set arbitrary mode

def cmd_mode(self, args):
        '''set arbitrary mode'''
        mode_mapping = self.master.mode_mapping()
        if mode_mapping is None:
            print('No mode mapping available')
            return
        if len(args) != 1:
            print('Available modes: ', mode_mapping.keys())
            return
        if args[0].isdigit():
            modenum = int(args[0])
        else:
            mode = args[0].upper()
            if mode not in mode_mapping:
                print('Unknown mode %s: ' % mode)
                return
            modenum = mode_mapping[mode]
        self.master.set_mode(modenum)