Update symlink naming PEP based on python-dev thread responses

pep-0394.txt

@@ -20,28 +20,42 @@ 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
-* ``python`` may refer to either, depending on distribution and system
+* ``python`` will refer to the same target as either ``python2`` or
+  ``python3``, depending on the specific distribution and system
 
 
 Recommendation
 ==============
 
-* ``*nix`` software distributions should 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. The
-  same applies for the more general ``python`` command, which should be
-  installed whenever any version of Python is installed and should invoke
-  some Python interpreter.
-* All new code that needs to invoke the Python interpreter should not specify
-  ``python``, but rather should specify either ``python2`` or ``python3`` (or
-  the more specific ``python2.x`` and ``python3.x`` versions; see the Notes).
+* Unix-like software distributions (including systems like Mac OS X and
+  Cygwin) should 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.
+* Similarly, the more general ``python`` command should be installed whenever
+  any version of Python is installed and should invoke the same version of
+  Python as either ``python2`` or ``python3``.
+* For the time being, it is recommended that ``python`` should refer to
+  ``python2``, except on distributions which include only ``python3`` in their
+  base install, or those that wish to push strongly for migration of user
+  scripts to Python 3.
+* In order to tolerate differences across platforms, all new code that needs
+  to invoke the Python interpreter should not specify ``python``, but rather
+  should specify either ``python2`` or ``python3`` (or the more specific
+  ``python2.x`` and ``python3.x`` versions; see the Notes).
   This distinction should be made in shebangs, when invoking from a shell
   script, when invoking via the system() call, or when invoking in any other
-  context. Note that, when reinvoking the interpreter from a Python script,
-  querying ``sys.executable`` remains the preferred approach.
+  context.
+* When reinvoking the interpreter from a Python script, querying
+  ``sys.executable`` to avoid hardcoded assumptions regarding the
+  interpreter location remains the preferred approach.
 
+These recommendations are the outcome of the relevant python-dev discussion in
+March 2011 [1] (NOTE: More accurately, they will be such once that "Draft"
+status disappears from the PEP header, it has been moved into the "Other
+Informational PEP" section in PEP 0 and the note has been deleted)
 
 Rationale
 =========
@@ -50,11 +64,12 @@ This is needed as, even though the majority of distributions still alias the
 ``python`` command to Python 2, some now alias it to Python 3. Some of
 the former also do not provide a ``python2`` command; hence, there is
 currently no way for Python 2 code (or any code that invokes the Python 2
-interpreter) to reliably run on all systems without modification, because both
-the ``python`` and the ``python2`` commands will fail on some systems. The
-recommendations in this PEP provide a very simple mechanism to restore
-cross-platform support, with minimal additional work required on the part
-of distribution maintainers.
+interpreter directly rather than via ``sys.executable``) to reliably run on
+all systems without modification, as the ``python`` command will invoke the
+wrong interpreter version on some systems, and the ``python2`` command will
+fail completely on others. The recommendations in this PEP provide a very
+simple mechanism to restore cross-platform support, with minimal additional
+work required on the part of distribution maintainers.
 
 
 Notes
@@ -62,8 +77,10 @@ Notes
 
 * Distributions can alias the ``python`` command to whichever version of the
   Python interpreter they choose (noting that, in the near term, most 3rd
-  party scripts will still expect this command to refer to Python 2.x).
-* The ``pythonX.X`` (e.g. ``python2.6``) utilities exist on some systems, on
+  party scripts will still expect this command to refer to Python 2.x). The
+  version chosen should also be available via either the ``python2`` or
+  ``python3`` command as appropriate.
+* The ``pythonX.X`` (e.g. ``python2.6``) commands exist on some systems, on
   which they invoke specific minor versions of the Python interpreter. It
   would be wise for distribution-specific packages to take advantage of these
   utilities if they exist, since it will prevent code breakage if the default
@@ -73,10 +90,13 @@ Notes
   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.
-* It would be wise for distribution-specific packages to always follow the
+* When the ``pythonX.X`` binaries are provided by a distribution, the
+  ``python2`` and ``python3`` commands should refer to one of those files
+  rather being provided as a separate binary file.
+* It is suggested that even distribution-specific packages follow the
   ``python2``/``python3`` convention, even in code that is not intended to
   operate on other distributions. This will prevent problems if the
-  distribution later decides to upgrade the version of the Python interpreter
+  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
@@ -88,10 +108,10 @@ Notes
   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.
-* The first recommendation can be ignored for systems on which the ``python``
-  command itself has traditionally been left undefined and users have always
-  had the responsibility of linking the ``python`` command to the Python
-  interpreter.
+* As an alternative to the recommendation presented above, some distributions
+  may choose to leave the ``python`` command itself undefined, leaving
+  sysadmins and users with the responsibility to choose their own preferred
+  version to be made available as the ``python`` command.
 * If the Python 2 interpreter becomes uncommon, scripts should nevertheless
   continue to use the ``python3`` convention rather that just ``python``. This
   will ease transition in the event that yet another major version of Python
@@ -108,17 +128,58 @@ A potential problem can arise if a script adhering to the
 these commands. This is mostly a non-issue, since the sysadmin can simply
 create these symbolic links and avoid further problems.
 
+
 Application to the CPython Reference Interpreter
 ================================================
 
-While technically a new feature, the ``make install`` command of the 2.7
-version of CPython will be adjusted to create the ``python2`` symlink in
-addition to the existing ``python`` symlink. This feature will first appear in
-CPython 2.7.2.
+While technically a new feature, the ``make install`` command and the Mac OS
+X installer in the 2.7 version of CPython will be adjusted to create the
+new ``python2`` command in addition to the existing ``python`` and
+``python2.7`` commands. This feature will first appear in CPython 2.7.2.
 
 The ``make install`` command in the CPython 3.x series will continue to
 install only the ``python3`` symlink for the foreseeable future.
 
+
+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``.
+
+
+Exclusions of MS Windows
+========================
+
+This PEP deliberately excludes any proposals relating to Microsoft Windows.
+The use of parallel installs on Windows suffers from numerous issues,
+including the "last installed wins" behaviour for handling of file
+associations, a lack of universal robust symlink support for easy aliasing of
+commands, the fact that the Python executable is not available on ``PATH`` by
+default, the fact that the ``python.exe`` and ``pythonw.exe`` names are
+used for both Python 2 and Python 3 binaries and the lack of distinction
+between the different Python versions when the Start menu shortcuts are
+divorced from their containing folders (e.g. when they appear in the
+"Recently Used" list.
+
+While these are questions well worth addressing, they do not have easy
+answers. The authors of this particular PEP aren't even inclined to try to
+begin answering them, but anyone that wants to tackle them should feel free
+to start working on their own PEP :)
+
+
+References
+==========
+
+[1] Support the /usr/bin/python2 symlink upstream (with bonus grammar class!)
+    (http://mail.python.org/pipermail/python-dev/2011-March/108491.html)
+
+
 Copyright
 ===========
 This document has been placed in the public domain.