Kivy AttributeError: 'NoneType' object has no attribute 'ids'

Question

I created a custom button in the kv file, i want to set disabled = True if app.root.ids.my_label == "disabled" else disabled=False. However i keep getting AttributeError: 'NoneType' object has no attribute 'ids. I no this can be done, i'm not just doing it right, i will be grateful for any help. Thanks!

test.py

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.lang import Builder
from kivy.properties import ObjectProperty

Builder.load_string('''
<CustomBtn@Button>
       disabled: True if app.root.ids.word.my_label == "disable" else False

<main>:
my_label:my_label
BoxLayout:
    orientation:"vertical"

    Label:
        id: my_label
        text: "Disabled"
    CustomBtn:
        text: "Btn1"
    CustomBtn:
        text: "Btn2"
    CustomBtn:
        text: "Btn3"
    Button:
        text: "Disable/Enable"
        on_press: root.disablebtn()
        ''')


class main(BoxLayout):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        my_label = ObjectProperty()

    def disablebtn(self):
        print(self.my_label)
        if self.my_label.text == "Disabled":
            self.my_label.text = "Enabled"
        else:
            self.my_label.text = "Disabled"


class CallApp(App):
    def build(self):
        return main()


CallApp().run()

Answer 1

You can do it by putting the expression under each CustomBtn , like this:

    CustomBtn:
        text: "Btn1"
        disabled: True if root.my_label.text == "Disabled" else False

rather than in the <CustomBtn>: rule.

Answer 2

Your original approach won't work because app.root is not set when your <CustomBtn@Button> rule is applied. Here is a modified version of your code, that does what you want without that problem:

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.lang import Builder
from kivy.properties import ObjectProperty
from kivy.uix.button import Button


class CustomBtn(Button):
    pass

Builder.load_string('''
<main>:
    my_label:my_label
    BoxLayout:
        orientation:"vertical"

        Label:
            id: my_label
            text: "Disabled"
        CustomBtn:
            text: "Btn1"
        CustomBtn:
            text: "Btn2"
        CustomBtn:
            text: "Btn3"
        Button:
            text: "Disable/Enable"
            on_press: root.disablebtn()
        ''')


class main(BoxLayout):
    my_label = ObjectProperty()

    def disablebtn(self):
        if self.my_label.text == "Disabled":
            self.my_label.text = "Enabled"
            self.disable_customBtns(False)
        else:
            self.my_label.text = "Disabled"
            self.disable_customBtns(True)

    def disable_customBtns(self, is_disabled):
        # Look for CustomBtn instances, and set their `disabled` value
        for child in self.walk():
            if isinstance(child, CustomBtn):
                child.disabled = is_disabled

    def on_my_label(self, *args):
        # this just sets the initial value of disabled to True
        self.disable_customBtns(True)



class CallApp(App):
    def build(self):
        return main()

CallApp().run()