a very simple calculator with kiviy for class

calculator.py

import kivy
kivy.require('1.0.6') # replace with your current kivy version !

from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.gridlayout import GridLayout

class MyApp(App):
    def build(self):
        self.last_result = 0
        self.last_action = ''
        
        self.main_box = BoxLayout(orientation='vertical')
        self.button_box = BoxLayout(orientation='horizontal')
        self.number_box = GridLayout(cols=3)
        self.action_box = BoxLayout(orientation='vertical')
        
        self.label = Label(text='0', font_size=20)

        self.main_box.add_widget(self.label)
        self.main_box.add_widget(self.button_box)
        self.button_box.add_widget(self.number_box)
        self.button_box.add_widget(self.action_box)

        self.numbers = []
        for i in (1,2,3,4,5,6,7,8,9,0):
            n = Button(text='%d' % i)
            n.bind(on_press=self.on_number_button_clicked)
            self.numbers.append(n)
            self.number_box.add_widget(n)
             
        for i in ('/', '*', '-', '+', '='):
            n = Button(text=i)
            n.bind(on_press=self.on_action_button_clicked)
            self.action_box.add_widget(n)
            

        return self.main_box

    def on_number_button_clicked(self, button):
        old = self.label.text
        if old == '0':
            self.label.text = button.text
        elif '=' in self.label.text:
            self.label.text = button.text
        else:
            self.label.text += button.text

    def on_action_button_clicked(self, button):
        if '=' in self.label.text:
            self.label.text = self.last_result
            if button.text != '=':
                self.label.text += button.text
        elif button.text == '=':
            if len(self.label.text) > 1:
                if self.label.text[-1] in ('+', '-'):
                    self.label.text += '0'
                if self.label.text[-1] in ('*', '/'):
                    self.label.text += '1'
            try:
                self.last_result = str(eval(self.label.text))
                self.label.text += '=' + self.last_result
            except ZeroDivisionError:
                self.last_result = '0'
                self.label.text += '=Error Diversion by zero'
        elif len(self.label.text) > 1 and self.label.text[-1] in ('/', '*', '-', '+'):
            self.label.text = self.label.text[:-1] + button.text
        else:
            self.label.text += button.text
 
if __name__ == '__main__':
    MyApp().run()