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

[Barry Warsaw]

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.

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.

AFAIK, there have been a few stabs at writing such a launcher, but nothing
that's far enough to be usable.

So for now, you don't have great options.

Cheers,
-Barry