def redis_string_to_list(value, sep=','):
    """convert a comma (or sep) separated string to list.

    Args:
        value: byte, a redis string.
        sep: string, which separator is used to separator items

    Return:
        a list of strings. Could be empty list if original string is None or
        empty.

    """
    return value.decode('utf-8').split(sep) if value else []