Calling a variable not working in kivy's kv-language - python-2.7

Question

My question is this, after I set a variable on one screen how do I call it on another?
Here is the code for what I am trying to do:
main.py
import kivy
kivy.require('1.10.0')
from kivy.app import App
from kivy.uix.screenmanager import Screen, ScreenManager
class Manager(ScreenManager):
pass
class FirstScreen(Screen):
pass
class SecondScreen(Screen):
pass
class ExampleApp(App):
def build(self):
return Manager()
if __name__ == '__main__':
ExampleApp().run()
example.kv
#: import sm kivy.uix.screenmanager
#: set Question 'not working'
<Manager>
transition: sm.FadeTransition()
FirstScreen:
SecondScreen:
<FirstScreen>
BoxLayout:
TextInput:
id: txt
multiline: False
Button:
text: 'Press Me'
on_release:
Question = txt.text
app.root.current = 'Next'
<SecondScreen>
name: 'Next'
Label:
text: Question
When this is run everything works as it should. With one exception. The Label on SecondScreen reads "not working" when it should read whatever I type into the text input on FirstScreen. Why can I not accomplish this task?

Answer 1

You have to provide id tag and use ids method or ObjectProperty to reference the variables. Please refer to the two example below for details.
Example 1 - Using ids method
An id is a weakref to the widget.
main.py
from kivy.app import App
from kivy.uix.screenmanager import Screen, ScreenManager
class Manager(ScreenManager):
pass
class FirstScreen(Screen):
pass
class SecondScreen(Screen):
pass
class ExampleApp(App):
def build(self):
return Manager()
if __name__ == '__main__':
ExampleApp().run()
example.kv
#:kivy 1.10.0
#:import FadeTransition kivy.uix.screenmanager.FadeTransition
#: set Question 'not working'
<Manager>
transition: FadeTransition()
FirstScreen:
SecondScreen:
id: second_screen
<FirstScreen>
BoxLayout:
TextInput:
id: txt
text: "Working!"
multiline: False
Button:
text: 'Press Me'
on_release:
root.manager.ids.second_screen.ids.label.text = txt.text
app.root.current = 'Next'
<SecondScreen>
name: 'Next'
Label:
id: label
text: Question
Output - Using ids method
Example 2 - Using ObjectProperty
The ‘best practice’ to use the ObjectProperty. This creates a direct reference, provides faster access and is more explicit.
main.py
from kivy.app import App
from kivy.uix.screenmanager import Screen, ScreenManager
from kivy.properties import ObjectProperty
class Manager(ScreenManager):
second_screen = ObjectProperty()
class FirstScreen(Screen):
pass
class SecondScreen(Screen):
label = ObjectProperty()
class ObjectPropertyApp(App):
def build(self):
return Manager()
if __name__ == '__main__':
ObjectPropertyApp().run()
objectproperty.kv
#:kivy 1.10.0
#:import FadeTransition kivy.uix.screenmanager.FadeTransition
#: set Question 'not working'
<Manager>
second_screen: second_screen
transition: FadeTransition()
FirstScreen:
SecondScreen:
id: second_screen
<FirstScreen>
BoxLayout:
TextInput:
id: txt
text: "Working!"
multiline: False
Button:
text: 'Press Me'
on_release:
root.manager.second_screen.label.text = txt.text
app.root.current = 'Next'
<SecondScreen>
name: 'Next'
label: label
Label:
id: label
text: Question
Output - Using ObjectProperty