gistfile1.py

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import NumericProperty, ReferenceListProperty, ObjectProperty, ListProperty, AliasProperty, StringProperty, DictProperty, BooleanProperty, StringProperty, OptionProperty
        
class MyCustomWidget(Widget):
    _val1 = NumericProperty(0)
    _val2 = NumericProperty(50)
    def AddVal1(self):
        valToAdd = 26
        self._val1 += valToAdd
    
class AppWidget(BoxLayout):
    pass

class TestApp(App):
    def build(self):
        return AppWidget()
    
if __name__ == "__main__":
    TestApp().run()

gistfile2.txt

<MyCustomWidget>:
    pos: 500, 50
    size: 150, 150
    Label:
        text: ("%d/%d" % (root._val1, root._val2))
        pos: (root.center_x, root.center_y)
        
<AppWidget>:
    orientation: 'vertical'
    MyCustomWidget:
        id: cw
    Label:
        text: str(cw._val1) + ' ' + str(cw._val2)
    Button:
        text: 'Add to val1'
        on_press: cw.AddVal1()