KivyTest.py

from kivy.app import App
from kivy.lang import Builder
from kivy.properties import ObjectProperty
from kivy.uix.label import Label
from kivy.uix.widget import Widget


UI = """
#:kivy 1.9.0

<ChildWidget>:
    grid: grid_layout
    GridLayout:
        id: grid_layout
        cols: 3

<MainWindow>:
    child: child
    ChildWidget:
        id: child
"""


class ChildWidget(Widget):
    grid = ObjectProperty(None)

    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.grid.add_widget(Label(text="Test"))
        # ^^^
        #Traceback (most recent call last):
        # File "/home/nathan/projects/KivyTest/src/ssce.py", line 42, in <module>
        #   SSCE().run()
        # File "/usr/lib/python3.4/site-packages/kivy/app.py", line 798, in run
        #   root = self.build()
        # File "/home/nathan/projects/KivyTest/src/ssce.py", line 39, in build
        #   return MainWindow()
        # File "/usr/lib/python3.4/site-packages/kivy/uix/widget.py", line 271, in __init__
        #   Builder.apply(self)
        # File "/usr/lib/python3.4/site-packages/kivy/lang.py", line 1872, in apply
        #   self._apply_rule(widget, rule, rule)
        # File "/usr/lib/python3.4/site-packages/kivy/lang.py", line 1983, in _apply_rule
        #   child = cls(__no_builder=True)
        # File "/home/nathan/projects/KivyTest/src/ssce.py", line 29, in __init__
        #   self.grid.add_child(Label(text="Test"))
        # AttributeError: 'NoneType' object has no attribute 'add_child'


class MainWindow(Widget):
    child = ObjectProperty(None)


class SSCE(App):
    def build(self):
        Builder.load_string(UI)
        return MainWindow()

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

From #kivy on irc.freenode.net:

<inclement> I think self.grid just isn't set until after the __init__
<inclement> Because when the kv is loaded, it has to instantiate ChildWidget before it adds the child
<me> So I would have to do something like Clock.schedule_once(self.__my_custom_init__)?
<inclement> I think so