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

Barry Warsaw barry at python.org
Fri Nov 13 17:57:27 EST 2015

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

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