'Always allow accessing the keys \'popup\', \'pick\', \'pick_file\' and
\'pick_folder\' as keys.'

def __missing__(self, key):

    if (key == u'popup'):
        return (self.get(IS_POPUP_VAR, False) == u'1')
    elif (key == u'pick'):
        return self.get(u'_pick', u'')
    elif key.startswith(u'pick_'):
        return (self.get(u'_pick', u'') == key.split(u'pick_')[1])
