I have 2 widgets, I need to copy all attributes (pos, size, canvas, etc.) from one widget to another somehow (and then move the last to new pos). Probably I can copy attributes one by one, but is there some built-in function?
It seems Python's copy makes only shell copy (I can't move duplicate etc.) and deepcopy fails.
One thing you can do is to make use of the copy function in python.
This does not copy all the pos/size values but you will have all the attributes. 
Ex:
from copy import copy
new_box = copy(self.current_box)
Hope this helps.
Can I copy attributes from one widget to another in Kivy?