def save(self):
        tmp_path = self.path + '.tmp'
        with open(tmp_path, 'w') as f:
            f.write(json.dumps(self.body_labels.items()))
        os.rename(tmp_path, self.path)
def _getN(self, n):
        n -= 1
        p = self.head
        while n:
            p = p.next
            n -= 1
        return p