def truncate_by_concating(s, max_bytes):
    """
    Ensure that the UTF-8 encoding of a string has not more than
    max_bytes bytes
    :param s: The string
    :param max_bytes: Maximal number of bytes
    :return: The cut string
    """

    def len_as_bytes(s):
        return len(s.encode(errors='replace'))

    if len_as_bytes(s) <= max_bytes:
        return s

    res = ""
    for c in s:
        old = res
        res += c
        if len_as_bytes(res) > max_bytes:
            res = old
            break
    return res