CODE EXAMPLE FOR PYTHON

select all Textinput kivy on selection

Selection is cancelled when TextInput is focused. 
If you need to show selection when TextInput is focused, 
you should delay (use Clock.schedule) the call to the functions for selecting text 
(select_all, select_text).

#: import Clock kivy.clock.Clock

TextInput:
  text: "Prefix"
  on_focus: Clock.schedule_once(lambda dt: self.select_all()) if self.focus else None