In Python 3.x, 'a' is just as much a Unicode string as 'ä'. I agree that binascii.a2b_hex should raise ValueError for argument 'ä', or ';' for that matter, as they are invalid values. There's an inconsistency in why some invalid characters raise ValueError and some raise binascii.Error. But since binascii.Error is a subclass of ValueError, it probably doesn't matter that much, so long as the documentation is clear.