kivy_background_to_all_instances_of_widget

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.lang import Builder
from kivy.properties import StringProperty

kv = '''
<MyWidget>:
    canvas:
        Rectangle:
            size: self.size
            pos: self.pos
            source: self.background
BoxLayout:
    MyWidget:
    MyWidget:
    MyWidget:

'''


class MyWidget(Widget):
    background = StringProperty('')

    instances = []

    def __init__(self, **kwargs):
        super(MyWidget, self).__init__(**kwargs)
        MyWidget.instances.append(self)

    @staticmethod
    def set_all_backgrounds(background):
        for widget in MyWidget.instances:
            widget.background = background


class TestApp(App):
    def build(self):
        root = Builder.load_string(kv)
        MyWidget.set_all_backgrounds('cover.jpg')
        return root

TestApp().run()