PEP: 394 Title: The "python" Command on Unix-Like Systems Author: Kerrick Staley , Alyssa Coghlan , Barry Warsaw , Petr Viktorin , Miro Hrončok , Carol Willing , Status: Active Type: Informational Created: 02-Mar-2011 Post-History: 04-Mar-2011, 20-Jul-2011, 16-Feb-2012, 30-Sep-2014, 28-Apr-2018, 26-Jun-2019 Resolution: https://mail.python.org/pipermail/python-dev/2012-February/116594.html Abstract ======== This PEP outlines the behavior of Python scripts when the ``python`` command is invoked. Depending on a distribution or system configuration, ``python`` may or may not be installed. If ``python`` is installed its target interpreter may refer to ``python2`` or ``python3``. End users may be unaware of this inconsistency across Unix-like systems. This PEP's goal is to reduce user confusion about what ``python`` references and what will be the script's behavior. The recommendations in the next section of this PEP will outline the behavior when: * using virtual environments * writing cross-platform scripts with shebangs for either ``python2`` or ``python3`` The PEP's goal is to clarify the behavior for script end users, distribution providers, and script maintainers / authors. Recommendation ============== Our recommendations are detailed below. We call out any expectations that these recommendations are based upon. For Python runtime distributors ------------------------------- * We expect Unix-like software distributions (including systems like macOS and Cygwin) to install the ``python2`` command into the default path whenever a version of the Python 2 interpreter is installed, and the same for ``python3`` and the Python 3 interpreter. * When invoked, ``python2`` should run some version of the Python 2 interpreter, and ``python3`` should run some version of the Python 3 interpreter. * If the ``python`` command is installed, it is expected to invoke either the same version of Python as the ``python3`` command or as the ``python2`` command. * Distributors may choose to set the behavior of the ``python`` command as follows: * ``python2``, * ``python3``, * not provide ``python`` command, allow ``python`` to be configurable by an end user or a system administrator. * The Python 3.x ``idle``, ``pydoc``, and ``python-config`` commands should likewise be available as ``idle3``, ``pydoc3``, and ``python3-config``; Python 2.x versions as ``idle2``, ``pydoc2``, and ``python2-config``. The commands with no version number should either invoke the same version of Python as the ``python`` command, or not be available at all. * When packaging third party Python scripts, distributors are encouraged to change less specific shebangs to more specific ones. This ensures software is used with the latest version of Python available, and it can remove a dependency on Python 2. The details on what specifics to set are left to the distributors; though. Example specifics could include: * Changing ``python`` shebangs to ``python3`` when Python 3.x is supported. * Changing ``python`` shebangs to ``python2`` when Python 3.x is not yet supported. * Changing ``python3`` shebangs to ``python3.8`` if the software is built with Python 3.8. * When a virtual environment (created by the :pep:`405` ``venv`` package or a similar tool such as ``virtualenv`` or ``conda``) is active, the ``python`` command should refer to the virtual environment's interpreter and should always be available. The ``python3`` or ``python2`` command (according to the environment's interpreter version) should also be available. For Python script publishers ---------------------------- * When reinvoking the interpreter from a Python script, querying ``sys.executable`` to avoid hardcoded assumptions regarding the interpreter location remains the preferred approach. * Encourage your end users to use a virtual environment. This makes the user's environment more predictable (possibly resulting in fewer issues), and helps avoid disrupting their system. * For scripts that are only expected to be run in an activated virtual environment, shebang lines can be written as ``#!/usr/bin/env python``, as this instructs the script to respect the active virtual environment. * In cases where the script is expected to be executed outside virtual environments, developers will need to be aware of the following discrepancies across platforms and installation methods: * Older Linux distributions will provide a ``python`` command that refers to Python 2, and will likely not provide a ``python2`` command. * Some newer Linux distributions will provide a ``python`` command that refers to Python 3. * Some Linux distributions will not provide a ``python`` command at all by default, but will provide a ``python3`` command by default. * When potentially targeting these environments, developers may either use a Python package installation tool that rewrites shebang lines for the installed environment, provide instructions on updating shebang lines interactively, or else use more specific shebang lines that are tailored to the target environment. * Scripts targeting both “*old systems*” and systems without the default ``python`` command need to make a compromise and document this situation. Avoiding shebangs (via the console_scripts Entry Points ([9]_) or similar means) is the recommended workaround for this problem. * Applications designed exclusively for a specific environment (such as a container or virtual environment) may continue to use the ``python`` command name. For end users of Python ----------------------- * While far from being universally available, ``python`` remains the preferred spelling for explicitly invoking Python, as this is the spelling that virtual environments make consistently available across different platforms and Python installations. * For software that is not distributed with (or developed for) your system, we recommend using a virtual environment, possibly with an environment manager like ``conda`` or ``pipenv``, to help avoid disrupting your system Python installation. These recommendations are the outcome of the relevant python-dev discussions in March and July 2011 ([1]_, [2]_), February 2012 ([4]_), September 2014 ([6]_), discussion on GitHub in April 2018 ([7]_), on python-dev in February 2019 ([8]_), and during the PEP update review in May/June 2019 ([10]_). 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 it entirely. This means either removing the ``python`` command or switching it to invoke Python 3. Some distributors also decided that their users were better served by ignoring the PEP's original recommendations, and provided system administrators with the freedom to configure their systems based on the needs of their particular environment. Current Rationale ================= As of 2019, activating a Python virtual environment (or its functional equivalent) prior to script execution is one way to obtain a consistent cross-platform and cross-distribution experience. Accordingly, publishers can expect users of the software to provide a suitable execution environment. Future Changes to this Recommendation ===================================== 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 will continue until January 2020. Migration Notes =============== This section does not contain any official recommendations from the core CPython developers. It's merely a collection of notes regarding various aspects of migrating to Python 3 as the default version of Python for a system. They will hopefully be helpful to any distributions considering making such a change. * The main barrier to a distribution switching the ``python`` command from ``python2`` to ``python3`` isn't breakage within the distribution, but instead breakage of private third party scripts developed by sysadmins and other users. Updating the ``python`` command to invoke ``python3`` by default indicates that a distribution is willing to break such scripts with errors that are potentially quite confusing for users that aren't familiar with the backwards incompatible changes in Python 3. For example, while the change of ``print`` from a statement to a builtin function is relatively simple for automated converters to handle, the SyntaxError from attempting to use the Python 2 notation in Python 3 may be confusing for users that are not aware of the change: .. code-block:: pytb $ python3 -c 'print "Hello, world!"' File "", line 1 print "Hello, world!" ^ SyntaxError: Missing parentheses in call to 'print'. Did you mean print("Hello, world!")? While this might be obvious for experienced Pythonistas, such scripts might even be run by people who are not familiar with Python at all. Avoiding breakage of such third party scripts was the key reason this PEP used to recommend that ``python`` continue to refer to ``python2``. * The error message ``python: command not found`` tends to be surprisingly actionable, even for people unfamiliar with Python. * The ``pythonX.X`` (e.g. ``python3.6``) commands exist on modern systems, on which they invoke specific minor versions of the Python interpreter. It can be useful for distribution-specific packages to take advantage of these utilities if they exist, since it will prevent code breakage if the default minor version of a given major version is changed. However, scripts intending to be cross-platform should not rely on the presence of these utilities, but rather should be tested on several recent minor versions of the target major version, compensating, if necessary, for the small differences that exist between minor versions. This prevents the need for sysadmins to install many very similar versions of the interpreter. * 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 strongly encouraged that distribution-specific packages use ``python3`` (or ``python2``) 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. * 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 versa. That way, if a sysadmin does decide to replace the installed ``python`` file, they can do so without inadvertently deleting the previously installed binary. * Even as the Python 2 interpreter becomes less common, it remains reasonable for scripts to continue to use the ``python3`` convention, rather than just ``python``. * If these conventions are adhered to, it will become the case that the ``python`` command is only executed in an interactive manner as a user convenience, or else when using a virtual environment or similar mechanism. Backwards Compatibility ======================= A potential problem can arise if a script adhering to the ``python2``/``python3`` convention is executed on a system not supporting these commands. This is mostly a non-issue, since the sysadmin can simply create these symbolic links and avoid further problems. It is a significantly more obvious breakage than the sometimes cryptic errors that can arise when attempting to execute a script containing Python 2 specific syntax with a Python 3 interpreter or vice versa. Application to the CPython Reference Interpreter ================================================ While technically a new feature, the ``make install`` and ``make bininstall`` command in the 2.7 version of CPython were adjusted to create the following chains of symbolic links in the relevant ``bin`` directory (the final item listed in the chain is the actual installed binary, preceding items are relative symbolic links): .. code-block:: text python -> python2 -> python2.7 python-config -> python2-config -> python2.7-config Similar adjustments were made to the macOS binary installer. This feature first appeared in the default installation process in CPython 2.7.3. The installation commands in the CPython 3.x series already create the appropriate symlinks. For example, CPython 3.2 creates: .. code-block:: text python3 -> python3.2 idle3 -> idle3.2 pydoc3 -> pydoc3.2 python3-config -> python3.2-config And CPython 3.3 creates: .. code-block:: text python3 -> python3.3 idle3 -> idle3.3 pydoc3 -> pydoc3.3 python3-config -> python3.3-config pysetup3 -> pysetup3.3 The implementation progress of these features in the default installers was managed on the tracker as issue #12627 ([3]_). Impact on PYTHON* Environment Variables ======================================= The choice of target for the ``python`` command implicitly affects a distribution's expected interpretation of the various Python related environment variables. The use of ``*.pth`` files in the relevant ``site-packages`` folder, the "per-user site packages" feature (see ``python -m site``) or more flexible tools such as ``virtualenv`` are all more tolerant of the presence of multiple versions of Python on a system than the direct use of ``PYTHONPATH``. Exclusion of MS Windows ======================= This PEP deliberately excludes any proposals relating to Microsoft Windows, as devising an equivalent solution for Windows was deemed too complex to handle here. :pep:`397` and the related discussion on the python-dev mailing list address this issue. References ========== .. [1] Support the /usr/bin/python2 symlink upstream (with bonus grammar class!) (https://mail.python.org/pipermail/python-dev/2011-March/108491.html) .. [2] Rebooting PEP 394 (aka Support the /usr/bin/python2 symlink upstream) (https://mail.python.org/pipermail/python-dev/2011-July/112322.html) .. [3] Implement PEP 394 in the CPython Makefile (https://github.com/python/cpython/issues/56836) .. [4] PEP 394 request for pronouncement (python2 symlink in \*nix systems) (https://mail.python.org/pipermail/python-dev/2012-February/116435.html) .. [5] Arch Linux announcement that their "python" link now refers Python 3 (https://www.archlinux.org/news/python-is-now-python-3/) .. [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 the ``python`` command to not be installed, and other minor edits (https://github.com/python/peps/pull/630) .. [8] Another update for PEP 394 -- The "python" Command on Unix-Like Systems (https://mail.python.org/pipermail/python-dev/2019-February/156272.html) .. [9] The console_scripts Entry Point (https://python-packaging.readthedocs.io/en/latest/command-line-scripts.html#the-console-scripts-entry-point) .. [10] May 2019 PEP update review (https://github.com/python/peps/pull/989) Copyright =========== This document has been placed in the public domain.