add app -> sample works

kivy/uix/screenmanager.py

@@ -47,9 +47,10 @@ a root widget for your own screen. Best way is to subclass.
 
 Here is an example with a 'Menu Screen', and a 'Setting Screen'::
 
+    from kivy.app import App
     from kivy.lang import Builder
     from kivy.uix.screenmanager import ScreenManager, Screen
-
+    
     # Create both screen. Please note the root.manager.current: this is how you
     # can control the ScreenManager from kv. Each screen have by default a
     # property manager that give you the instance of the ScreenManager used.
@@ -61,7 +62,7 @@ Here is an example with a 'Menu Screen', and a 'Setting Screen'::
                 on_press: root.manager.current = 'settings'
             Button:
                 text: 'Quit'
-
+    
     <SettingsScreen>:
         BoxLayout:
             Button:
@@ -70,18 +71,26 @@ Here is an example with a 'Menu Screen', and a 'Setting Screen'::
                 text: 'Back to menu'
                 on_press: root.manager.current = 'menu'
     """)
-
+    
     # Declare both screen
     class MenuScreen(Screen):
         pass
-
+    
     class SettingsScreen(Screen):
         pass
-
+    
     # Create the screen manager
     sm = ScreenManager()
     sm.add_widget(MenuScreen(name='menu'))
     sm.add_widget(SettingsScreen(name='settings'))
+    
+    class TestApp(App):
+    
+        def build(self):
+            return sm 
+    
+    if __name__ == '__main__':
+        TestApp().run()
 
 
 Changing transition