Elements' Sizes Don't Update On Application Loading In Kivy

I have on_size() event, it works when I resize window, but I can't make it work on application loading correctly (it works, but not like it must): from random import random, randin

Solution 1:

As I update my class's children's sizes depending on labels and buttons inside of it, I've added to them on_size event:

defon_size(self, *args):
    self.parent.parent.on_size()


classSideLabel(Label):
    on_size = on_size


classSideButton(Button):
    on_size = on_size

And also I have replaced Buttons and Labels on these classes in kv. It seems to work correctly now.