[Python-ideas] PEP 4XX: Adding sys.implementation

Fri Apr 27 08:36:26 CEST 2012

I've written up a PEP for the sys.implementation idea.  Feedback is welcome!

You'll notice some gaps which I'll be working on to fill in over the
next couple days.  Don't mind the gaps. <wink>  They are in less
critical (?) portions and I wanted to get this out to you before the
weekend.  Thanks!

-eric

--------------------------------------------------------------

PEP: 4XX
Title: Adding sys.implementation
Version: $Revision$
Last-Modified: $Date$
Author: Eric Snow <ericsnowcurrently at gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 26-April-2012
Python-Version: 3.3

Abstract
========

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``.

Motivation
==========

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.

Proposal
========

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`` is 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 they would need to be defined:

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
--------------

XXX same as sys.version_info?

Rationale
=========

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.

With the single-namespace-under-sys so straightforward, no alternatives
have been considered for this PEP.

Discussion
==========

The topic of ``sys.implementation`` came up on the python-ideas list in
2009, where the reception was broadly positive [1]_.  I revived the
discussion recently while working on a pure-python ``imp.get_tag()`` [2]_.
The messages in `issue 14673`_ are also relevant.

Use-cases
=========

``platform.python_implementation()``
------------------------------------

"explicit is better than implicit"

The platform module guesses the python implementation by looking for
clues in a couple different sys variables [3]_.  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 Lib/import.c any code that
did not need to be there.

The cache tag defined in Lib/import.c was hard-coded to
``"cpython" MAJOR MINOR`` [4]_.  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 how the implementation name is gotten, the version to use
for the cache tag is more likely to be the implementation version rather
than the language version.  That implementation version is not readily
identified anywhere in the standard library.

Implementation-Specific Tests
-----------------------------

XXX

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

Jython's ``os.name`` Hack
-------------------------

XXX

http://hg.python.org/cpython/file/2f563908ebc5/Lib/test/support.py#l512

Impact on CPython
=================

XXX

Feedback From Other Python Implementators
=========================================

IronPython
----------

XXX

Jython
------

XXX

PyPy
----

XXX

Past Efforts
============

XXX PEP 3139
XXX PEP 399

Open Issues
===========

* What are the long-term objectives for sys.implementation?

  - pull in implementation detail 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.

Implementation
==============

The implementatation of this PEP is covered in `issue 14673`_.

References
==========

.. [1]

   http://mail.python.org/pipermail/python-dev/2009-October/092893.html

.. [2]

   http://mail.python.org/pipermail/python-ideas/2012-April/014878.html

.. [3]

   http://hg.python.org/cpython/file/2f563908ebc5/Lib/platform.py#l1247

.. [4]

   http://hg.python.org/cpython/file/2f563908ebc5/Python/import.c#l121

.. _issue 14673

   http://bugs.python.org/issue14673

Copyright
=========

    This document has been placed in the public domain.

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