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On POSIX (but not on Mac OS X), Python3 calls get_codeset() to get the file system encoding. If this function fails, sys.getfilesystemencoding() returns None. PyUnicode_DecodeFSDefaultAndSize() fallbacks to utf-8 whereas subprocess fail:
 ...
 File "/home/SHARE/SVN/py3k/Lib/subprocess.py", line 670, in __init__
 restore_signals, start_new_session)
 File "/home/SHARE/SVN/py3k/Lib/subprocess.py", line 1101, in _execute_child
 executable_list = (fs_encode(executable),)
 File "/home/SHARE/SVN/py3k/Lib/subprocess.py", line 1088, in fs_encode
 return s.encode(fs_encoding, 'surrogateescape')
TypeError: encode() argument 1 must be string, not None
We have two choices: raise a fatal error if get_codeset() failed, or fallback to utf-8.
On Windows and Mac OS X, get_codeset() shouldn't be called because the result is just dropped. We should call _PyCodec_Lookup(Py_FileSystemDefaultEncoding) instead to ensure that the file system encoding can be loaded.