Confused about root object and class object and their referencing

[suhsin85]

Here are the py and kv codes for a simple app that's aimed at helping me understand referencing objects using kivy properties. My questions are:

1. Is CustomLayout effectively the root object, since its first button calls for root.move_slider() when pressed?
2. Can I swap the order of CustomLayout and TestLayout around so that TestLayout becomes the root object because it is on top of the hierarchy and have the app still work the same?
3. When trying to reference kivy ids within the python file, how do I know when to reference using "self.ids." vs "self.parent.ids."?
4. I thought root objects aren't enclosed in angle brackets, why is CustomLayout being done so?

py code:

...  
class TestLayout(FloatLayout):
    pass  
class CustomLayout(BoxLayout):  
    def move_slider(self):  
        if self.ids._slider.value <= self.ids._slider.max - 5:  
            self.ids._slider.value += 5  
class CustomButton(Button):  
    def set_text(self):  
        self.parent.ids._label2.text = 'CHANGED'  
class TestApp(App):  
    def build(self):  return TestLayout()  
if  __name__  ==  '__main__':  TestApp().run() 

kv code:

<CustomLayout>:  
    size_hint: .5, .5  
    pos_hint: {'x': .5, 'top': 1}  
    Button:  
        text: str(_slider.orientation)  
        on_press: root.move_slider()  
    BoxLayout:  
        orientation: 'vertical'  
        Button:  text: str(_slider.orientation)  
        Slider:  
            id:  _slider  
            min: 100  
            max: 200  
            value: 150  
            `orientation:` 'vertical'
        Button:  
            text: str(_slider.value)  
    Button:  text: str(_slider.orientation)  
<TestLayout>:  
    Label:  
        text:  _label2.text  
        size_hint: .5, .5  
        pos_hint: {'x': 0, 'top': 1}  
    CustomLayout:  
    CustomButton:  
        id:  _customButton  
        text:  _label2.text  
        size_hint: .5, .5  
        pos_hint: {'x': 0, 'y': 0}  
        on_press: self.set_text()  
    Label:  
        id:  _label2  
        text: 'label 2'  
        size_hint: .5, .5  
        pos_hint: {'x': .5, 'y': 0}  

[ElliotGarbus]

Is CustomLayout effectively the root object, since its first button calls for root.move_slider() when pressed?

Root is an overloaded term in kivy. There are 2 meanings of root:

• The root widget of the app. The root widget of the app is the base widget of the app. It is the widget returned by App.build(), in your code this is TestLayout.
• In a kv rule, root is a keyword that refers to the base widget of the rule..

The use of root, and other important kv constructs are defined here: https://kivy.org/doc/stable/api-kivy.lang.html#module-kivy.lang

Can I swap the order of CustomLayout and TestLayout around so that TestLayout becomes the root object because it is on top of the hierarchy and have the app still work the same?
You can change the order in the kv file. Changning the order of the rules does not change the way the widget tree is rendered.

When trying to reference kivy ids within the python file, how do I know when to reference using "self.ids." vs "self.parent.ids."?
Each kivy rule has it's own ids dictionary. If self refers to the kivy rule you are in use self.ids, if the parent of self has the ids, then use self.parent.ids.

I thought root objects aren't enclosed in angle brackets, why is CustomLayout being done so?
See the answer to your first question. You have defined a rule for CustomLayout. In your code, you did not instance your root widget in kv, you instanced the root widget in python.