'Get/set the value of this element, using the ``value`` attribute.
Also, if this is a checkbox and it has no value, this defaults
to ``\'on\'``.  If it is a checkbox or radio that is not
checked, this returns None.'

def _value__get(self):

    if self.checkable:
        if self.checked:
            return (self.get('value') or 'on')
        else:
            return None
    return self.get('value')
