PEP: 291
Title: Backward Compatibility for Standard Library
Version: $Revision: 41645 $
Last-Modified: $Date: 2005-12-11 15:33:53 -0800 (Sun, 11 Dec 2005) $
Author: Neal Norwitz <neal at>
Status: Active
Type: Informational
Created: 06-Jun-2002
Python-Version: 2.3


    This PEP describes the packages and modules in the 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.


    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 CVS 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,
                   plus all features below

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

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

Backward Compatible Packages, Modules, and Tools

    Package/Module     Maintainer(s)          Python Version     Notes
    --------------     -------------          --------------     -----
    bsddb              Greg Smith                  2.1
                       Barry Warsaw
    compiler           Jeremy Hylton               2.1
    decimal            Raymond Hettinger           2.3           [2]
    email              Barry Warsaw                2.1 / 2.3     [1]
    logging            Vinay Sajip                 1.5.2
    sre                Fredrik Lundh               2.1
    subprocess         Peter Astrand               2.2
    xml (PyXML)        Martin v. Loewis            2.0
    xmlrpclib          Fredrik Lundh               2.1
    modulefinder       Thomas Heller               2.2
                       Just van Rossum
    platform           Marc-Andre Lemburg          1.5.2

    Tool                         Maintainer(s)   Python Version
    ----                         -------------   --------------


    [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.


    This document has been placed in the public domain.