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PEP 394: Allow the python command to not be installed, and other minor edits #630

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Merged
encukou merged 4 commits into from
Apr 28, 2018
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PEP 394: Allow the python command to not be installed, and other minor edits #630

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PEP 394: Allow changing the ``python`` command in some cases
encukou Apr 25, 2018
43ce426
Address Guido's review comments
encukou Apr 26, 2018
3dee5df
Change the Pull Request name
encukou Apr 27, 2018
67f943b
Add myself as an author
encukou Apr 28, 2018
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Address Guido's review comments
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@encukou
encukou committed Apr 26, 2018
commit 43ce4266bc1c0bb71b61038d0a9b65bbf710ddfd
47 changes: 12 additions & 35 deletions pep-0394.txt
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Expand Up @@ -23,8 +23,8 @@ Python interpreter (i.e. the version invoked by the ``python`` command).
* ``python2`` will refer to some version of Python 2.x.
* ``python3`` will refer to some version of Python 3.x.
* for the time being, all distributions *should* ensure that ``python``,
if installed, refers to the same target as ``python2``, unless the system
administrator or user deliberately override this.
if installed, refers to the same target as ``python2``, unless the user
deliberately overrides this or a virtual environment is active.
* however, end users should be aware that ``python`` refers to ``python3``
on at least Arch Linux (that change is what prompted the creation of this
PEP), so ``python`` should be used in the shebang line only for scripts
Expand All @@ -47,11 +47,7 @@ Recommendation
* If the ``python`` command is installed, it should invoke the same version of
Python as the ``python2`` command (however, note that some distributions
have already chosen to have ``python`` implement the ``python3``
command, and system administrators may be allowed to change the default;
see further recommendations and the `Rationale`_ and `Migration Notes`_
below).
* The ``python`` command should be available whenever ``python2`` is available,
except in controlled environments as discussed below.
command; see the `Rationale`_ and `Migration Notes`_ below).
* The Python 2.x ``idle``, ``pydoc``, and ``python-config`` commands should
likewise be available as ``idle2``, ``pydoc2``, and ``python2-config``,
with the original commands invoking these versions by default, but possibly
Expand All @@ -68,8 +64,8 @@ Recommendation
compatible with both Python 2.x and 3.x. Such scripts may continue to use
``python`` on their shebang line.
* When packaging software that is source compatible with both versions,
distributions may change such ``python`` shebangs to ``python3`` (or
``python2``). This ensures software is used with the latest version of
distributions may change such ``python`` shebangs to ``python3``.
This ensures software is used with the latest version of
Python available, and it can remove a dependency on Python 2.

@gvanrossum gvanrossum Apr 25, 2018

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(Then why the "or python2" ?)

* When reinvoking the interpreter from a Python script, querying
``sys.executable`` to avoid hardcoded assumptions regarding the
Expand All @@ -81,11 +77,6 @@ Recommendation
(All software in such a controlled environment must use ``python3`` or
``python2`` rather than ``python``, which means scripts that deliberately
use ``python`` need to be modified for such environments.)
* System administrators and users may be empowered to change the meaning of
the ``python`` command to ``python3``. This should be a deliberate action
by the administrator. Related documentation should include a discussion
of the effects -- for example, a link to the "Migration Notes" section of
this PEP.
* When a virtual environment (created by the PEP 405 ``venv`` package or a

@gvanrossum gvanrossum Apr 25, 2018

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Yes on this bullet.

similar tool) is active, the ``python`` command should refer to the
virtual environment's interpreter. In other words, activating a virtual
Expand All @@ -94,7 +85,7 @@ Recommendation

These recommendations are the outcome of the relevant python-dev discussions
in March and July 2011 ([1]_, [2]_), February 2012 ([4]_),
September 2014 ([6]_), and April 2018 (XXX).
September 2014 ([6]_), and discussion on GitHub in April 2018 ([7]_).


Rationale
Expand All @@ -116,11 +107,6 @@ on the part of distribution maintainers.
Future Changes to this Recommendation
=====================================

It is anticipated that there will eventually come a time where the third
party ecosystem surrounding Python 2 becomes irrelevant in most use cases,
at which point this recommendation should be updated to suggest that the
``python`` symlink refer to ``python3`` rather than ``python2``.

This recommendation will be periodically reviewed over the next few years,
and updated when the core development team judges it appropriate. As a
point of reference, regular maintenance releases for the Python 2.7 series
Expand Down Expand Up @@ -175,15 +161,13 @@ making such a change.
* When the ``pythonX.X`` binaries are provided by a distribution, the
``python2`` and ``python3`` commands should refer to one of those files
rather than being provided as a separate binary file.
* It is suggested that distribution-specific packages use ``python2`` or
``python3`` rather than ``python``, even in code that is not intended to
* It is strongly encouraged that distribution-specific packages use ``python2``
or ``python3`` rather than ``python``, even in code that is not intended to
operate on other distributions. This will reduce problems if the
distribution later decides to change the version of the Python interpreter
that the ``python`` command invokes, or if a sysadmin installs a custom
``python`` command with a different major version than the distribution
default. Distributions can test whether they are fully following this
convention by changing or removing the ``python`` command on a test box
and checking to see if anything breaks.
default.
* If the above point is adhered to and sysadmins are permitted to change the
``python`` command, then the ``python`` command should always be implemented
as a link to the interpreter binary (or a link to a link) and not vice
Expand Down Expand Up @@ -212,16 +196,6 @@ attempting to execute a script containing Python 2 specific syntax with a
Python 3 interpreter.


Forward Compatibility
=====================

A problem can arise if a script compatible with both Python 2 and 3, which
has ``python`` in its shebang line, is invoked on a system that does not have
Python 2 (and thus, the ``python`` command) installed.
This is mostly a non-issue -- as with any other case of a required interpreter
not being installed, the sysadmin can install the ``python`` command.


Application to the CPython Reference Interpreter
================================================

Expand Down Expand Up @@ -302,6 +276,9 @@ References
.. [6] PEP 394 - Clarification of what "python" command should invoke
(https://mail.python.org/pipermail/python-dev/2014-September/136374.html)

.. [7] PEP 394: Allow changing the `python` command in some cases

@gvanrossum gvanrossum Apr 27, 2018

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I suggest changing the PR title to match the contents.

(https://github.com/python/peps/pull/630)

Copyright
===========
This document has been placed in the public domain.