open: 'ascii', 'backslashreplace' not behaving as expected - why?

Veek M <[email protected]>
Newsgroups comp.lang.python
Organization A noiseless patient Spider
Message-ID <[email protected]>
So in kate, i Ctrl-Shift-U and type ffff to enter a unicode codepoint  of 
0xffff. Then at the REPL prompt i do 
fh = open('/tmp/x', 'rt', -1, 'ascii', 'backslashreplace', None)

and i get 
fh.readline()
'\\xef\\xbf\\xbf\n'

Since I wrote two bytes 0xff and 0xff into Kate - why am i getting 0xef 
0xbf and 0xbf ?
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