The encoding used impacts the result:

>>> s = 'abc\udcc3\udca9'
>>> s.encode('ascii', 'surrogateescape').decode('ascii', 'replace')
'abc��'
>>> s.encode('utf-8', 'surrogateescape').decode('utf-8', 'replace')
'abcé'

The original string ('abc\udcc3\udca9') was obtained by decoding a valid utf-8 string with the 'ascii' codec and the 'surrogateescape' error handler.

If anything, the default encoding should probably be sys.getfilesystemencoding().