Here's an update to the PEP. Though I have indirect or old feedback already, I'd love to hear from the other main Python implementations, particularly regarding the version variable. Thanks.
PEP: 421 Title: Adding sys.implementation Version: $Revision$ Last-Modified: $Date$ Author: Eric Snow email@example.com Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 26-April-2012 Post-History: 26-April-2012
This PEP introduces a new variable for the ``sys`` module: ``sys.implementation``. The variable holds consolidated information about the implementation of the running interpreter. Thus ``sys.implementation`` is the source to which the standard library may look for implementation-specific information.
The proposal in this PEP is in line with a broader emphasis on making Python friendlier to alternate implementations. It describes the new variable and the constraints on what that variable contains. The PEP also explains some immediate use cases for ``sys.implementation``.
For a number of years now, the distinction between Python-the-language and CPython (the reference implementation) has been growing. Most of this change is due to the emergence of Jython, IronPython, and PyPy as viable alternate implementations of Python.
Consider, however, the nearly two decades of CPython-centric Python (i.e. most of its existance). That focus had understandably contributed to quite a few CPython-specific artifacts both in the standard library and exposed in the interpreter. Though the core developers have made an effort in recent years to address this, quite a few of the artifacts remain.
Part of the solution is presented in this PEP: a single namespace on which to consolidate implementation specifics. This will help focus efforts to differentiate the implementation specifics from the language. Additionally, it will foster a multiple-implementation mindset.
We will add ``sys.implementation``, in the ``sys`` module, as a namespace to contain implementation-specific information.
The contents of this namespace will remain fixed during interpreter execution and through the course of an implementation version. This ensures behaviors don't change between versions which depend on variables in ``sys.implementation``.
``sys.implementation`` will be a dictionary, as opposed to any form of "named" tuple (a la ``sys.version_info``). This is partly because it doesn't have meaning as a sequence, and partly because it's a potentially more variable data structure.
The namespace will contain at least the variables described in the `Required Variables`_ section below. However, implementations are free to add other implementation information there. Some possible extra variables are described in the `Other Possible Variables`_ section.
This proposal takes a conservative approach in requiring only two variables. As more become appropriate, they may be added with discretion.
Required Variables --------------------
These are variables in ``sys.implementation`` on which the standard library would rely, meaning implementors must define them:
name the name of the implementation (case sensitive).
version the version of the implementation, as opposed to the version of the language it implements. This would use a standard format, similar to ``sys.version_info`` (see `Version Format`_).
Other Possible Variables ------------------------
These variables could be useful, but don't necessarily have a clear use case presently:
cache_tag a string used for the PEP 3147 cache tag (e.g. 'cpython33' for CPython 3.3). The name and version from above could be used to compose this, though an implementation may want something else. However, module caching is not a requirement of implementations, nor is the use of cache tags.
repository the implementation's repository URL.
repository_revision the revision identifier for the implementation.
build_toolchain identifies the tools used to build the interpreter.
url (or website) the URL of the implementation's site.
site_prefix the preferred site prefix for this implementation.
runtime the run-time environment in which the interpreter is running.
gc_type the type of garbage collection used.
Version Format --------------
A main point of ``sys.implementation`` is to contain information that will be used in the standard library. In order to facilitate the usefulness of a version variable, its value should be in a consistent format across implementations.
XXX Subject to feedback
As such, the format of ``sys.implementation['version']`` must follow that of ``sys.version_info``, which is effectively a named tuple. It is a familiar format and generally consistent with normal version format conventions.
The status quo for implementation-specific information gives us that information in a more fragile, harder to maintain way. It's spread out over different modules or inferred from other information, as we see with ``platform.python_implementation()``.
This PEP is the main alternative to that approach. It consolidates the implementation-specific information into a single namespace and makes explicit that which was implicit. The ``sys`` module should old the new namespace because ``sys`` is the depot for interpreter-centric variables and functions.
With the single-namespace-under-sys so straightforward, no alternatives have been considered for this PEP.
The topic of ``sys.implementation`` came up on the python-ideas list in 2009, where the reception was broadly positive _. I revived the discussion recently while working on a pure-python ``imp.get_tag()`` _. The messages in `issue #14673`_ are also relevant.
"explicit is better than implicit"
The ``platform`` module guesses the python implementation by looking for clues in a couple different ``sys`` variables _. However, this approach is fragile. Beyond that, it's limited to those implementations that core developers have blessed by special-casing them in the ``platform`` module.
With ``sys.implementation`` the various implementations would *explicitly* set the values in their own version of the ``sys`` module.
Aside from the guessing, another concern is that the ``platform`` module is part of the stdlib, which ideally would minimize implementation details such as would be moved to ``sys.implementation``.
Any overlap between ``sys.implementation`` and the ``platform`` module would simply defer to ``sys.implementation`` (with the same interface in ``platform`` wrapping it).
Cache Tag Generation in Frozen Importlib ----------------------------------------
PEP 3147 defined the use of a module cache and cache tags for file names. The importlib bootstrap code, frozen into the Python binary as of 3.3, uses the cache tags during the import process. Part of the project to bootstrap importlib has been to clean out of `Python/import.c` any code that did not need to be there.
The cache tag defined in `Python/import.c` was hard-coded to ``"cpython" MAJOR MINOR`` _. For importlib the options are either hard-coding it in the same way, or guessing the implementation in the same way as does ``platform.python_implementation()``.
As long as the hard-coded tag is limited to CPython-specific code, it's livable. However, inasmuch as other Python implementations use the importlib code to work with the module cache, a hard-coded tag would become a problem..
Directly using the ``platform`` module in this case is a non-starter. Any module used in the importlib bootstrap must be built-in or frozen, neither of which apply to the ``platform`` module. This is the point that led to the recent interest in ``sys.implementation``.
Regardless of the outcome for the implementation name used, another problem relates to the version used in the cache tag. That version is likely to be the implementation version rather than the language version. However, the implementation version is not readily identified anywhere in the standard library.
Implementation-Specific Tests -----------------------------
Currently there are a number of implementation-specific tests in the test suite under ``Lib/test``. The test support module (`Lib/test/support.py`_) provides some functionality for dealing with these tests. However, like the ``platform`` module, ``test.support`` must do some guessing that ``sys.implementation`` would render unnecessary.
Jython's ``os.name`` Hack -------------------------
Feedback From Other Python Implementators =========================================
Past Efforts ============
PEP 3139 --------
This PEP from 2008 recommended a clean-up of the ``sys`` module in part by extracting implementation-specific variables and functions into a separate module. PEP 421 is a much lighter version of that idea. While PEP 3139 was rejected, its goals are reflected in PEP 421 to a large extent, though with a much lighter approach.
PEP 399 -------
This informational PEP dictates policy regarding the standard library, helping to make it friendlier to alternate implementations. PEP 421 is proposed in that same spirit.
Open Issues ===========
* What are the long-term objectives for ``sys.implementation``?
- possibly pull in implementation details from the main ``sys`` namespace and elsewhere (PEP 3137 lite).
* Alternatives to the approach dictated by this PEP?
* ``sys.implementation`` as a proper namespace rather than a dict. It would be it's own module or an instance of a concrete class.
The implementatation of this PEP is covered in `issue #14673`_.
..  Examples of implementation-specific handling in test.support:
| http://hg.python.org/cpython/file/2f563908ebc5/Lib/test/support.py#l509 | http://hg.python.org/cpython/file/2f563908ebc5/Lib/test/support.py#l1246 | http://hg.python.org/cpython/file/2f563908ebc5/Lib/test/support.py#l1252 | http://hg.python.org/cpython/file/2f563908ebc5/Lib/test/support.py#l1275
.. _issue #14673: http://bugs.python.org/issue14673
.. _Lib/test/support.py: http://hg.python.org/cpython/file/2f563908ebc5/Lib/test/support.py
This document has been placed in the public domain.