Nesting Box Layouts in Kivy

main.py

from kivy.app import App
from kivy.uix.togglebutton import ToggleButton
from kivy.uix.widget import Widget
from kivy.uix.boxlayout import BoxLayout

class HBoxWidget(Widget):
    def __init__(self, **kwargs):
        super(HBoxWidget, self).__init__(**kwargs)

class VBoxWidget(Widget):
    def __init__(self, **kwargs):
        super(VBoxWidget, self).__init__(**kwargs)
        
class ContainerBox(BoxLayout):
    def __init__(self, **kwargs):
        super(ContainerBox, self).__init__(**kwargs)

class TestApp(App):
    def build(self):
        return ContainerBox() 
     
if __name__ == '__main__':
    TestApp().run()

test.kv

<HBoxWidget>:
	BoxLayout:
		size: root.size
		pos: root.pos
		id: boxlayout_h
		orientation: 'horizontal'
		ToggleButton:
			text: 'h1'
			group: 'test'
		ToggleButton:
			text: 'h2'
			group: 'test'

<VBoxWidget>:
	BoxLayout:
		orientation: 'vertical'
		size: root.size
		pos: root.pos
		ToggleButton:
			text: 'v1'
			group: 'test'
		ToggleButton:
			text: 'v2'
			group: 'test'
			
<ContainerBox>:
	orientation: 'horizontal'
	HBoxWidget:
	VBoxWidget:

Thanks to your code. I can build my layout with boxlaytout and nesting gridlayout :-)