Requesting@gvanrossum since you originally added this.

Sorry, I no longer do such code reviews.

This looks like an improvement to me, although it would be more complete to explicitly allow passing None even whenstring is a byte string.

FWIW I would write something like

Ifstring is astr object,encoding anderrors specify how to deal with non-ASCII characters. Ifencoding is None, the 'utf-8' encoding is used as default. Iferrors is None, the 'strict' error handler is used as default. All otherencoding anderror values accepted bystr.encode are also allowed.

Ifstring is abytes object,encoding anderrors must be None, or a TypeError is raised.