Clash Royale CLAN TAG

Python Kivy: bind a command inside popup executes command

I want to have a popup for confirming that the user really wants to quit the app. Now when I try to bind commands to the two buttons, I can only add the dismiss directly inside the function, not via a callback. That may be ok.
But I can only call my closing routine through a callback, not inside the function. When I bind quit_app() inside this function it gets directly executed when opening the popup. Why? It just should bind, not execute.

class pop():
def callback(instance):
if instance.id == 'quit':
aempApp.quit_app(aempApp, 1)

def up(self):
print('popup')
qbutton = Button(text='Quit', id='quit')
abutton = Button(text='Return to Program', id='return')
blayout = BoxLayout()
blayout.add_widget(qbutton)
blayout.add_widget(abutton)
self.popup = kivy.uix.popup.Popup(title='Quit Program?', content=blayout, size_hint=(None, None), size=(400, 400))
self.popup.open()
abutton.bind(on_release=self.popup.dismiss)
#qbutton.bind(on_release=aempApp.quit_app(aempApp, 1))
qbutton.bind(on_release=aempApp.pop.callback)


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