I don’t think the API should be expanded to accept arbitrary bytes-like objects as format strings. Struct formats are strings of ASCII-compatible characters, but not arbitrary chunks of memory.

I think the main question is whether it is okay to break compatibility (Victor’s pull request, or my format-str.patch), or whether there has to be a backwards-compatible deprecation of the existing bytes attribute. FWIW I am okay with breaking compatibility, since the main documentation already implies it should be a text string.