Linking a Kivy Button to Function

Question

There is something in kivy I do not understand, and am hoping that someone could shed light. I have done a lot of reading in this topic but it just doesn't seem to be connecting in my head.

My issue comes from linking a function to a kivy button. Right now I am trying to learn how to do a simple function:

def Math():
    print 1+1

What I would like to do something more complex:

def Math(a,b):
    print a^2 + b^2

Where a and b are input labels from kivy, and upon clicking a button the answer will be printed.

This is what I have so far:

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen, NoTransition
from kivy.uix.widget import Widget
from kivy.uix.floatlayout import FloatLayout


#######``Logics``#######
class Math(FloatLayout):
    def add(self):
        print 1+1

#######``Windows``#######
class MainScreen(Screen):
    pass

class AnotherScreen(Screen):
   pass

class ScreenManagement(ScreenManager):
   pass


presentation = Builder.load_file("GUI_Style.kv")

class MainApp(App):
    def build(self):
       return presentation

if __name__ == "__main__":
    MainApp().run()

This is my kivy language file:

import NoTransition kivy.uix.screenmanager.NoTransition

ScreenManagement:
    transition: NoTransition()
    MainScreen:
    AnotherScreen:

<MainScreen>:
    name: "main"
    FloatLayout:
        Button:
            on_release: app.root.current = "other"
            text: "Next Screen"
            font_size: 50
            color: 0,1,0,1
            font_size: 25
            size_hint: 0.3,0.2
            pos_hint: {"right":1, "top":1}

<AnotherScreen>:
    name: "other"
    FloatLayout:
        Button:
            color: 0,1,0,1
            font_size: 25
            size_hint: 0.3,0.2
            text: "add"
            pos_hint: {"x":0, "y":0}
            on_release: root.add
        Button:
            color: 0,1,0,1
            font_size: 25
            size_hint: 0.3,0.2
            text: "Back Home"
            on_release: app.root.current = "main"
            pos_hint: {"right":1, "top":1}

Answer 1

<AnotherScreen>:
    name: "other"
    FloatLayout: 
        Button:
            ...
            on_release: root.add <-- here *root* evaluates to the top widget in the rule.

Which is an AnotherScreen instance, but it doesn't have an add method.

class Math(FloatLayout):
    def add(self):
        print 1+1

Here you have declared a Math class by inheriting from the FloatLayout, which is a uix component -a widget-. And you defined a method on this class add. Still you haven't used it. In the kv file you used FloatLayout.

Now in order for you to access a function in kv, most of the time you'll acces it as a method of a uix component, by using either root/self or app, you can also import it e.g:

#: import get_color_from_hex kivy.utils.get_color_from_hex
<ColoredWidget>:
    canvas:
        Color: 
            rgba: get_color_from_hex("DCDCDC")
        Rectangle:
            size: self.size
            pos: self.pos 

So you can't do it like this:

<AnotherScreen>:
    name: "other"
    Math:
        id: math_layout
        Button:
            ...
            on_release: math_layout.add()

or like this:

class AnotherScreen(Screen):
   def add(self):
       print(1+1)

If you still have problems conserning this topic, i'll be glad to provide more help .