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In the following example, sys.path[0] should be '/home/SHARE/SVN/py3k\udcc3\udca9' (my locale and filesystem encodings are utf-8):
$ cd /home/SHARE/SVN/py3ké
$ echo "import sys; print(sys.path[0])" > x.py
$ ./python x.py
/home/SHARE/SVN/py3ké
$ PYTHONFSENCODING=ascii ./python x.py
/home/SHARE/SVN/py3ké
The problem is that PySys_SetArgvEx() inserts argv[0] at sys.path[0], but argv[0] is decoded using the locale encoding (by _Py_char2wchar() in main()), whereas paths of sys.path are supposed to be encodable (and decoded) by sys.getfilesystemencoding().
argv array should be decoded using the filesystem encoding (see issue #9992) or argv[0] should be redecoded (encode to the locale encoding, and decode from the filesystem encoding, see issue #9630).