switchingwidgets.kv

#:kivy 1.5.1
#:import kivy kivy


<FirstWidget>
    GridLayout:
        size: root.size
        rows: 2

        Label:
            text: 'First Widget'

        Button:
            text: 'Go to Second'
            on_press: app.root.widget_switcher(self.text)

<SecondWidget>
    GridLayout:
        size: root.size
        rows: 2

        Label:
            text: 'Second Widget'

        Button:
            text: 'Go to First

switchingwidgets.py

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.floatlayout import FloatLayout


class FirstWidget(Widget):
    def build(self):
        print "made it to first"

class SecondWidget(Widget):
    def build(self):
        print "made it to second"

class SwitchingWidgets(App):
    def build(self):
        parent = FloatLayout()
        first = FirstWidget()
        second = SecondWidget()

        parent.add_widget(first)

        return parent

    def widget_switcher(self, string):
        print string

if __name__ == '__main__':
    SwitchingWidgets().run(