kivy，无法访问app.root属性

Question

I have the following codes in my main.py and spend.kv as follows

main.py

from kivy.app import App
from kivy.core.window import Window
from kivy.uix.gridlayout import GridLayout
from kivy.uix.relativelayout import RelativeLayout
from kivy.uix.button import Button
from kivy.uix.screenmanager import ScreenManager, Screen

class Manager(ScreenManager):
   currency = '$'

class SpendApp(App):
   def build(self):
      control = Manager()
      return control

class First(Screen):
   pass

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

spend.kv

<Manager>:
    First


<First>:
    GridLayout:
        cols: 1
        Label: 
            text: 'Total spending'
            height: '48dp'
            size_hint_y: None
         Amount:
            height: '38dp'
            size_hint_y: None
            font_color: 1,0,0,1



<Amount@Label>:
   text: app.root.currency + '0.0'

when i run this the program crashes with the following error:

AttributeError: 'NoneType' object has no attribute 'currency'

I know this is because of my referance to app.root.currency in my spend.kv file:

<Amount@Label>:
   text: app.root.currency + '0.0'

so is there a way to make this reference correctly, without receiving this error?

Answer 1

If you use following code:

<Amount@Label>:
    text: str(root) # 

you'll discover that the root of Amount object is the Amount object itself because when you're defining it it's not in any hierarchy yet. You can access root widged only inside actual hierarchy:

<First>:
    GridLayout:
        cols: 1
        Label: 
            text: str(root)

In this hierarchy, root object is definied as a object of First class, which is actually an instance of Screen widget, so you have to use manager attribute in order to acces your Manager class:

<First>:
    GridLayout:
        cols: 1
        Label: 
            text: root.manager.currency  + '0.0'