gistfile1.txt

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.lang import Builder
from kivy.properties import StringProperty

Builder.load_string('''
<Simple>:
    Label:
        #text: 'Hello World' # THIS IS WORKING
        text: root.sometext
''')

class Simple(BoxLayout):
    sometext = StringProperty('')

    def __init__(self, **kwargs):
        super(Simple, self).__init__(**kwargs)

        self.sometext = 'Hello from Init.'

class TApp(App):
    def build(self):
        return Simple()


TApp().run()