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For completeness sake: Apple's Cocoa APIs do not renormalize strings, that is: I've created a file named 'één' in the Terminal, then (using a python 3.2 build):
# Terminal input seems NFC:
>>> len('één')
3
# Output from os.listdir isn't:
>>> os.listdir('.')
['één']
>>> len(_[0])
5
# Output from the Cocoa equivalant also isn't:
>>> import Foundation
>>> mgr = Foundation.NSFileManager.defaultManager()
>>> mgr.directoryContentsAtPath_('.')
(
 "e\U0301e\U0301n"
)
>>> len(_[0])
5
BTW. fsdecode(fsencode(x)) cannot in general be a no-op, unicode normalizations can screw things up (with the now withdrawn proposal the expression wouldn't be a no-op for NFD strings).