Since 29ba6c399090 stdout and stderr flush upon exit. But any errors on flushing were swallowed. Since 4ca497f4819c (issue5319) an error is printed if stdout flushing fails.

FlushFile doesn't provide all methods and attributes of io.TextIOBase. But it is not required that sys.stdout should be an instance of io.TextIOBase or provide the flush() method. I suppose this is either not a bug or documentation issue.