Why is my kivy program not calling the function from another class? - python-3.x

Question

I think this is more of a python question than a kivy question.
class Keyboard is sharing a method from class GUI. I created a GUI instance called self.a to connected the 2 classes in class Keyboard. Also, I created a keyboard class instance, self.key in class MainApp.
When I use this method, print ("Return button is pressed") the "return" button was able to do the print statement. I understand why it works. When I use the self.a.up() in the method, the return button does not call the up() method from class GUI. It's probably a small detail or a concept that I am missing. This is not an error from the rest of the program. Please help and thanks in advance. Code:
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.button import Button
from kivy.lang.builder import Builder
from kivy.properties import ObjectProperty
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.textinput import TextInput
from kivy.modules import keybinding
from kivy.core.window import Window
class GUI(BoxLayout):
main_display = ObjectProperty()
text_input = ObjectProperty()
def plus_1(self):
self.value = int(self.main_display.text)
self.main_display.text = str(self.value + 1)
print (self.main_display.text)
def minus_1(self):
self.value = int(self.main_display.text)
self.main_display.text = str(self.value - 1)
def up(self):
self.main_display.text = self.text_input.text
self.text_input.text = ''
class Keyboard(Widget):
def __init__(self,):
super().__init__()
self.a = GUI()
self.keyboard = Window.request_keyboard(None, self)
self.keyboard.bind(on_key_down=self.on_keyboard_down)
def on_keyboard_down(self, keyboard, keycode, text, modifiers):
if keycode[1] == 'enter':
self.a.up()
# print ("Return button is pressed")
return True
class MainApp(App):
def build(self):
key = Keyboard()
return GUI()
if __name__=="__main__":
app = MainApp()
app.run()

Answer 1

I think it works fine but in another object that you can't see. The problem is that you show one object of GUI class on screen. And for the Keyboard class you have created another object that you can't see. So the solution is to use one object of GUI class and operate with it in both of MainApp and Keyboard classes. Something like:
class MainApp(App):
def build(self):
# here we create an object...
self.gui = GUI()
# ...send it to Keyboard class
key = Keyboard(self.gui)
# and return it
return self.gui
class Keyboard(Widget):
def __init__(self, gui):
super().__init__()
# and here we receive that object instead of creating new one
self.a = gui