Okay, I was trying to confirm your proposal in Python 3 terms, because in Python 2, str has a different meaning and I was confused.

I agree that the existence of the decoding mode is a design bug, so how would you feel about deprecating it, at least in the documentation? I.e. in Python 3, deprecate usage like str(buffer, "utf-8") in favour of buffer.decode("utf-8") or using the codecs module directly. If this was done, it would clearly remove the need for an encoding parameter to str() in all cases. I would be in favour of deprecating the complementary bytes() and bytearray() encoding modes as well.

Do you have an example use case in Python 3 that would benefit from always allowing an encoding parameter? I can understand that your to_unicode() function could be useful in Python 2. But in Python 3, byte strings tend to hold raw data that is not necessarily textual at all. There are some places (warts in my opinion) such as the binascii module where ASCII-encoded byte strings are common, but I still don’t think this proposal would be very helpful with that.