PEP 394: Allow the `python` command to not be installed, and other minor edits (GH-630)
* Allow the `python` command to not be installed, and other minor edits - Distributions can omit the ``python`` command even when ``python2`` is installed in test environments, build systems, and other controlled environments where being explicit is valued more than user experience. - Users, by a deliberate action, change ``python`` to invoke Python 3. Activating a venv counts as such an action. - Other edits and clarifications * Add myself as an author
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pep-0394.txt
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pep-0394.txt
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@ -4,7 +4,8 @@ Version: $Revision$
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Last-Modified: $Date$
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Author: Kerrick Staley <mail@kerrickstaley.com>,
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Nick Coghlan <ncoghlan@gmail.com>,
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Barry Warsaw <barry@python.org>
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Barry Warsaw <barry@python.org>,
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Petr Viktorin <encukou@gmail.com>
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Status: Active
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Type: Informational
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Content-Type: text/x-rst
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@ -22,8 +23,9 @@ Python interpreter (i.e. the version invoked by the ``python`` command).
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* ``python2`` will refer to some version of Python 2.x.
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* ``python3`` will refer to some version of Python 3.x.
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* for the time being, all distributions *should* ensure that ``python``
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refers to the same target as ``python2``.
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* for the time being, all distributions *should* ensure that ``python``,
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if installed, refers to the same target as ``python2``, unless the user
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deliberately overrides this or a virtual environment is active.
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* however, end users should be aware that ``python`` refers to ``python3``
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on at least Arch Linux (that change is what prompted the creation of this
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PEP), so ``python`` should be used in the shebang line only for scripts
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@ -43,8 +45,7 @@ Recommendation
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* When invoked, ``python2`` should run some version of the Python 2
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interpreter, and ``python3`` should run some version of the Python 3
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interpreter.
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* The more general ``python`` command should be installed whenever
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any version of Python 2 is installed and should invoke the same version of
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* If the ``python`` command is installed, it should invoke the same version of
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Python as the ``python2`` command (however, note that some distributions
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have already chosen to have ``python`` implement the ``python3``
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command; see the `Rationale`_ and `Migration Notes`_ below).
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@ -62,14 +63,30 @@ Recommendation
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context.
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* One exception to this is scripts that are deliberately written to be source
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compatible with both Python 2.x and 3.x. Such scripts may continue to use
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``python`` on their shebang line without affecting their portability.
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``python`` on their shebang line.
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* When packaging software that is source compatible with both versions,
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distributions may change such ``python`` shebangs to ``python3``.
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This ensures software is used with the latest version of
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Python available, and it can remove a dependency on Python 2.
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* When reinvoking the interpreter from a Python script, querying
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``sys.executable`` to avoid hardcoded assumptions regarding the
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interpreter location remains the preferred approach.
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* In controlled environments aimed at expert users, where being explicit
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is valued over user experience (for example, in test environments and
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package build systems), distributions may choose to not provide the
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``python`` command even if ``python2`` is available.
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(All software in such a controlled environment must use ``python3`` or
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``python2`` rather than ``python``, which means scripts that deliberately
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use ``python`` need to be modified for such environments.)
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* When a virtual environment (created by the PEP 405 ``venv`` package or a
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similar tool) is active, the ``python`` command should refer to the
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virtual environment's interpreter. In other words, activating a virtual
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environment counts as deliberate user action to change the default
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``python`` interpreter.
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These recommendations are the outcome of the relevant python-dev discussions
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in March and July 2011 ([1]_, [2]_), February 2012 ([4]_) and
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September 2014 ([6]_).
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in March and July 2011 ([1]_, [2]_), February 2012 ([4]_),
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September 2014 ([6]_), and discussion on GitHub in April 2018 ([7]_).
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Rationale
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@ -91,11 +108,6 @@ on the part of distribution maintainers.
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Future Changes to this Recommendation
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=====================================
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It is anticipated that there will eventually come a time where the third
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party ecosystem surrounding Python 3 is sufficiently mature for this
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recommendation to be updated to suggest that the ``python`` symlink
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refer to ``python3`` rather than ``python2``.
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This recommendation will be periodically reviewed over the next few years,
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and updated when the core development team judges it appropriate. As a
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point of reference, regular maintenance releases for the Python 2.7 series
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@ -150,15 +162,13 @@ making such a change.
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* When the ``pythonX.X`` binaries are provided by a distribution, the
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``python2`` and ``python3`` commands should refer to one of those files
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rather than being provided as a separate binary file.
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* It is suggested that even distribution-specific packages follow the
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``python2``/``python3`` convention, even in code that is not intended to
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* It is strongly encouraged that distribution-specific packages use ``python2``
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or ``python3`` rather than ``python``, even in code that is not intended to
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operate on other distributions. This will reduce problems if the
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distribution later decides to change the version of the Python interpreter
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that the ``python`` command invokes, or if a sysadmin installs a custom
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``python`` command with a different major version than the distribution
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default. Distributions can test whether they are fully following this
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convention by changing the ``python`` interpreter on a test box and checking
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to see if anything breaks.
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default.
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* If the above point is adhered to and sysadmins are permitted to change the
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``python`` command, then the ``python`` command should always be implemented
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as a link to the interpreter binary (or a link to a link) and not vice
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@ -267,6 +277,10 @@ References
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.. [6] PEP 394 - Clarification of what "python" command should invoke
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(https://mail.python.org/pipermail/python-dev/2014-September/136374.html)
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.. [7] PEP 394: Allow the `python` command to not be installed, and other
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minor edits
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(https://github.com/python/peps/pull/630)
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Copyright
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===========
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This document has been placed in the public domain.
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