how to assign function to buttonn in kivy code example

Example: kivy button on press call function

app.py:

import kivy
from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button

class Launch(BoxLayout):
    def __init__(self, **kwargs):
        super(Launch, self).__init__(**kwargs)

    def say_hello(self): #<---- function that the button is calling
        print "hello"


class App(App):
    def build(self):
        return Launch()


if __name__ == '__main__':
    App().run()
    
app.kv:

#:kivy 1.9.1

<Launch>:
    BoxLayout:
        Button:
            size:(80,80)
            size_hint:(None,None)
            text:"Click me"
            on_press: say_hello() #<--- calls the say_hello() function