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CLA signed.

Would be nice to add tests.

Python implementation in _pyio.py may need a change too.

I'll look into tests.

The Python implementation in Lib/_pyio.py should be fine:

 209   │     try:
 210   │         line_buffering = False
 211   │         if buffering == 1 or buffering < 0 and raw.isatty():
 212   │             buffering = -1
 213   │             line_buffering = True

Since it is left to right. But maybe I am missing something else here?

Regarding CLA: It is signed, I got the copy via E-Mail and my Github account is linked in the Python Bugtracker .. I'll double check it again (maybe it takes a while?).

Also regarding the Misc/News, is that worth a news entry, it's just a rather small bugfix/improvement?

A news entry may save hours for somebody if his program will fail when run on different computer with older Python because of this change.

The Python implementation looks fine to me too.

I changed the code you commented on, I turned the now redundant buffering < 0 && isatty into just isatty and assigned 0 as a default to isatty.

I also added a news entry, I hope that is proper English and makes sense for a bystander.

I am still having troubles with writing tests for this though. My initial idea was to replace the FileIO class with my own "spy" like so:

        FileIO = self.io.FileIO

        class SpyFileIO(FileIO):
            _isatty = False

            def isatty(self):
                SpyFileIO._isatty = True
                return False

        self.io.FileIO = SpyFileIO
        try:
            f = self.open(TESTFN, 'w', buffering=-1)
        finally:
            self.io.FileIO = FileIO
            f.close()

        self.assertTrue(SpyFileIO._isatty)

This works fine for the _pyio module, but it does not work for _io - the C module. I assume because the C module doesn't look up the reference from the module for the FileIO type every time (please correct me when I am wrong!). I don't have a better idea on how I could verify the desired behaviour in a test than monkey patch FileIO here. Please point me in the right direction.

Any news on this? Anything I should do?