[Python-Dev] Clarification of PEP 394 for scripts that run under Python 2 and 3

[Nick Coghlan]

On 14 November 2015 at 08:57, Barry Warsaw <barry at python.org> wrote:
> On Nov 13, 2015, at 10:32 PM, Damien George wrote:
>
>>1. What is the true intent of PEP 394 when only Python 3 is installed?  Is
>>"python" available or not to run scripts compatible with 2.x and 3.x?
>>
>>2. Is it possible to write a shebang line that supports all variations of
>>Python installations on *nix machines?
>>
>>3. If the answer to 2 is no, then what is the recommended way to support
>>all Python installations with one standalone script?
>
> It's important to remember that PEP 394 is an informational PEP, still under
> active (if dormant) discussion.  It is also just a recommendation, and can't
> force any downstream redistributors to do one thing or another.  Still, the
> intent is to provide a set of guidelines for the majority of *nix distributors
> to (eventually) adopt.
>
> I'll also point you to the recently created linux-sig where this topic is
> being discussed.
>
> https://mail.python.org/pipermail/linux-sig/2015-October/000000.html
>
> As you've noticed, Arch took a particular approach, but speaking as part of
> the Debian/Ubuntu community, don't expect that ecosystem to go down the same
> path.  It's highly unlikely /usr/bin/python will ever point to anything other
> than Python 2, even when only Python 3 is installed by default.  That might
> change once Python 2.7 is actually EOL'd, and that won't happen for quite a
> long time.  PEP 373 currently says there will be bug fix releases until 2020,
> and I'd expect security-only releases for some time after that.

This is the same situation for the Fedora/RHEL/CentOS ecosystem - for
the time being, we think failure -> "yum install /usr/bin/python" is
the cleanest user experience we can readily offer for Python 3 only
systems. However, we don't think it's an acceptable long term answer,
and something *else* needs to be done with /usr/bin/python than just
making it a symlink to Python 3 (since the latter will lead to cryptic
error messages for Python 2 only scripts).

> In general, the discussions on linux-sig and elsewhere is coalescing around a
> launcher-type approach, where you'd put something like /usr/bin/py in your
> shebang if your script really is bilingual.  Then `py` can try to figure out
> what's available on your system, what your preference is, etc. and then
> execute your script using that version-specific binary.  I would expect the
> launcher to eventually be provided by the upstream Python development
> community.

A couple of other links folks may find worth reading for background:

* LWN summary of the discussion at this year's language summit:
https://lwn.net/Articles/640296/
* Geoffrey Thomas's launcher concept:
https://ldpreload.com/blog/usr-bin-python-23

This thread prompted me to do an update pass over
https://wiki.python.org/moin/Python3LinuxDistroPortingStatus, adding
these background links to the Python symlink section, and updating the
Fedora porting status link to refer to the recently published porting
DB: http://portingdb-encukou.rhcloud.com/

Regards,
Nick.

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