PEP: 387
Title: Backwards Compatibility Policy
Version: $Revision$
Last-Modified: $Date$
Author: Benjamin Peterson <benjamin@python.org>
BDFL-Delegate: Brett Cannon (on behalf of the steering council)
Discussions-To: https://discuss.python.org/t/pep-387-backwards-compatibilty-policy/
Status: Active
Type: Process
Content-Type: text/x-rst
Created: 18-Jun-2009
Post-History: 19-Jun-2009, 12-Jun-2020


Abstract
========

This PEP outlines Python's backwards compatibility policy.


Rationale
=========

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

This PEP takes the perspective that "backwards incompatibility" means
preexisting code ceases to comparatively function after a change. It is
acknowledged that this is not a concrete definition, but the expectation
is people in general understand what is meant by
"backwards incompatibility", and if they are unsure they may ask the
Python development team and/or steering council for guidance.


Backwards Compatibility Rules
=============================

This policy applies to all public APIs.  These include:

- Syntax and behavior of these constructs as defined by the reference
  manual.

- The C-API.

- Function, class, module, attribute, and method names and types.

- Given a set of arguments, the return value, side effects, and raised
  exceptions of a function.  This does not preclude changes from
  reasonable bug fixes.

- The position and expected types of arguments and returned values.

- Behavior of classes with regards to subclasses: the conditions under
  which overridden methods are called.

- Documented exceptions and the semantics which lead to their raising.

- Exceptions commonly raised in EAFP scenarios.

Others are explicitly not part of the public API.  They can change or
be removed at any time in any way.  These include:

- Function, class, module, attribute, method, and C-API names and
  types that are prefixed by "_" (except special names).

- Anything documented publicly as being private.

- Imported modules (unless explicitly documented as part of the public
  API; e.g. importing the ``bacon`` module in the ``spam`` does not
  automatically mean ``spam.bacon`` is part of the public API unless
  it is documented as such).

- Inheritance patterns of internal classes.

- Test suites.  (Anything in the ``Lib/test`` directory or test
  subdirectories of packages.)

- Backward compatibility rules do not apply to any module or API that is
  explicitly documented as **Provisional** per PEP 411.


Basic policy for backwards compatibility
----------------------------------------

* In general, incompatibilities should have a large benefit to
  breakage ratio, and the incompatibility should be easy to resolve in
  affected code.  For example, adding an stdlib module with the same
  name as a third party package is generally not acceptable.  Adding
  a method or attribute that conflicts with 3rd party code through
  inheritance, however, is likely reasonable.

* Unless it is going through the deprecation process below, the
  behavior of an API *must* not change in an incompatible fashion
  between any two consecutive releases.  Python's yearly release
  process (:pep:`602`) means that the deprecation period must last at
  least two years.

* Similarly a feature cannot be removed without notice between any two
  consecutive releases.

* The steering council may grant exceptions to this policy. In
  particular, they may shorten the required deprecation period for a
  feature. Exceptions are only granted for extreme situations such as
  dangerously broken or insecure features or features no one could
  reasonably be depending on (e.g., support for completely obsolete
  platforms).


Making Incompatible Changes
===========================

Making an incompatible change is a gradual process performed over
several releases:

1. Discuss the change.  Depending on the degree of 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.  If behavior is changing, 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 where the old and new versions of the API will
   coexist for many releases [#warnings]_. Compiler warnings are also
   acceptable. The warning message should include the release the
   incompatibility is expected to become the default and a link to an
   issue that users can post feedback to.

3. Wait for the warning to appear in at least two major Python
   versions. It's fine to wait more than two releases.

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 having reached the declared version. Remove the old
   version and warning.


References
==========

.. [#tiobe] TIOBE Programming Community Index

   http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html

.. [#warnings] The warnings module

   http://docs.python.org/library/warnings.html


Copyright
=========

This document has been placed in the public domain.



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