> 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.

When you say Python 2, I assume you mean CPython 2, right?
Because - AFAICT - files got flushed only by accident, not by design.
For example, I suspect that Jython doesn't flush files on exit (since
the JVM doesn't), and I guess IronPython neither.

> However I'm complaining about the current behavior: files are flushed *most of the time*.

That's the problem with implementation-defined behavior ;-)