Проблема с программой на Python kivy

from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.label import Label
from kivy.uix.widget import Widget
 
from kivy.uix.gridlayout import GridLayout
from kivy.uix.boxlayout import BoxLayout
 
from kivy.config import Config
 
Config.set('graphics', 'resizable', 0)
Config.set('graphics', 'width', 400)
Config.set('graphics', 'height', 500)
 
class Calculator(App):
    def __init__(self):
        App.init(self)
        self.signs = '/*-+.'
        self.past = '1'
    def update_label(self):
        self.lbl.text = self.formula
    def add_number(self, instance):
        if self.formula == '0':
            self.formula = ''
        self.formula += str(instance.text)
        self.update_label()
    def add_operation(self, instance):
        if self.past  not in self.signs or str(instance.text) not in self.signs:
            if str(instance.text) == 'X' and self.past not in self.signs:
                self.formula += '*'
            else:
                self.formula += str(instance.text)
            self.update_label()
            self.past = str(instance.text)
    def calc_result(self, instance):
        self.formula = str(eval(self.lbl.text))
        self.lbl.text = str(eval(self.lbl.text))
 
    def build(self):
        self.formula = '0'
        bl = BoxLayout( orientation = 'vertical', padding = 25)
        calc = GridLayout(cols = 4, spacing = 3, size_hint = (1, .6))
 
        self.lbl = Label(text = '0', font_size = 40, size_hint = (1, .4), halign = 'right', valign = 'center', text_size = (400 - 50, 500 * .4 - 50))
        bl.add_widget(self.lbl)
        calc.add_widget(Button(text = '7', on_press = self.add_number))
        calc.add_widget(Button(text = '8', on_press = self.add_number))
        calc.add_widget(Button(text = '9', on_press = self.add_number))
        calc.add_widget(Button(text = 'X', on_press = self.add_operation))
 
        calc.add_widget(Button(text = '4', on_press = self.add_number))
        calc.add_widget(Button(text = '5', on_press = self.add_number))
        calc.add_widget(Button(text = '6', on_press = self.add_number))
        calc.add_widget(Button(text = '-', on_press = self.add_operation))
 
        calc.add_widget(Button(text = '1', on_press = self.add_number))
        calc.add_widget(Button(text = '2', on_press = self.add_number))
        calc.add_widget(Button(text = '3', on_press = self.add_number))
        calc.add_widget(Button(text = '+', on_press = self.add_operation))
 
        calc.add_widget(Button(text = '0', on_press = self.add_number))
        calc.add_widget(Button(text = ',', on_press = self.add_number))
        calc.add_widget(Button(text = '=', on_press = self.calc_result))
        calc.add_widget(Button(text = '/', on_press = self.add_operation))
 
        bl.add_widget(calc)
        return bl
 
if __name__ == '__main__':
    Calculator().run()

App.__init__(self)