How Can I Refer To Kivy's Root Widget From Python?

Question

In Kivy language, it is possible to refer to the root widget with something like

<RootWidget>:
    BoxLayout:
        SomeButton:
            on_press: print root

but trying to access root from Python is impossible

class SomeButton(Button):
    def __init__(self, **kwargs):
        super(SomeButton, self).__init__(**kwargs)
        self.text = "Button"
        self.font_size = 15
    def on_press(self, *args):
        print root

and will result in

NameError: global name 'root' is not defined

or if using self.root,

AttributeError: 'SomeButton' object has no attribute 'root'

Answer 1

If you want the actual root widget from the App then I use the following from within any Widget class...

from kivy.app import App
...
class myWidget(BoxLayout):
    app= App.get_running_app()
    app.root.whatever-you-want

Answer 2

Inside a kv file, root always refers to a parent with angle brackets. There can therefore be multiple roots which you can refer to in a kv file, depending on where you are in the file.

# Root here refers to the parent class in angle brackets
<SomeClass>:
    BoxLayout:
        Label:
            text: root.label_text

# and further down in the same kv file, this other
# class is also a root.. here root refers to
# this class
<SomeOtherClass/Widget/LayoutEtc>:
    BoxLayout:
        Label:
            text: root.label_text

In a python file then, these classes could be represented like so:

class SomeClass:

    label_text = StringProperty("I'm a label")

    def __init__(**kwargs):
        super(SomeClass, self).__init__(**kwargs)
        b = BoxLayout()
        l = Label(text=self.label_text)
        b.add_widget(l)
        self.add_widget(b)
        # now we're set up like the first class in the above kv file

Now look above and compare how the kv file assigned the text to the label, and how it's done in the python file above. In kv it was root.label_text, but above, the class uses self. As in, text=self.label_text. It's also used when adding the boxlayout, self.add_widget(b). self is a way of referring to the current instance of the class.

That's how you basically refer to what would be 'root' in the kv file, but in the python file.

If you don't know why self is used, then I advise learning about classes in python, as that's where the explanation to that lies.