[Python-Dev] PEP 394 request for pronouncement (python2 symlink in *nix systems)

[Nick Coghlan]

On Tue, Feb 14, 2012 at 8:08 AM, Barry Warsaw <barry at python.org> wrote:
> On Feb 13, 2012, at 12:31 PM, Nick Coghlan wrote:
>
>>I think Antoine makes a good point about ease of introspection when
>>you have multiple versions in the same series installed, so I'd be
>>fine with:
>>- updating the PEP recommendation to say that either form of link is
>>fine (with hard links marginally faster, but harder to introspect)
>>- noting that python.org releases will consistently use symlinks for
>>easier introspection via "ls -l"
>>- updating Makefile.pre.in to ensure that we really do consistently use symlinks
>
> +1, and +1 for the PEP to be accepted.
>
>>This does mean that launching Python may involve a slightly longer
>>symlink chain in some cases (python -> python2 -> python2.7), but the
>>impact of that is always going to be utterly dwarfed by other startup
>>costs.
>
> Agreed about startup times.  However, does the symlink chain have to go in
> this order?  Couldn't python -> python2.7 and python2 -> python2.7?  OTOH, I
> seriously doubt removing one level of symlink chasing will have any noticeable
> effect on startup times.

I considered that, but thought it would be odd to make people
double-key a manual default version change within a series. It seemed
more logical to have "python" as a binary "python2/3" switch and then
have the python2/3 symlinks choose which version is the default for
that series. (I'll add that rationale to the PEP, though)

> One other thing I'd like to see the PEP address is a possible migration
> strategy to python->python3.  Even if that strategy is "don't do it, man!".
> IOW, can a distribution change the 'python' symlink once it's pointed to
> python2?  What is the criteria for that?  Is it up to a distribution?  Will
> the PEP get updated when our collective wisdom says its time to change the
> default?  etc.

I have no idea, and I'm not going to open that can of worms for this
PEP. We need to say something about the executable aliases so that
people can eventually write cross-platform python2 shebang lines, but
how particular distros actually manage the transition is going to
depend more on their infrastructure and community than it is anything
to do with us.

> Also, if Python 2.7 is being changed to add this feature, why can't Python 3.2
> also be changed?  (And if there's a good reason for not doing it there, that
> should be added to the PEP.)

Because Python 3.2 already installs itself as python3 and doesn't
touch the python symlink. Aside from potentially cleaning up the
choice of symlinks vs hardlinks in a couple of cases, the PEP really
doesn't alter Python 3 deployment at all.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia