This doesn't work, because FocusBehavior doesn't seem to be defined yet. I get:

       File "C:\kivy-dev\kivy\kivy\uix\behaviors.py", line 421, in <module>
         class FocusBehavior(object):
       File "C:\kivy-dev\kivy\kivy\uix\behaviors.py", line 549, in FocusBehavior
         focus_next = ObjectProperty(None, allownone=True, baseclass=FocusBehavior)
     NameError: name 'FocusBehavior' is not defined