main.py

from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.gridlayout import GridLayout
from kivy.uix.label import Label       
class MyCalculator(App):
    def build(self):
        widget_root = BoxLayout(orientation = "vertical")
        label_op = Label(size_hint_y = 0.75, font_size = 51)
        symbol_button = ('1', '2', '3', '+',
                         '4', '5', '6', '*',
                         '7', '8', '9', '.',
                         '0', '/', '-', '=')
        grid_button = GridLayout(cols = 4, size_hint_y = 2)
        
        for symbol in symbol_button:
            grid_button.add_widget(Button(text=symbol))
            
        button_clear = Button(text="CE", size_hint_y = None, height = 100)
        
        def text_print_button(instance):
            label_op.text += instance.text
        
        for button in grid_button.children[1:]:
            button.bind(on_press = text_print_button)
            
        def label_text_size(label, new_height):
            label.fontsize = 0.5*label.height
        label_op.bind(height = label_text_size)
        
        def result(instance):
            try:
                label_op.text = str(eval(label_op.text))
            except Exception as e:
                print(e)
                
        grid_button.children[0].bind(on_press = result)
        
        def label_clear(instance):
            label_op.text = " "
        button_clear.bind(on_press = label_clear)
        
        widget_root.add_widget(label_op)
        widget_root.add_widget(grid_button)
        widget_root.add_widget(button_clear)
        return widget_root
    
if __name__ == '__main__':  
    MyCalculator().run()