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.

from kivy.app import App from kivy.uix.button import Button from kivy.uix.label import Label class Test(App): def press(self,instance): print("Pressed") def build(self): butt=Button(text="Click") butt.bind(on_press=self.press) #dont use brackets while calling function return butt Test().run() 

say_hello is a method of the Launch class. In your kv rule, the Launch class is the root widget, so it can be accessed using the root keyword:

on_press: root.say_hello() 

Note also that you have to actually call the function, not just write its name - everything to the right of the colon is normal Python code.

I believe that the solution utilizing the bind function (given by Ender Look) will lead to the following error:

TypeError: pressed() takes 1 positional argument but 2 were given 

To solve this issue you have to allow the say_hello() module to accept the touch input as well, although it isn't needed.

For anyone is looking for a pure Python approach here is a general example.

To call a function without arguments:

def my_function(instance): ... my_button.bind(on_press=my_function) 

To call a function with arguments:

from functools import partial def my_function(instance, myarg): ... my_button.bind(on_press=partial(my_function,"a string")) 

If you would like to add function to any element in Kivy just use logic like here.

In your .py file:

class LoginButton(Button): def __init__(self, **kwargs): super().__init__(**kwargs) self.text = "Login" def login(self): print("logged_in") 

And then in .kv file:

LoginButton: id: "login" on_press: self.login() size_hint: None, None size: 500, 50 pos_hint: {"center_x": 0.5} 

It works for me :)

function with object as argument in kv-file

in kv:

on_press: root.say_hello(self) 

in py:

def say_hello(self,obj): print(obj) 

from kivy.app import App from kivy.uix.button import Button class MyApp(App): def build(self): return Button(text="Hello, Kivy!", on_press=self.on_button_click) def on_button_click(self, instance): instance.text = "Button Pressed!" if __name__ == '__main__': MyApp().run() 

you just added root before say_hello() function. like this

on_press: root.say_hello() 

it should working