def _remove_key_prefix(json_, sep):
    """
    去除key在第一个sep之前的部分
    """
    key_path = next(iter(json_))
    next_sep_pos = key_path.find(sep) + 1
    ret = {}
    for key, value in json_.items():
        ret[key[next_sep_pos:]] = value
    return ret
