class Screen1(Screen):
    pass

class Screen2(Screen):
    pass

class MyManager(ScreenManager):
    pass

class PopUp(Popup):
    def changeText(self,nameStr):
        self.ids.label.text = "You are on Screen %s!" %nameStr #this is text that I want to display

class PrimaryApp(App):
    def build(self):
        return MyManager()

PrimaryApp().run()

#:import Factory kivy.factory.Factory
<MyManager>:
    Screen1:
        id: screen1
    Screen2:
        id: screen2

<Screen1>:
    name: "one"
    GridLayout:
        id: grid
        rows: 2
        Button:
            id: button1
            text: "Go to Screen Two"
            on_release: root.manager.current = "two"
        Button:
            id: button2
            text: "Display Popup"
            on_release:
                Factory.PopUp().changeText(root.name)
                Factory.PopUp().open()
<Screen2>:
    name: "two"
    GridLayout:
        id: grid
        rows: 2
        Button:
            id: button1
            text: "Go to Screen One"
            on_release: root.manager.current = "one"
        Button:
            id: button2
            text: "Display Popup"
            on_release:
                Factory.PopUp().changeText(root.name)
                Factory.PopUp().open()


<PopUp>:
    id:pop
    size_hint: (.5,.5)
    title: "Notice!"
    Label:
        id: label
        text: "PLACEHOLDER TEXT" #this is not the code I want displayed