Bind function to Kivy button

I don't think any of the answers are very clear. Neither explains that problem is that the callback given to on_press gets called with a parameter, the instance of button, so LoginScreen.auth must accept a parameter after the self:

def auth(self, button):
    print('button pressed:', instance)

The problem is not that on_press must be given via Button.bind or that the callback must be a function, it can be a bound method, and the docs cited by other answer and by comments link to ButtonbBhavior which indicates that OP use of on_press in constructor was fine:

self.hello = Button(text="hello", on_press=self.auth)

would have worked if auth had been as described above.

If you read the Button documentation, the key seems to be to use the bind function:

def callback(instance):
    print('The button <%s> is being pressed' % instance.text)

btn1 = Button(text='Hello world 1')
btn1.bind(on_press=callback)