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Python Enhancement Proposals

PEP 291 – Backward Compatibility for the Python 2 Standard Library

Neal Norwitz <nnorwitz at>

Table of Contents


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.


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
  • Greg Smith
  • 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


  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.


Last modified: 2023-09-09 17:39:29 GMT