[Python-Dev] draft pep: backwards compatibility

Benjamin Peterson benjamin at python.org
Fri Jun 19 04:17:44 CEST 2009

Backwards compatibility seems to be an issue that arises a lot here. I
think we all have an idea of it is, but we need some hard place to
point to. So here's my attempt:

PEP: 387
Title: Backwards Compatibility Policy
Version: $Revision$
Last-Modified: $Date$
Author: Benjamin Peterson <benjamin at python.org>
Status: Draft
Type: Process
Content-Type: text/x-rst
Created: 18-Jun-2009


This PEP outlines Python's backwards compatibility policy.


As one of the most used programming languages today [#tiobe]_, the Python core
language and its standard library play a critcal role in thousands of
applications and libraries.  This is fantastic; it is probably one of a language
designer's most wishful dreams.  However, it means the development team must be
very careful not to break this existing 3rd party code with new releases.

Backwards Compatibility Rules

This policy applys to all public APIs.  These include the C-API, the standard
library, and the core language including syntax and operation as defined by the
reference manual.

This is the basic policy for backwards compatibility:

* The behavior of an API *must* not change between any two consecutive releases.

* A feature cannot be removed without notice between any two consecutive

* Addition of a feature which breaks 3rd party libraries or applications should
  have a large benefit to breakage ratio, and/or the incompatibility should be
  trival to fix in broken code.

Making Incompatible Changes

It's a fact: design mistakes happen.  Thus it is important to be able to change
APIs or remove misguided features.  This is accomplished through a gradual
process over several releases:

1. Discuss the change.  Depending on the size of the incompatibility, this could
   be on the bug tracker, python-dev, python-list, or the appropriate SIG.  A
   PEP or similar document may be written.  Hopefully users of the affected API
   will pipe up to comment.

2. Add a warning [#warnings_]_.  If behavior is changing, a the API may gain a
   new function or method to perform the new behavior; old usage should raise
   the warning.  If an API is being removed, simply warn whenever it is entered.
   DeprecationWarning is the usual warning category to use, but
   PendingDeprecationWarning may be used in special cases were the old and new
   versions of the API will coexist for many releases.

3. Wait for a release.

4. See if there's any feedback.  Users not involved in the original discussions
   may comment now after seeing the warning.  Perhaps reconsider.

5. The behavior change or feature removal may now be made default or permanent
   in the next release.  Remove the old version and warning.


.. [#tiobe] TIOBE Programming Community Index


.. [#warnings] The warnings module



This document has been placed in the public domain.

   Local Variables:
   mode: indented-text
   indent-tabs-mode: nil
   sentence-end-double-space: t
   fill-column: 70
   coding: utf-8

More information about the Python-Dev mailing list