182 lines
6.1 KiB
Plaintext
182 lines
6.1 KiB
Plaintext
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.
|
||
|
||
* For changes that are unable to raise a deprecation warning, consult
|
||
with the steering council.
|
||
|
||
* 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 minor Python
|
||
versions of the same major version, or one minor version in an older
|
||
major version (e.g. for a warning in Python 3.10, you either wait
|
||
until at least Python 3.12 or Python 4.0 to make the change).
|
||
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.
|
||
|
||
6. If a warning cannot be provided to users, consult with the steering
|
||
council.
|
||
|
||
|
||
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.
|
||
|
||
|
||
|
||
..
|
||
Local Variables:
|
||
mode: indented-text
|
||
indent-tabs-mode: nil
|
||
sentence-end-double-space: t
|
||
fill-column: 70
|
||
coding: utf-8
|
||
End:
|