PEP 529 isn't implemented for io.FileIO, and I think it should be. If a UTF-8 path is passed to open(), it ends up calling C _open instead of decoding the path and calling C _wopen. Also, if a pathlike object is passed to io.FileIO, it calls PyUnicode_FSConverter on it, converting it to UTF-8 and then passing it to _open. For example:

    >>> p = r'C:\Temp\αβψδ'
    >>> os.path.exists(p)
    True

    >>> open(p.encode())
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    FileNotFoundError: [Errno 2] No such file or directory: b'C:\\Temp\\\xce\xb1\xce\xb2\xcf\x88\xce\xb4'

    >>> io.FileIO(pathlib.Path(p))
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    FileNotFoundError: [Errno 2] No such file or directory: WindowsPath('C:/Temp/αβψδ')

The Windows implementation should mirror the POSIX implementation via PyUnicode_FSDecoder.

Merging this in now, along with a few other patches. This is the test I'm adding, in case anyone spots any problems with it before I push (copied from the test for pure-ASCII bytes):

    @unittest.skipIf(sys.getfilesystemencoding() != 'utf-8',
                     "test only works for utf-8 filesystems")
    def testUtf8BytesOpen(self):
        # Opening a UTF-8 bytes filename
        try:
            fn = TESTFN_UNICODE.encode("utf-8")
        except UnicodeEncodeError:
            self.skipTest('could not encode %r to utf-8' % TESTFN_UNICODE)
        f = self.FileIO(fn, "w")
        try:
            f.write(b"abc")
            f.close()
            with open(TESTFN_UNICODE, "rb") as f:
                self.assertEqual(f.read(), b"abc")
        finally:
            os.unlink(TESTFN_UNICODE)

New changeset 0bf72810f8ea by Steve Dower in branch '3.6':
Issue #29409: Implement PEP 529 for io.FileIO (Patch by Eryk Sun)
https://hg.python.org/cpython/rev/0bf72810f8ea

New changeset a5538734cc87 by Steve Dower in branch 'default':
Merge issue #28164 and issue #29409
https://hg.python.org/cpython/rev/a5538734cc87

Committed, but I'll leave this open for a bit in case anyone wants to comment on the commit.

New changeset fe7e868bad5bffaf863c80fa2bcc4e3bf87a413a by Steve Dower in branch 'master':
Issue #29409: Implement PEP 529 for io.FileIO (Patch by Eryk Sun)
https://github.com/python/cpython/commit/fe7e868bad5bffaf863c80fa2bcc4e3bf87a413a

New changeset 965f8d68a8dcc2ebb2480fe7e9121ac7efdee54e by Steve Dower in branch 'master':
Merge issue #28164 and issue #29409
https://github.com/python/cpython/commit/965f8d68a8dcc2ebb2480fe7e9121ac7efdee54e

New changeset fe7e868bad5bffaf863c80fa2bcc4e3bf87a413a by Steve Dower in branch '3.6':
Issue #29409: Implement PEP 529 for io.FileIO (Patch by Eryk Sun)
https://github.com/python/cpython/commit/fe7e868bad5bffaf863c80fa2bcc4e3bf87a413a