Python | How to use Multiple kv files in kivy

# Multiple .kv file Python code

import kivy 
  
# base Class of your App inherits from the App class.   
# app:always refers to the instance of your application  
from kivy.app import App

# The GridLayout arranges children in a matrix.
# It takes the available space and divides
# it into columns and rows, then adds
# widgets to the resulting “cells”.
from kivy.uix.gridlayout import GridLayout

# Builder is a global Kivy instance used
# in widgets that you can use to load other
# kv files in addition to the default ones.
from kivy.lang import Builder


# Loading Multiple .kv files 
Builder.load_file('box1.kv')
Builder.load_file('box2.kv')
Builder.load_file('box3.kv')


# Creating main kv file class
class main_kv(GridLayout):
    pass

# Create App class
class MainApp(App):
    def build(self):
        self.x = 150
        self.y = 400
        return main_kv()

# run the App
if __name__=='__main__':
    MainApp().run()

# Creating the main .kv files
# the difference is that it is
# the heart of the Application
# Other are just Organs

<main_kv>:

    # Assigning Grids
    cols: 3

    # Creating AnchorLayout
    AnchorLayout:
        anchor_x: 'left'
        anchor_y: 'center'

        # Canvas creation
        canvas:
            Color:
                rgb: [1, 0, 0]
            Rectangle:
                pos: self.pos
                size: self.size
        
        Box1:
            size_hint: [None, None]
            size: [app.x, app.y]

    AnchorLayout:
        anchor_x: 'center'
        anchor_y: 'center'
        canvas:
            Color:
                rgb: [0, 1, 0]
            Rectangle:
                pos: self.pos
                size: self.size
        Box2:
            size_hint: [None, None]
            size: [app.x, app.y]

    AnchorLayout:
        anchor_x: 'right'
        anchor_y: 'center'
        canvas:
            Color:
                rgb: [0, 0, 1]
            Rectangle:
                pos: self.pos
                size: self.size
        Box3:
            size_hint: [None, None]
            size: [app.x, app.y] 

# Creating 1st .kv file  

<Box1@BoxLayout>:
    Button:
        text: 'B1a'
    Button:
        text: 'B1b'

# Creating 2nd .kv file

<Box2@BoxLayout>:
    Button:
        text: 'B2a'
    Button:
        text: 'B2b'

# Creating 3rd .kv file 

<Box3@BoxLayout>:
    Button:
        text: 'B3a'
    Button:
        text: 'B3b'