Need to pass variable to new screen, then read it and use it

Question

How can I change the color/image of the second screen based on the button I press using both Kivy and Python? Here is the code that illustrates my problem.

Python:

startvalue = 0


class Start(Screen):
    pass


class End(Screen):
    global startvalue

    def which_color(self):
        if startvalue == 1:
            self.bg_image.source = 'blue.png'
        elif startvalue == 2:
            self.bg_image.source = 'red.png'
        elif startvalue == 3:
            self.bg_image.source = 'yellow.jpg'
        elif startvalue == 4:
            self.bg_image.source = 'green.jpg'

Kivy:

#: import startvalue __kivyseeing__.startvalue

ScreenManagement:
    Start:
    End:

<Start>:
    startvalue: startvalue
    id: start
    name: 'start'

    BoxLayout:

        Button:
            name: 'btn1'
            id: btn1
            text: "btn1"
            on_release:
                startvalue = 1
                app.root.current = 'end'

        Button:
            name: 'btn2'
            id: btn2
            text: "btn2"
            on_release:
                startvalue = 2
                app.root.current = 'end'

        Button:
            name: 'btn3'
            id: btn3
            text: "btn3"
            on_release:
                startvalue = 3
                app.root.current = 'end'

        Button:
            name: 'btn4'
            id: btn4
            text: "btn4"
            on_release:
                startvalue = 4
                app.root.current = 'end'


<End>:
    id: end
    name: "end"
    endbtn: endbtn
    bg_image: bg_image

    on_pre_enter: self.which_color()

    Image:
        id: bg_image
        source: ""
        allow_stretch: True

    Button:
        name: 'endbtn'
        id: endbtn
        text: "end"
        size_hint_y: .35
        size_hint_x: .25
        on_release: app.root.current = 'start'

I realize that globals can be pretty dangerous and I'd rather not use them but I don't see how else it can work. Also, I'd like to know how to change Python globals in Kivy and then use them in the second class in order to determine the screen background.

Answer 1

The solution is to use Kivy NumericProperty and hook it up. Please refer to the example below for details.

Example

main.py

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.properties import NumericProperty


class ScreenManagement(ScreenManager):
    pass


class Start(Screen):
    startvalue = NumericProperty(0)


class End(Screen):

    def which_color(self, startvalue):
        print("startvalue={}".format(startvalue))
        if startvalue == 1:
            self.bg_image.source = 'blue.png'
        elif startvalue == 2:
            self.bg_image.source = 'red.png'
        elif startvalue == 3:
            self.bg_image.source = 'yellow.jpg'
        elif startvalue == 4:
            self.bg_image.source = 'green.jpg'


class TestApp(App):
    def build(self):
        return ScreenManagement()


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

test.kv

#:kivy 1.10.0

<ScreenManagement>:
    Start:
        id: start
    End:
        startvalue: start.startvalue

<Start>:
    name: 'start'

    BoxLayout:

        Button:
            name: 'btn1'
            id: btn1
            text: "btn1"
            on_release:
                root.startvalue = 1
                app.root.current = 'end'

        Button:
            name: 'btn2'
            id: btn2
            text: "btn2"
            on_release:
                root.startvalue = 2
                app.root.current = 'end'

        Button:
            name: 'btn3'
            id: btn3
            text: "btn3"
            on_release:
                root.startvalue = 3
                app.root.current = 'end'

        Button:
            name: 'btn4'
            id: btn4
            text: "btn4"
            on_release:
                root.startvalue = 4
                app.root.current = 'end'


<End>:
    id: end
    name: "end"
    endbtn: endbtn
    bg_image: bg_image

    on_pre_enter: self.which_color(root.startvalue)

    Image:
        id: bg_image
        source: ""
        allow_stretch: True

    Button:
        name: 'endbtn'
        id: endbtn
        text: "end"
        size_hint_y: .35
        size_hint_x: .25
        on_release: app.root.current = 'start'

Output

Answer 2

You try to use the value from the first screen:

<End>:
    id: end
    name: "end"
    endbtn: endbtn
    bg_image: bg_image

    on_pre_enter: 
        self.which_color(self.manager.get_screen('start').startvalue)

And in the python file:

def which_color(self, startvalue):
    ...