def decode_bytes(obj):
    """If the argument is bytes, decode it.

    :param Object obj: A string or byte object
    :return: A string representation of obj
    :rtype: str

    """
    if isinstance(obj, bytes):
        return obj.decode('utf-8')
    elif isinstance(obj, str):
        return obj
    else:
        raise ValueError("ERROR: {} is not bytes or a string.".format(obj))