp root for ActionApp() class  
myApp = ActionApp()

# run function runs the whole program 
# i.e run() method which calls the 
# target function passed to the constructor. 
myApp.run()

# Base widget from Rootwidget class in .py file
<RootWidget>:

    # used to change the label text
    # as in rootwidget class
    lbl:my_label

    # child that is an instance of the BoxLayout
    BoxLayout:
        orientation: 'vertical'
        size: [1, .25]

    Button:
        text:'Click Me'
        font_size:"50sp"
        color: [0, 255, 255, .67]
        on_press: root.btn_clk()

    Label:
        # id is limited in scope to the rule
        # it is declared in. An id is a weakref
        # to the widget and not the widget itself.
        id: my_label
        text: 'No click Yet'
        color: [0, 84, 80, 19]