unicode(s, enc).encode(enc) == s ?

[mario]

Thanks a lot Martin and Marc for the really great explanations! I was
wondering if it would be reasonable to imagine a utility that will
determine whether, for a given encoding, two byte strings would be
equivalent. But I think such a utility will require *extensive*
knowledge about many bizarrities of many encodings -- and has little
chance of being pretty!

In any case, it goes well beyond the situation that triggered my
original question in the first place, that basically was to provide a
reasonable check on whether round-tripping a string is successful --
this is in the context of a small utility to guess an encoding and to
use it to decode a byte string. This utility module was triggered by
one that Skip Montanaro had written some time ago, but I wanted to add
and combine several ideas and techniques (and support for my usage
scenarios) for guessing a string's encoding in one convenient place. I
provide a write-up and the code for it here:

http://gizmojo.org/code/decodeh/

I will be very interested in any remarks any of you may have!

Best regards, mario