@Glenn
" The _defined_ encoding of the original stream is irrelevant, in the same manner that if it is a text stream, that is irrelevant.  The stream is binary, and latin-1, or it is non-standard"

I wish it could be as simple, but I'm afraid it's not. On my PC, sys.stdin.encoding is cp-1252. I tested a multipart/form-data with an INPUT field, and I entered the euro character, which is encoded  \x80 in cp-1252

If I use the encoding defined for sys.stdin (cp-1252) to decode the bytes received on sys.stdin.buffer, I get the correct value in the cgi script ; if I set the encoding to latin-1 in FieldStorage, since \x80 maps to undefined in latin-1, I get a UnicodeEncodeError if I try to print the value ("character maps to <undefined>")