Updating images between kivy screens

Question

I've come to a problem in a larger app im writing that seems dumb but I can't figure it out. I want to have the user select an image path on screen1 and then have the path load onto screen2. This presents 2 problems.

1. if the path is first empty, kivy runs the second window anyway, and cv2 reads an empty path. I would like to keep the path empty and prevent the window from running until the path is loaded from the previous window

2. my code isn't even loading the data from the previous window. I know i could update the image source directly, but I am seeking to update the path variable

Here is a test code and test image

from kivy.app import App
from kivy.uix.relativelayout import RelativeLayout
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.lang import Builder
import cv2

class WindowManager(ScreenManager):
    pass

class Window1(Screen, RelativeLayout):
    button_size = (100, 100)
    button_pos = (400, 400)

class Window2(Screen,RelativeLayout):
    path=''
    print(cv2.imread(path).shape)


KV = Builder.load_string("""

WindowManager:
    Window1:
    Window2:

<Window1>:
    name: 'one'
    RelativeLayout:
        Button:
            allow_stretch: True
            keep_ratio: True
            size_hint: None,None
            size: root.button_size
            pos: root.button_pos
            on_press: 
                root.manager.screens[1].path='/MRI-scan.jpg'
    
            on_release:
                app.root.current = "two" 
                root.manager.transition.direction = "left"  
<Window2>: 
    name: 'two'
    
    RelativeLayout:  
        Image:
            id: my_image
            size_hint: 0.5,0.5
            pos: (0,0)
            source: root.path

""")


class TApp(App):

    def build(self):
        return KV

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

Answer 1

I was able to solve the problem with one shortcoming. Anyone who figures out this shortcoming, I'll check mark your answer. I can switch between images using the path and I did not even need to use StringProperty. However, I cannot start with path=''. I need to start with a file initially. Here's the solution. Anyone who can provide an answer by starting with an empty string, I'll give a check mark to you. I solved the problem with on_pre_enter.

from kivy.app import App
from kivy.uix.relativelayout import RelativeLayout
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.lang import Builder
import cv2

class WindowManager(ScreenManager):
    pass

class Window1(Screen, RelativeLayout):
    button_size = (100, 100)
    button_pos = (400, 400)

    btn2_size=(100,100)
    btn2_pos=(400,300)

    def on_touch_up(self, touch):
        if self.ids.btn1.collide_point(*touch.pos):
            self.manager.get_screen('two').path='img1'

        if self.ids.btn2.collide_point(*touch.pos):
            self.manager.get_screen('two').path = 'img2'

class Window2(Screen,RelativeLayout):
    btn3_size = (100, 100)
    btn3_pos = (400, 200)
    path='img start' #===> this is where I want to start with a blank string 
    print(cv2.imread(path).shape)
    def on_pre_enter(self, *args):
        self.manager.get_screen('two').ids['my_image'].source=self.path
        print(print(cv2.imread(self.path).shape)
)
KV = Builder.load_string("""

WindowManager:
    Window1:
    Window2:

<Window1>:
    name: 'one'
    RelativeLayout:
        Button:
            id: btn1
            text: 'one'
            allow_stretch: True
            keep_ratio: True
            size_hint: None,None
            size: root.button_size
            pos: root.button_pos 
            on_release:
                app.root.current = "two" 
                root.manager.transition.direction = "left" 
                
        Button:
            id: btn2
            text: 'two'
            allow_stretch: True
            keep_ratio: True
            size_hint: None,None
            size: root.btn2_size
            pos: root.btn2_pos
            on_release:
                app.root.current = "two" 
                root.manager.transition.direction = "left" 
               
<Window2>: 
    name: 'two'
    
    RelativeLayout:  
        Image:
            id: my_image
            size_hint: 0.5,0.5
            pos: (0,0)
            source: root.path
    
        Button:
            id: btn3
            text: 'previous'
            allow_stretch: True
            keep_ratio: True
            size_hint: None,None
            size: root.btn3_size
            pos: root.btn3_pos
          
    
            on_release:
                app.root.current = "one" 
                root.manager.transition.direction = "right"     
      
""")


class TApp(App):

    def build(self):
        return KV

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