How to get, use and change values from a .kv file

Question

So, I've been working on a very simple kivy app and am having trouble with the backend. How would you go about getting the values from text_input and changing label values? I've watched a few tutorials but their methods vary and I get error messages. Here is my python code:

import kivy
from kivy.app import App

kivy.require('1.9.1')


class MyWindowApp(App):
    pass


window = MyWindowApp()
window.run()

And here is the .kv file:

Screen:
    side: side
    FloatLayout:
    Label:
        text: "Side:"
        pos_hint: {"x": 0.1, "y": 0.7}
        text_size: self.size
    Label:
        text: "Volume:"
        pos_hint: {"x": 0.1, "y": 0.65}
        text_size: self.size
    Label:
        text: "Surface Area:"
        pos_hint: {"x": 0.1, "y": 0.6}
        text_size: self.size
    TextInput:
        size_hint: (.4, None)
        height: 26
        multiline: False
        pos_hint: {"x": 0.24, "y": 0.7}
        id: side
    Label:
        text: "0cm"
        id: volume
        pos_hint: {"x": 0.27, "y": 0.65}
        text_size: self.size
    Label:
        text: "0cm"
        id: surface_area
        pos_hint: {"x": 0.355, "y": 0.6}
        text_size: self.size

Answer 1

As mentioned in the other answer, place Builder.load_file or Builder.load_string in your build method.

To handle changes e.g. if the user hits enter in the side input use on_text_validate: app.on_side_change(self) or place a button to trigger the calculation and use on_press method.

The on_side_change method in your class MyApp will handle the change. It will be called once the user hits enter. See the example code below, for a basic calculation.

For getting/setting values from the labels/inputs you can use ObjectProperty or StringProperty from kivy.properties. In the below code I'm using StringProperty. Important you need the StringProperty in your app class and use it in the kv file.

For the mode of your calculation I've added a property, so you can use it in your calculation method. The mode is also used in the kv file so it's displaying the current mode.

The switcher dictionary is the Python way to do a switch/case statement.

Example code

from kivy.app import App
from kivy.lang import Builder
from kivy.properties import StringProperty

kv = """
Screen:
    side: side
    GridLayout:
        rows: 1
        cols:2
        spacing:0

        GridLayout:
            rows: 5
            cols:1
            Button:
                text: "Cube"
                # on_press: app.mode = self.text
                on_press: app.setMode(self)
            Button:
                text: "Cuboid"
                on_press: app.setMode(self)
            Button:
                text: "Cylinder"
                on_press: app.setMode(self)
            Button:
                text: "Cone"
                on_press: app.setMode(self)
            Button:
                text: "Sphere"
                on_press: app.setMode(self)

        FloatLayout:
            Label:
                text: "The Volume and surface area of a {}:".format(app.mode)
                pos_hint: {"x":0.1, "y":0.8}
                text_size: self.size
            Label:
                text:"Side:"
                pos_hint: {"x":0.1, "y":0.7}
                text_size: self.size
            Label:
                text:"Volume:"                
                pos_hint: {"x":0.1, "y":0.65}
                text_size: self.size
            Label:
                text:"Surface Area:"                
                pos_hint: {"x":0.1, "y":0.6}
                text_size: self.size
            TextInput:
                size_hint: (.4, None)
                height: 26
                multiline: False
                pos_hint: {"x":0.24, "y":0.7}
                id: side
                text: app.sideText
                on_text_validate: app.on_side_change(self)
            Label:                
                text: app.volume
                pos_hint: {"x":0.27, "y":0.65}
                text_size: self.size
            Label:
                text: app.area
                pos_hint: {"x":0.355, "y":0.6}
                text_size: self.size
"""

class MyApp(App):
    sideText = StringProperty("")
    area = StringProperty("0 cm²")
    volume = StringProperty("0 cm³")
    mode = StringProperty("Cube")

    def build(self):
        return Builder.load_string(kv)

    def setMode(self, btn):
        self.mode = btn.text

    def on_side_change(self, instance):
        print(instance.text)
        result = 0

        try:
            value = float(instance.text)
        except:
            # failed to convert 
            return

        def cubeCalc(val):
            return {
                "volume": val * val * val,
                "area": val * val
            }

        switcher = {
            "Cube": cubeCalc
        }
        method = switcher.get(self.mode, "Unknown mode")
        if method is not "Unknown mode":
            result = method(value) # result is a dictionary with volume & area
            #print(result)
            print(self.volume)
            self.volume = "{:.2f} cm³".format(result["volume"])
            self.area = "{:.2f} cm²".format(result["area"])

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