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Hi,
I just noticed that version output generated via the
**'version' action** of the **argparse** module
is routed to stderr. I'd expect regular output to go to stdout instead.
The current behavior also seems inconsistent to me because --help
prints to stdout.
Best,
Wolfgang

Yes, it seems like an oversight to me. Printing --version to stdout is more customary (Python itself does it, and most other tools do too).
A question comes up about backwards compatibility. I would definitely not change it in 2.x - it's just not worth it. As for 3.x, should this go into 3.3 too or just 3.4?

Only on 3.4.
Python prints the version on stdout since 3.4 -- before it used stderr:
3.3$ ./python -V 2> /dev/null
3.3$ ./python -V > /dev/null
Python 3.3.2+
3.4$ ./python -V 2> /dev/null
Python 3.4.0a1+
3.4$ ./python -V > /dev/null
This might also explain why argparse uses stderr (other modules/scripts in the stdlib might do the same too).

On Wed, Sep 4, 2013 at 6:26 AM, Ezio Melotti <report@bugs.python.org> wrote:
>
> Ezio Melotti added the comment:
>
> Only on 3.4.
> Python prints the version on stdout since 3.4 -- before it used stderr:
> 3.3$ ./python -V 2> /dev/null
> 3.3$ ./python -V > /dev/null
> Python 3.3.2+
>
> 3.4$ ./python -V 2> /dev/null
> Python 3.4.0a1+
> 3.4$ ./python -V > /dev/null
>
> This might also explain why argparse uses stderr (other modules/scripts in
> the stdlib might do the same too).
>
Ah, right. On 3.4 Python's main.c uses printf for --version; on earlier
versions it's fprintf(stderr...)
I guess it's a no-brainer then; 3.4 has to be changed, but not earlier
versions. I'll whip up a quick patch.

The Python executable change is from #18338 

Patch attached

> This might also explain why argparse uses stderr (other modules/scripts in the stdlib might do the same too).
Lib/trace.py, Tools/pynche/Main.py, and Tools/i18n/pygettext.py write to the stdout.
Lib/smtpd.py and Tools/i18n/msgfmt.py write to the stderr.
The optparse module also writes to the stdout.

You misplace Misc/NEWS entry in wrong section -- "What's New in Python 3.4.0 Alpha 1".
I think this change (as change of issue18338) worths the mentioning in Doc/whatsnew/3.4.rst.
Did you run all test suite? This change can affect other tests.

New changeset ec9a4b77f37b by Eli Bendersky in branch 'default':
Issue #18920: argparse's default version action (for -v, --version) should
http://hg.python.org/cpython/rev/ec9a4b77f37b 

Thanks, I moved the NEWS entry to the right place. Yes, all tests pass. I'll update whatsnew separately.

New changeset 587bdb940524 by Eli Bendersky in branch 'default':
Update whatsnew/3.4 wrt. --version going to stdout. #18338, #18920, #18922
http://hg.python.org/cpython/rev/587bdb940524 

Is there a workaround for python2? The issue is that autotools is checking that installed tools support --help and --version and it expects those to stdout:
bad=0; pid=$$; list="gtkdoc-check gtkdoc-depscan gtkdoc-fixxref gtkdoc-mkdb gtkdoc-mkhtml gtkdoc-mkhtml2 gtkdoc-mkman gtkdoc-mkpdf gtkdoc-rebase gtkdoc-scan gtkdoc-scangobj gtkdocize"; for p in $list; do \
 case ' ' in \
 *" $p "* | *" ../../$p "*) continue;; \
 esac; \
 f=`echo "$p" | sed 's,^.*/,,;s,x,x,'`; \
 for opt in --help --version; do \
 if "/home/ensonic/projects/gnome/gtk-doc/gtk-doc-1.27.1/_inst/bin/$f" $opt >c${pid}_.out \
 2>c${pid}_.err </dev/null \
 && test -n "`cat c${pid}_.out`" \
 && test -z "`cat c${pid}_.err`"; then :; \
 else echo "$f does not support $opt" 1>&2; bad=1; fi; \
 done; \
done; rm -f c${pid}_.???; exit $bad
gtkdoc-check does not support --version
gtkdoc-fixxref does not support --version
gtkdoc-mkdb does not support --version
gtkdoc-mkhtml does not support --version
gtkdoc-mkhtml2 does not support --version
gtkdoc-mkman does not support --version
gtkdoc-mkpdf does not support --version
gtkdoc-rebase does not support --version
gtkdoc-scan does not support --version
gtkdoc-scangobj does not support --version
Makefile:637: recipe for target 'installcheck-binSCRIPTS' failed

You can implement your own version action which outputs to stdout and use it for -V and --version.