Python Kivy: how to call a function on button click?

Question

i'm pretty new at using kivy library.

I have an app.py file and an app.kv file , my problem is that I can't call a function on button press.

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):
        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

Answer 1

Mode:.kv

It's very simple, say_hello belongs to the Launch class so in order to use it in your .kv file you have to write root.say_hello. Note that say_hello is a function that you want to execute so you don't have to forget the () ---> root.say_hello().

Also, if say_hello were in App class you should use App.say_hello() because it belongs to the app. (Note: even if your App class were class MyFantasicApp(App): it would always be App.say_hello() or app.say_hello() I don't remember, sorry).

#:kivy 1.9.1

<Launch>:
    BoxLayout:
        Button:
            size:(80,80)
            size_hint:(None,None)
            text:"Click me"
            on_press: root.say_hello()

Mode: .py

You can bind the function.

from kivy.uix.button import Button # You would need futhermore this
class Launch(BoxLayout):
    def __init__(self, **kwargs):
        super(Launch, self).__init__(**kwargs)
        mybutton = Button(
                            text = 'Click me',
                            size = (80,80),
                            size_hint = (None,None)
                          )
        mybutton.bind(on_press = self.say_hello) # Note: here say_hello doesn't have brackets.
        Launch.add_widget(mybutton)

    def say_hello(self):
        print "hello"

Why use bind? Sorry, no idea. But you it's used in the kivy guide.