loewis> That's indeed exactly what I had proposed 
loewis> - only that you shouldn't repeat the .encode('ascii') 
loewis>  all over the place, (...)

If you can only use bytes 0..127, it can not used for binary protocols 
and so I don't think that it's really useful. If your protocol is 
ASCII text, use explicit conversion to ASCII.

I also not fan on functions having different result type 
(format->bytes or str, it depends...).