>> It looks to be a feature of the standard C library, at least the GNU libc.
> Yes, it's guaranteed by POSIX/ANSI (see man exit).

Hum, POSIX (2004) is not so strict:
"Whether open streams are flushed or closed, or temporary files are
removed is implementation-defined."
http://pubs.opengroup.org/onlinepubs/009695399/functions/exit.html

2013/4/29 Charles-François Natali <report@bugs.python.org>:
>
> Charles-François Natali added the comment:
>
>> "When you say Python 2, I assume you mean CPython 2, right?
>> Because - AFAICT - files got flushed only by accident, not by design."
>>
>> It looks to be a feature of the standard C library, at least the GNU libc. Its libio library installs an exit handler flushing all open files. You can see it if you set a breaking on write() using gdb:
>
> Yes, it's guaranteed by POSIX/ANSI (see man exit).
> I was refering to the fact that the automatic flushing of files upon
> exit is a mere side effect of the implementation based atop stdio
> stream in cpython 2. It's no guaranteed by any Python spec (and I
> can't really think of any platform other than C that makes such
> guarantee).
>
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