python - Why isn't kv binding of the screen change working? -

i've defined 2 buttons: 1 in kv , 1 in python. located in different screens , used navigate between them. found strange button defined in python switched screen, while 1 defined in kv did not. perhaps i'm not accessing app class method properly?
here code of issue:

from kivy.app import app kivy.lang import builder kivy.uix.screenmanager import screen, screenmanager kivy.uix.button import button  builder.load_string(''' <myscreen1>:     button:         id: my_bt         text: "back"         on_release: app.back ''')   class myscreen1(screen):     pass  class testapp(app):     def here(self, btn):         self.sm.current = "back"      def back(self, btn):         self.sm.current = "here"      def build(self):         self.sm = screenmanager()         s1 = screen(name = "here")         bt = button(text = "here",                     on_release = self.here)         s2 = myscreen1(name = "back")         #s2.ids['my_bt'].bind(on_release = self.back)         self.sm.add_widget(s1)         s1.add_widget(bt)         self.sm.add_widget(s2)         return self.sm  testapp().run() 

so if define switching function in kv (on_release), can't go "here" screen. if uncomment line in python , comment on_release: app.back instead, works fine.
i'm pretty sure correct way access current app, since doesn't give me errors (which means method located)

that's subtle difference between kv , python: in kv have write callback function call (a python expression), in case:

on_release: app.back(self)