PEP 394 – The “python” Command on Unix-Like Systems
- Kerrick Staley <mail at kerrickstaley.com>, Nick Coghlan <ncoghlan at gmail.com>, Barry Warsaw <barry at python.org>, Petr Viktorin <encukou at gmail.com>, Miro Hrončok <miro at hroncok.cz>, Carol Willing <willingc at gmail.com>
- 04-Mar-2011, 20-Jul-2011, 16-Feb-2012, 30-Sep-2014, 28-Apr-2018, 26-Jun-2019
- Python-Dev message
Table of Contents
- History of this PEP
- Current Rationale
- Future Changes to this Recommendation
- Migration Notes
- Backwards Compatibility
- Application to the CPython Reference Interpreter
- Impact on PYTHON* Environment Variables
- Exclusion of MS Windows
This PEP outlines the behavior of Python scripts when the
Depending on a distribution or system configuration,
python may or may not be installed.
python is installed its target interpreter may refer to
End users may be unaware of this inconsistency across Unix-like systems.
This PEP’s goal is to reduce user confusion about what
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
The PEP’s goal is to clarify the behavior for script end users, distribution providers, and script maintainers / authors.
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
python2command into the default path whenever a version of the Python 2 interpreter is installed, and the same for
python3and the Python 3 interpreter.
- When invoked,
python2should run some version of the Python 2 interpreter, and
python3should run some version of the Python 3 interpreter.
- If the
pythoncommand is installed, it is expected to invoke either the same version of Python as the
python3command or as the
- Distributors may choose to set the behavior of the
pythoncommand as follows:
- not provide
pythonto be configurable by an end user or a system administrator.
- The Python 3.x
python-configcommands should likewise be available as
python3-config; Python 2.x versions as
python2-config. The commands with no version number should either invoke the same version of Python as the
pythoncommand, 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:
python3when Python 3.x is supported.
python2when Python 3.x is not yet supported.
python3.8if the software is built with Python 3.8.
- When a virtual environment (created by the PEP 405
venvpackage or a similar tool such as
conda) is active, the
pythoncommand should refer to the virtual environment’s interpreter and should always be available. The
python2command (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.executableto 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
pythoncommand that refers to Python 2, and will likely not provide a
- Some newer Linux distributions will provide a
pythoncommand that refers to Python 3.
- Some Linux distributions will not provide a
pythoncommand at all by default, but will provide a
python3command by default.
- Older Linux distributions will provide a
- 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
pythoncommand need to make a compromise and document this situation. Avoiding shebangs (via the console_scripts Entry Points () 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
For end users of Python
- While far from being universally available,
pythonremains 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
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 (, ), February 2012 (), September 2014 (), discussion on GitHub in April 2018 (), on python-dev in February 2019 (), and during the PEP update review in May/June 2019 ().
History of this PEP
In 2011, the majority of distributions
python command to Python 2, but some started switching it to
Python 3 (). 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:
pythoncommand 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).
pythoncommand should always invoke Python 2 (to prevent hard-to-diagnose errors when Python 2 code is run on Python 3).
python3commands 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.
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.
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
python3isn’t breakage within the distribution, but instead breakage of private third party scripts developed by sysadmins and other users. Updating the
pythoncommand to invoke
python3by 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
$ python3 -c 'print "Hello, world!"' File "<string>", 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
pythoncontinue to refer to
- The error message
python: command not foundtends to be surprisingly actionable, even for people unfamiliar with Python.
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.Xbinaries are provided by a distribution, the
python3commands 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
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
pythoncommand invokes, or if a sysadmin installs a custom
pythoncommand with a different major version than the distribution default.
- If the above point is adhered to and sysadmins are permitted to change the
pythoncommand, then the
pythoncommand 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
pythonfile, 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
python3convention, rather than just
- If these conventions are adhered to, it will become the case that the
pythoncommand is only executed in an interactive manner as a user convenience, or else when using a virtual environment or similar mechanism.
A potential problem can arise if a script adhering to the
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
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):
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:
python3 -> python3.2 idle3 -> idle3.2 pydoc3 -> pydoc3.2 python3-config -> python3.2-config
And CPython 3.3 creates:
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 ().
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
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.
This document has been placed in the public domain.
Last modified: 2022-01-21 11:03:51 GMT