Python - Kivy - TextInput, Button

import kivy
from kivy.app import App
from kivy.uix.gridlayout import GridLayout
from kivy.uix.label import Label
from kivy.uix.textinput import TextInput
from kivy.uix.button import Button


class LoginScreen(GridLayout):

    def __init__(self, **kwargs):
        super(LoginScreen, self).__init__(**kwargs)
        self.cols = 2
        self.add_widget(Label(text='Livro1:'))
        self.bookone = TextInput(multiline=False)
        self.add_widget(self.bookone)

        self.add_widget(Label(text='Livro2:'))
        self.booktwo = TextInput(multiline=False)
        self.add_widget(self.booktwo)

        self.add_widget(Button(text='Submit2', on_press=self.callback ))
        self.add_widget(Button(text='Submit1', on_press=self.callback ))

    def callback(self, args): # need self, args
        print('[DEBUG] args:', args)
        print('[DEBUG] args.text:', args.text)

        if args.text == 'Submit1':
            txt = self.bookone.text # get text from input
        elif args.text == 'Submit2':
            txt = self.booktwo.text # get text from input
        else:
            txt = '???'

        print('I am being pressed', args.text, txt)


class MyApp(App):

    def build(self):
        return LoginScreen()


if __name__ == '__main__':
    MyApp().run()