Accessing Kivy Layout size in canvas instructions

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I have the following code:

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout


class TestGUI(BoxLayout):
    pass


class TestApp(App):
    def build(self):
        return TestGUI()


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

and the corresponding .kv file:

#:kivy 1.9.1

<TestGUI>:
    temp_size: (0.5 * x for x in self.size)
    canvas.before:
        Color:
            rgba: (1, 0, 0, 1)
        Rectangle:
            size: self.temp_size

The code does not run, throwing TypeError: 'NoneType' object is not iterable due to the last line of code in the kv file. It seems that self.size is not initialized when temp_size is declared, but this raises a few questions. Why am I allowed to iterate through self.size when declaring temp_size if it is a NoneType object? Also, why does the code work perfectly fine when I replaces the last line with size: (0.5 * x for x in self.size) instead of using a variable? Also, how can I circumvent this issue and assign variables based on the value of self.size?

asked 45 secs ago

Kootling

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Accessing Kivy Layout size in canvas instructions