222 lines
8.0 KiB
Plaintext
222 lines
8.0 KiB
Plaintext
PEP: 387
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Title: Backwards Compatibility Policy
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Version: $Revision$
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Last-Modified: $Date$
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Author: Benjamin Peterson <benjamin@python.org>
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PEP-Delegate: Brett Cannon <brett@python.org>
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Status: Active
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Type: Process
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Content-Type: text/x-rst
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Created: 18-Jun-2009
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Post-History: `19-Jun-2009 <https://mail.python.org/archives/list/python-dev@python.org/thread/YAZQHFUCYEYIX5MIG6LFJLCVQ5ORVUM6/>`__,
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`12-Jun-2020 <https://discuss.python.org/t/pep-387-backwards-compatibilty-policy/4421>`__,
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`16-Jun-2023 <https://discuss.python.org/t/formalize-the-concept-of-soft-deprecation-dont-schedule-removal-in-pep-387-backwards-compatibility-policy/27957>`__
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Abstract
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========
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This PEP outlines Python's backwards compatibility policy.
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Rationale
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=========
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As one of the most used programming languages today [#tiobe]_, the
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Python core language and its standard library play a critical role in
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millions of applications and libraries. This is fantastic. However, it
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means the development team must be very careful not to break this
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existing 3rd party code with new releases.
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This PEP takes the perspective that "backwards incompatibility" means
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preexisting code ceases to comparatively function after a change. It is
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acknowledged that this is not a concrete definition, but the expectation
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is people in general understand what is meant by
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"backwards incompatibility", and if they are unsure they may ask the
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Python development team and/or steering council for guidance.
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Backwards Compatibility Rules
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=============================
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This policy applies to all public APIs. These include:
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- Syntax and behavior of these constructs as defined by the reference
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manual.
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- The C-API.
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- Function, class, module, attribute, and method names and types.
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- Given a set of arguments, the return value, side effects, and raised
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exceptions of a function. This does not preclude changes from
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reasonable bug fixes.
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- The position and expected types of arguments and returned values.
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- Behavior of classes with regards to subclasses: the conditions under
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which overridden methods are called.
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- Documented exceptions and the semantics which lead to their raising.
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- Exceptions commonly raised in EAFP scenarios.
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Others are explicitly not part of the public API. They can change or
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be removed at any time in any way. These include:
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- Function, class, module, attribute, method, and C-API names and
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types that are prefixed by "_" (except special names).
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- Anything documented publicly as being private.
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Note that if something is not documented at all, it is *not*
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automatically considered private.
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- Imported modules (unless explicitly documented as part of the public
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API; e.g. importing the ``bacon`` module in the ``spam`` does not
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automatically mean ``spam.bacon`` is part of the public API unless
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it is documented as such).
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- Inheritance patterns of internal classes.
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- Test suites. (Anything in the ``Lib/test`` directory or test
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subdirectories of packages.)
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- Backward compatibility rules do not apply to any module or API that is
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explicitly documented as **Provisional** per :pep:`411`.
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Basic policy for backwards compatibility
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----------------------------------------
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* In general, incompatibilities should have a large benefit to
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breakage ratio, and the incompatibility should be easy to resolve in
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affected code. For example, adding an stdlib module with the same
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name as a third party package is generally not acceptable. Adding
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a method or attribute that conflicts with 3rd party code through
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inheritance, however, is likely reasonable.
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* Unless it is going through the deprecation process below, the
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behavior of an API *must* not change in an incompatible fashion
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between any two consecutive releases. Python's yearly release
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process (:pep:`602`) means that the deprecation period must last at
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least two years.
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* Similarly a feature cannot be removed without notice between any two
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consecutive releases.
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* For changes that are unable to raise a deprecation warning, consult
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with the steering council.
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* The steering council may grant exceptions to this policy. In
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particular, they may shorten the required deprecation period for a
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feature. Exceptions are only granted for extreme situations such as
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dangerously broken or insecure features or features no one could
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reasonably be depending on (e.g., support for completely obsolete
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platforms).
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Soft Deprecation
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================
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A soft deprecation can be used when using an API which should no longer
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be used to write new code, but it remains safe to continue using it in
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existing code. The API remains documented and tested, but will not be
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developed further (no enhancement).
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The main difference between a "soft" and a (regular) "hard" deprecation
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is that the soft deprecation does not imply scheduling the removal of
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the deprecated API.
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Another difference is that a soft deprecation does not issue a warning:
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it's only mentioned in the documentation, whereas usually a "hard"
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deprecation issues a ``DeprecationWarning`` warning at runtime. The
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documentation of a soft deprecation should explain why the API should be
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avoided, and if possible propose a replacement.
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If the decision is made to deprecate (in the regular sense) a feature
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that is currently soft deprecated, the deprecation must follow the
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`Backwards Compatibility Rules`_ (i.e., there is no exception because
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the feature is already soft deprecated).
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Making Incompatible Changes
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===========================
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Making an incompatible change is a gradual process performed over
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several releases:
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1. Discuss the change. Depending on the degree of incompatibility,
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this could be on the bug tracker, python-dev, python-list, or the
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appropriate SIG. A PEP or similar document may be written.
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Hopefully users of the affected API will pipe up to comment.
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2. Add a warning to the current ``main`` branch.
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If behavior is changing, the API may gain a new
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function or method to perform the new behavior; old usage should
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raise the warning. If an API is being removed, simply warn
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whenever it is entered. ``DeprecationWarning`` is the usual
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warning category to use, but ``PendingDeprecationWarning`` may be
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used in special cases where the old and new versions of the API will
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coexist for many releases [#warnings]_. The warning message should
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include the release the incompatibility is expected to become the
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default and a link to an issue that users can post feedback to.
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For C API, a compiler warning generated by the ``Py_DEPRECATED`` macro
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is also acceptable.
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3. Wait for the warning to appear in at least two minor Python
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versions of the same major version, or one minor version in an older
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major version (e.g. for a warning in Python 3.10.0, you either wait
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until at least Python 3.12 or Python 4.0 to make the change).
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It's fine to wait more than two releases, for example:
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- If the expected maintenance overhead and security risk of the
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deprecated behavior is small (e.g. an old function is reimplemented
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in terms of a new, more general one), it can stay indefinitely
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(or until the situation changes).
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- If the deprecated feature is replaced by a new one, it should
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generally be removed only after the last Python version
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*without* the new feature reaches end of support.
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4. See if there's any feedback. Users not involved in the original
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discussions may comment now after seeing the warning. Perhaps
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reconsider.
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5. The behavior change or feature removal may now be made default or
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permanent having reached the declared version. Remove the old
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version and warning.
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6. If a warning cannot be provided to users, consult with the steering
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council.
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References
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==========
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.. [#tiobe] TIOBE Programming Community Index
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http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html
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.. [#warnings] The warnings module
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http://docs.python.org/library/warnings.html
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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