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
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
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(),
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
compiler Jeremy Hylton 2.1
decimal Raymond Hettinger 2.3 
email Barry Warsaw 2.1 / 2.3 
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
---- ------------- --------------
 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.
 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.