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

[Petr Viktorin]

On 4/12/19 4:53 PM, Miro Hrončok 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.