How to make calculator using kivy | Python

# Program to create a calculator 
  
# Program to Show how to create a switch 
# import kivy module    
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 
   
# this restrict the kivy version i.e  
# below this kivy version you cannot  
# use the app or software  
kivy.require('1.9.0') 

# for making multiple buttons to arranging
# them we are using this
from kivy.uix.gridlayout import GridLayout

# for the size of window
from kivy.config import Config

# Setting size to resizable
Config.set('graphics', 'resizable', 1)
## Config.set('graphics', 'width', '400')
## Config.set('graphics', 'height', '400')


# Creating Layout class
class CalcGridLayout(GridLayout):
 
    # Function called when equals is pressed
    def calculate(self, calculation):
        if calculation:
            try:
                # Solve formula and display it in entry
                # which is pointed at by display
                self.display.text = str(eval(calculation))
            except Exception:
                self.display.text = "Error"
 
 # Creating App class
class CalculatorApp(App):
 
    def build(self):
        return CalcGridLayout()
 
# creating object and running it 
calcApp = CalculatorApp()
calcApp.run()
 

# Custom button
<CustButton@Button>:
    font_size: 32
 
# Define id so I can refer to the CalcGridLayout
# class functions
# Display points to the entry widget
<CalcGridLayout>:
    id: calculator
    display: entry
    rows: 6
    padding: 10
    spacing: 10

    
 
    # Where input is displayed
    BoxLayout:
        TextInput:
            id: entry
            font_size: 32
            multiline: False
 
    # When buttons are pressed update the entry
    BoxLayout:
        spacing: 10
        CustButton:
            text: "7"
            on_press: entry.text += self.text
        CustButton:
            text: "8"
            on_press: entry.text += self.text
        CustButton:
            text: "9"
            on_press: entry.text += self.text
        CustButton:
            text: "+"
            on_press: entry.text += self.text
 
    BoxLayout:
        spacing: 10
        CustButton:
            text: "4"
            on_press: entry.text += self.text
        CustButton:
            text: "5"
            on_press: entry.text += self.text
        CustButton:
            text: "6"
            on_press: entry.text += self.text
        CustButton:
            text: "-"
            on_press: entry.text += self.text
 
    BoxLayout:
        spacing: 10
        CustButton:
            text: "1"
            on_press: entry.text += self.text
        CustButton:
            text: "2"
            on_press: entry.text += self.text
        CustButton:
            text: "3"
            on_press: entry.text += self.text
        CustButton:
            text: "*"
            on_press: entry.text += self.text
 
    # When equals is pressed pass text in the entry
    # to the calculate function
    BoxLayout:
        spacing: 10
        CustButton:
            text: "AC"
            on_press: entry.text = ""
        CustButton:
            text: "0"
            on_press: entry.text += self.text
        CustButton:
            text: "="
            on_press: calculator.calculate(entry.text)
        CustButton:
            text: "/"
            on_press: entry.text += self.text
    BoxLayout:
        CustButton:
            font_size: 20
            text: "Scientific calculator"
            on_press: entry.text = ""