kivy button on press call function

1app.py:
2
3import kivy
4from kivy.app import App
5from kivy.uix.boxlayout import BoxLayout
6from kivy.uix.button import Button
7
8class Launch(BoxLayout):
9    def __init__(self, **kwargs):
10        super(Launch, self).__init__(**kwargs)
11
12    def say_hello(self): #<---- function that the button is calling
13        print "hello"
14
15
16class App(App):
17    def build(self):
18        return Launch()
19
20
21if __name__ == '__main__':
22    App().run()
23    
24app.kv:
25
26#:kivy 1.9.1
27
28<Launch>:
29    BoxLayout:
30        Button:
31            size:(80,80)
32            size_hint:(None,None)
33            text:"Click me"
34            on_press: say_hello() #<--- calls the say_hello() function

1import kivy
2from kivy.uix.widget import Widget
3from kivy.lang import Builder
4from kivymd.app import MDApp
5
6Builder.load_file('foo.kv')
7
8class layout1(Widget):
9  pass
10class App(MDApp):
11  def build(self):
12    self.theme_cls.theme_style = "Dark"
13    self.theme_cls.primary_palette = "BlueGray" 
14    return layout1()
15  def themeSwitch(self):# function that will be called from layout1() class
16    pass
17
18
19#kv file from here on.
20<layout1>
21	BoxLayout:
22		Button:
23          size_hint:(None,None)  
24          size:(80,80)
25          text:"Click me"
26          on_press: app.themeSwitch()

1button.bind(self.function)