Which is the best way to link Widgets between .kv and .py file

Question

I need some help in order to understand Kivy library. If i have to create a questionary like this example :

But with a button in order to change page that will stay in the same position how I have to proceed? How I link lots of widgets together and the .kv file with the .py file? I'm trying but without any results.

For exemple i've written this code with a button and a colored background but it doesn't work :

.py Part :

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.widget import Widget
from kivy.uix.boxlayout import BoxLayout

class Principale(BoxLayout):
    pass


class TryApp(App):
    def build(self):
        self.root = Builder.load_file('questionario.kv')
        Principale()


if __name__ == '__main__':
    PongApp().run()

.kv Part:

#:kivy 1.0.9
<Principale>:
    canvas:
        Color:
            rgb : 0.2549,0.73333,0.12156
        Rectangle:
            size:self.size
    size_hint : 1,1
    orientation:"vertical"
    padding : 30

    Button:
        text: "ciao"
        size_hint : 0.12,0.12

Thanks a lot

Answer 1

class TryApp(App):
    def build(self):
        self.root = Builder.load_file('questionario.kv')
        Principale()

You need to return Principale here.

return Principale()

Not just create it.

Also, you called your App TryApp but you're trying to run something called PongApp

So this:

PongApp().run()

Should be:

TryApp().run()

So it should look like this in the end:

class TryApp(App):
    def build(self):
        self.root = Builder.load_file('questionario.kv')
        return Principale()


if __name__ == '__main__':
    TryApp().run()

Note: It is harmless but you don't need to declare the kv as root. It already knows in this case.