from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import  StringProperty
from kivy.clock import Clock


class FirstLayout(BoxLayout):
    r = 0

    def __init__(self, **kwargs):
        super(FirstLayout, self).__init__(**kwargs)
        self.change = self.ids.temp_label

    def my_callback(self, *args):
        self.r += 1
        print self.r
        t = str(self.r)
        self.change.text = t


class TryApp(App):
    def build(self):
        Clock.schedule_interval(FirstLayout().my_callback, 1)
        return FirstLayout()


app = TryApp()
app.run()

<FirstLayout>:
    orientation: 'vertical'
    Label:
        id: temp_label
        text: 'something'
    Button:
        on_press: root.my_callback()