from kivy.clock import Clock

class app(App):
    def build(self):
        self.layout = Layout()
        self.name = Label(text = "john")
        self.layout.add_widget(self.name)
        self.current_i = 0
        Clock.schedule_interval(self.update, 1)
        return self.layout

    def update(self, *args):
        self.name.text = str(self.current_i)
        self.current_i += 1
        if self.current_i >= 50:
            Clock.unschedule(self.update)