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.clock import *

class LoginScreen(GridLayout):
    def __init__(self, **kwargs):
        super(LoginScreen, self).__init__(**kwargs)
        self.cols = 2
        self.add_widget(Label(text='User Name'))
        self.username = TextInput(multiline=False)
        self.add_widget(self.username)
        self.add_widget(Label(text='password'))
        self.password = TextInput(password=True, multiline=False)
        self.add_widget(self.password)

stop= 0

class MyApp(App):

    def build(self):

        return LoginScreen()

    def my_callback(dt):
        global stop
        if stop == 1:
            print ('Last call of my callback, bye bye !')
            return False
        print ('My callback is called')

    def stop_my_callback(dt):
        global stop
        stop = 1

    def start_my_callback(dt):
        event = Clock.schedule_interval(my_callback, 29 / 30.)
        Clock.schedule_once(stop_my_callback, 2)

    Clock.schedule_once(start_my_callback, 5)

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

Traceback (most recent call last):
   File "C:UsersenzoDesktopteste.py", line 44, in <module>
     MyApp().run()
   File "C:UsersenzoAppDataLocalProgramsPythonPython37libsite-packageskivyapp.py", line 826, in run
     runTouchApp()
   File "C:UsersenzoAppDataLocalProgramsPythonPython37libsite-packageskivybase.py", line 502, in runTouchApp
     EventLoop.window.mainloop()
   File "C:UsersenzoAppDataLocalProgramsPythonPython37libsite-packageskivycorewindowwindow_sdl2.py", line 727, in mainloop
     self._mainloop()
   File "C:UsersenzoAppDataLocalProgramsPythonPython37libsite-packageskivycorewindowwindow_sdl2.py", line 460, in _mainloop
     EventLoop.idle()
   File "C:UsersenzoAppDataLocalProgramsPythonPython37libsite-packageskivybase.py", line 337, in idle
     Clock.tick()
   File "C:UsersenzoAppDataLocalProgramsPythonPython37libsite-packageskivyclock.py", line 581, in tick
     self._process_events()
   File "kivy_clock.pyx", line 384, in kivy._clock.CyClockBase._process_events
   File "kivy_clock.pyx", line 414, in kivy._clock.CyClockBase._process_events
   File "kivy_clock.pyx", line 412, in kivy._clock.CyClockBase._process_events
   File "kivy_clock.pyx", line 167, in kivy._clock.ClockEvent.tick
   File "C:UsersenzoDesktopteste.py", line 38, in start_my_callback
     event = Clock.schedule_interval(my_callback, 29 / 30.)
 NameError: name 'my_callback' is not defined