Multirelease effort: Moving to Python 3

[Toshio Kuratomi]

On Fri, Jul 19, 2013 at 08:20:02AM +0200, Marcela Mašláňová wrote:
> On 07/19/2013 05:44 AM, Toshio Kuratomi wrote:
> >
> >On Jul 18, 2013 5:42 PM, "Michael Catanzaro" <mike.catanzaro at gmail.com
> ><mailto:mike.catanzaro at gmail.com>> wrote:
> > >
> > > On Thu, 2013-07-18 at 09:53 -0700, Toshio Kuratomi wrote:
> > > > /usr/bin/python should refer to python2 --
> > > > http://www.python.org/dev/peps/pep-0394/  I'd be -1 to changing this
> > > But when python2 is no longer installed by default, surely you want to
> > > get a python prompt when you type 'python'?
> >
> >Yes, and for a long time, I'm going to want to get a python2 prompt.
> >Which means installing the package for python2, not having python3 start
> >on that case.
> >
> >
> >
> Why do you want to use old version as default? That's very
> conservative approach for Fedora ;-)

This is the wrong mental model of the relationship between python2 and
python3.  As a comparison this argument is akin to saying, "C is the old
version of C++.  We should be porting all our code to use the C++ compiler
in Fedora"  C is much more compatible to C++ than python2 is to python3 so
this would be a less drastic change.

python2 and python3 are separate languages.  There is a lot of similarity
between the two and with recent enough versions of python2 (2.7) and python3
(python3.4) and some external libraries (python-six) and by sticking to some
specific coding styles ( http://python3porting.com/noconv.html ) and by
sometimes resorting to having separate files for some python2-specific
routines vs python3-specific routines you can write code that is valid and
runs under either language.  That does not mean that they are the same
language.

> Upstream plans to support it until 2015 (maybe little longer). Fedora
> needs to be prepared for such step, so it's the right time to start
> working on it.
> 
I am wholeheartedly in favor of "working on it".  But things like switching
the default /usr/bin/python are not "working on it".  Those are backwards
incompatible, end user visible, changes of expectations that we do not need
to implement now in order to be a leader.  When people invoke python, they
expect python2.  When they want python3, they expect to type python3 to get
that.  At the moment, causing /usr/bin/python to invoke python3 is just
going to cause confusion and end user confusion (arch is the only linux
distro that sets /usr/bin/python to point to python3).

As an example here, let's suppose that we were to stop shipping firefox by
default and started shipping chromium instead.  Would we make
/usr/bin/firefox invoke /usr/bin/chromium?  Would we have a firefox icon
that started chromium when clicked?  When applied here, it does seem to make
more sense for end user's to notice that there isn't a firefox icon and
either consciously open up a different web browser or else install firefox
to get their preferred browsing experience, right?  /usr/bin/python is the
same way.  People have tons of scripts on their systems with
#!/usr/bin/python shebang lines.  If we change /usr/bin/python to
point to /usr/bin/python3 then we'll force them to deal with either porting,
going through all their scripts and replacing them with /usr/bin/python2, or
switching to another distribution which doesn't change the meanings of  well
known interpreter paths for no gain.

All this is not to say that there might not be a time in the future when it
makes sense to have python3 assume the /usr/bin/python name.  However, that
time is *after* the collective user consciousness has stopped using
/usr/bin/python, not before.  When we can go to pycon and talks are assumed
to be targeting python3 rather than python2.  After people wanting to do
a simple calculation on the CLI stop doing /usr/bin/python -c 'print 5 + 6'.
This is probably after alternate python implementations like pypy and jython
either die off or finally make the switch to implementing python3.  My guess
is this will be after there's a RHEL release that includes /usr/bin/python3
in the default install since RHEL and CentOS are major parts of our
ecosystem and will have to be able to use the python3 syntax on both their
RHEL servers and Fedora machines.  To some extent, this also depends on what
other large distros set as their expectations: Debian and Ubuntu currently
point /usr/bin/python to python2.  AFAIK, only Arch is pointing to python3.
My intuition is this will be at least two years after we stop shipping
/usr/bin/python (pointing to python2) by default but this could be wrong --
it's about the expectations of the Fedora community and the Linux and python
development communities as a whole.

-Toshio
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 198 bytes
Desc: not available
URL: <http://lists.fedoraproject.org/pipermail/devel/attachments/20130719/f5e78eca/attachment.sig>