> I would just say that str(bytesobj, encoding, errors) is equivalent to bytesobj.decode(encoding, errors) (assuming it really is).

Good suggestion.  And yes, code is shared in the following way:

http://hg.python.org/cpython/file/d3c7ebdc71bb/Objects/bytesobject.c#l2306

One thing that would need to be addressed in the str() version is if bytesobj is a PEP 3118 character buffer, after which it falls back to bytesobj.decode(encoding, errors).  I will update the patch so people can see how it looks.