def _validate_format_string(format_string):
    """
    Ensure that either little endian (<) or big endian (>) is specified.
    """
    if not format_string.startswith('<') and not format_string.startswith('>'):
        raise ValueError(
            'Byte order must be specified. See https://docs.python.org/3/library/struct.html#byte-order-size-and-alignment'
            )
