save waypoints to a file

def save(self, filename):
        '''save waypoints to a file'''
        f = open(filename, mode='w')
        f.write("QGC WPL 110\n")
        for w in self.wpoints:
            if getattr(w, 'comment', None):
                f.write("# %s\n" % w.comment)
            f.write("%u\t%u\t%u\t%u\t%f\t%f\t%f\t%f\t%f\t%f\t%f\t%u\n" % (
                w.seq, w.current, w.frame, w.command,
                w.param1, w.param2, w.param3, w.param4,
                w.x, w.y, w.z, w.autocontinue))
        f.close()