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[Python-Dev] Update PEP 394: Distributions can choose what does python command mean


I think this is reasonable. Thanks for making the rationale clear!

On Fri, Apr 12, 2019 at 8:02 AM Petr Viktorin <encukou at gmail.com> wrote:

> On 4/12/19 4:53 PM, Miro Hron?ok wrote:
> > Hello.
> >
> > Based on discussions in [1], Petr Viktorin and me have drafted a new
> > update [2] to the PEP 394 (The "python" Command on Unix-Like Systems).
> >
> > The update gives distributors the opportunity to decide where does the
> > "python" command lead to, whether it is present etc.
> >
> > Please, see the PR [2] for the suggested changes.
> >
> > [1]:
> https://mail.python.org/pipermail/python-dev/2019-February/156272.html
> > [2]: https://github.com/python/peps/pull/989
>
> The text is available at
> https://github.com/hroncok/peps/blob/pep394-2019/pep-0394.txt
>
> As a summary, I'll paste the rationale sections here:
>
> History of this PEP
> ===================
>
> In 2011, the majority of distributions
> aliased the ``python`` command to Python 2, but some started switching it
> to
> Python 3 ([5]_). As some of the former distributions did not provide a
> ``python2`` command by default, there was previously no way for Python 2
> code
> (or any code that invokes the Python 2 interpreter directly rather than via
> ``sys.executable``) to reliably run on all Unix-like systems without
> modification, as the ``python`` command would invoke the wrong interpreter
> version on some systems, and the ``python2`` command would fail completely
> on others. This PEP originally provided a very simple mechanism
> to restore cross-platform support, with minimal additional work required
> on the part of distribution maintainers. Simplified, the recommendation
> was:
>
> 1. The ``python`` command was preferred for code compatible with both
>     Python 2 and 3 (since it was available on all systems, even those that
>     already aliased it to Python 3).
> 2. The ``python`` command should always invoke Python 2 (to prevent
>     hard-to-diagnose errors when Python 2 code is run on Python 3).
> 3. The ``python2`` and ``python3`` commands should be available to specify
>     the version explicitly.
>
> However, these recommendations implicitly assumed that Python 2 would
> always be
> available. As Python 2 is nearing its end of life in 2020 (PEP 373, PEP
> 404),
> distributions are making Python 2 optional or removing entirely.
> This means either removing the ``python`` command or switching it to invoke
> Python 3, invalidating respectively the first or second recommendation.
> Also, some distributors decided that their users are better served by
> ignoring the PEP's recommendations, making the PEP's supposedly
> cross-platform recommendations on ``python`` and ``python2`` in shebangs
> increasingly unreliable.
>
>
> Current Rationale
> =================
>
> As of 2019, nearly all new systems include Python 3 and the ``python3``
> command. This makes the ``python3`` command the best general choice for
> code that can run on either Python 3.x or 2.x, even though it is not
> available everywhere.
>
> The recommendation is skewed toward current and future systems, leaving
> behind ?*old systems*? (like RHEL 6 or default Python installed on macOS).
> On these systems, Python software is rarely updated and any recommendations
> this PEP makes would likely be ignored.
>
> Also, since distributors often ignored recommendations the PEP gave
> regarding the ``python`` command (for what they saw as legitimate special
> needs), this PEP now gives them broad control over the command.
> Correspondingly, users are advised to not use the ``python`` command
> in cross-platform code.
> Instead, this PEP specifies the expected behavior of the ``python3`` and
> ``python2`` commands, which is not controversial.
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