Python - Personalizar en kv objeto de py

#Archivo py

class Principal(ScreenManager):

    def __init__(self, **kwargs):

        super(Principal, self).__init__(**kwargs)

        self._principal=Screen(name='Principal')

        self._layout=AnchorLayout()

        self._boton=Button(text='Hola')

        self._layout.add_widget(self._boton)

        self._principal.add_widget(self._layout)

        self.add_widget(self._principal)

#Archivo kv

#:kivy 1.11.1

<Principal>:

    root._boton.text:'hola2'    #Esta linea me lanza error. Como hago para cambiar el texto del boton?

from kivy.app import App

from kivy.uix.screenmanager import ScreenManager

from kivy.properties import StringProperty

from kivy.uix.anchorlayout import AnchorLayout

from kivy.uix.button import Button

class Principal(ScreenManager):

    text_button = StringProperty('Valor por defecto')

    def __init__(self, **kwargs):

        super(Principal, self).__init__(**kwargs)

        self._principal=Screen(name='Principal')

        self._layout=AnchorLayout()

        self._boton=Button(text=self.text_button)

        self._layout.add_widget(self._boton)

        self._principal.add_widget(self._layout)

        self.add_widget(self._principal)

class TestApp(App):

    def build(self):

        return Principal()

if __name__ == '__main__':

    TestApp().run()

<Principal>:

    text_button: 'Ejemplo'