Accessing Kivy layout size in kvlang

Question

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?

Answer 1

What happens here is basically this:

[x for x in None]

and why is it like that you can read in this bug report. It's because the canvas isn't available yet and the properties seem to be calculated after it's created, therefore - if no canvas, then value == garbage.

To fix it you have to do it in the canvas itself like this and not outside in a property, because the canvas is built first:

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

I'm not sure what do you mean by the last question, though I think it might be either using properties after the canvas is already available, or using values from Config if you intend to use Window and not Widget size, or set the property in __init__ and use it later in kv.

The latter one might be the best thing for you, but don't forget to:

• initialize with some values (ListProperty([0, 0])) otherwise the Rectangle will just kill the app
• use super(), for inheritance reasons

Example:

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import ListProperty
Builder.load_string('''
<TestGUI>:
    canvas.before:
        Color:
            rgba: (1, 0, 0, 1)
        Rectangle:
            size: self.temp_size
''')


class TestGUI(BoxLayout):
    temp_size = ListProperty([0, 0])

    def __init__(self, **kwargs):
        super(TestGUI, self).__init__(**kwargs)
        print(self.size)
        self.temp_size = [0.5 * x for x in self.size]


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


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