PEP: 291
Title: Backward Compatibility for the Python 2 Standard Library
Author: Neal Norwitz <nnorwitz@gmail.com>
Status: Superseded
Type: Informational
Created: 06-Jun-2002
Python-Version: 2.3
Post-History:
Superseded-By: 387


Abstract
========

This PEP describes the packages and modules in the Python 2
standard library which should remain backward compatible with
previous versions of Python.  If a package is not listed here,
then it need only remain compatible with the version of Python it
is distributed with.

This PEP has no bearing on the Python 3 standard library.


Rationale
=========

Authors have various reasons why packages and modules should
continue to work with previous versions of Python.  In order to
maintain backward compatibility for these modules while moving the
rest of the standard library forward, it is necessary to know
which modules can be modified and which should use old and
possibly deprecated features.

Generally, authors should attempt to keep changes backward
compatible with the previous released version of Python in order
to make bug fixes easier to backport.

In addition to a package or module being listed in this PEP,
authors must add a comment at the top of each file documenting
the compatibility requirement.

When a major version of Python is released, a Subversion branch is
created for continued maintenance and bug fix releases.  A package
version on a branch may have a different compatibility requirement
than the same package on the trunk (i.e. current bleeding-edge
development).  Where appropriate, these branch compatibilities are
listed below.


Features to Avoid
=================

The following list contains common features to avoid in order
to maintain backward compatibility with each version of Python.
This list is not complete!  It is only meant as a general guide.

Note that the features below were implemented in the version
following the one listed.  For example, features listed next to
1.5.2 were implemented in 2.0.

=======  ======================================================
Version  Features to Avoid
=======  ======================================================
1.5.2    string methods, Unicode, list comprehensions,
         augmented assignment (eg, +=), zip(), import x as y,
         dict.setdefault(), print >> f,
         calling f(\*args, \**kw), plus all features below

2.0      nested scopes, rich comparisons,
         function attributes, plus all features below

2.1      use of object or new-style classes, iterators,
         using generators, nested scopes, or //
         without from __future__ import ... statement,
         isinstance(X, TYP) where TYP is a tuple of types,
         plus all features below

2.2      bool, True, False, basestring, enumerate(),
         {}.pop(), PendingDeprecationWarning,
         Universal Newlines, plus all features below
         plus all features below

2.3      generator expressions, multi-line imports,
         decorators, int/long unification, set/frozenset,
         reversed(), sorted(), "".rsplit(),
         plus all features below

2.4      with statement, conditional expressions,
         combined try/except/finally, relative imports,
         yield expressions or generator.throw/send/close(),
         plus all features below

2.5      with statement without from __future__ import,
         io module, str.format(), except as,
         bytes, b'' literals, property.setter/deleter
=======  ======================================================


Backward Compatible Packages, Modules, and Tools
================================================

==============  ==================  ==============  =====
Package/Module  Maintainer(s)       Python Version  Notes
==============  ==================  ==============  =====
2to3            Benjamin Peterson   2.5
bsddb           - Greg Smith        2.1
                - Barry Warsaw
compiler        Jeremy Hylton       2.1
decimal         Raymond Hettinger   2.3             [2]
distutils       Tarek Ziade         2.3
email           Barry Warsaw        2.1 / 2.3       [1]
pkgutil         Phillip Eby         2.3
platform        Marc-Andre Lemburg  1.5.2
pybench         Marc-Andre Lemburg  1.5.2           [3]
sre             Fredrik Lundh       2.1
subprocess      Peter Astrand       2.2
wsgiref         Phillip J. Eby      2.1
xml (PyXML)     Martin v. Loewis    2.0
xmlrpclib       Fredrik Lundh       2.1
==============  ==================  ==============  =====


====  =============  ==============
Tool  Maintainer(s)  Python Version
====  =============  ==============
None
====  =============  ==============


Notes
-----

1. The email package version 2 was distributed with Python up to
   Python 2.3, and this must remain Python 2.1 compatible.  email
   package version 3 will be distributed with Python 2.4 and will
   need to remain compatible only with Python 2.3.

2. Specification updates will be treated as bugfixes and backported.
   Python 2.3 compatibility will be kept for at least Python 2.4.
   The decision will be revisited for Python 2.5 and not changed
   unless compelling advantages arise.

3. pybench lives under the Tools/ directory. Compatibility with
   older Python versions is needed in order to be able to compare
   performance between Python versions. New features may still
   be used in new tests, which may then be configured to fail
   gracefully on import by the tool in older Python versions.


Copyright
=========

This document has been placed in the public domain.