It used to be a consistently reliable behavior in Python 2 (and we made it so in PyPy too), provided of course that the process exits normally; but it no longer is in Python 3.  Well I can see the reasons for not flushing files, if it's clearly documented somewhere as a change of behavior from Python 2.

However I'm complaining about the current behavior: files are flushed *most of the time*.  That's a behavior that is clearly misleading, or so I would think.  I'm rather sure that there are many small scripts and large programs out there relying on automatic flushing, and then one day they'll hit a case where the file is not flushed and get the worst kind of error: a file unexpectedly truncated at 99% of its length, in a way that cannot be reproduced by small examples.

Feel free to close anyway as not-a-bug; I won't fight the Python 3 behavior, because Python 2 works as expected.