python-peps/pep-0689.rst

PEP: 689
Title: Unstable C API tier
Author: Petr Viktorin <encukou@gmail.com>
Discussions-To: https://mail.python.org/archives/list/python-dev@python.org/thread/PQXSP7E2B6KNXTJ2AERWMKKX42YP5D6O/
Status: Deferred
Type: Standards Track
Content-Type: text/x-rst
Requires: 523
Created: 22-Apr-2022
Python-Version: 3.12
Post-History: `27-Apr-2022 <https://mail.python.org/archives/list/python-dev@python.org/thread/PQXSP7E2B6KNXTJ2AERWMKKX42YP5D6O/>`__,


Deferral note
===============

This PEP was accepted provided that function names in the unstable API start
with leading underscores. Unfortunately, this uncovered wider disagreement
about the meaning of leading underscores. The PEP's author is not comfortable
pushing the PEP forward before we agree on what a leading underscore means.


Abstract
========

Some functions and types of the C-API are designated *unstable*,
meaning that they will not change in patch (bugfix/security) releases,
but may change between minor releases (e.g. between 3.11 and 3.12) without
deprecation warnings.


Motivation & Rationale
======================

The Python C-API is currently divided into `three tiers <https://devguide.python.org/c-api/>`__:

- Limited API, with high compatibility expectations
- Public API, which follows the :pep:`backwards compatibility policy
  <387>`, and requires deprecation warnings before changes
- Private (internal) API, which can change at any time.

Tools requring access to CPython internals (e.g. advanced
debuggers and JIT compilers) are often built for minor series releases
of CPython, and assume that the C-API internals used do not change
in patch releases. To support these tools, we need a tier between the
Public and Private C-API, with guarantees on stability throughout
the minor-series release.


Setting Stability Expectations
------------------------------

Currently, there are no guarantees for the internal API -- that is, anything
that requires ``Py_BUILD_CORE`` or is named with a leading underscore.
This API can change without warning at any time, and code that uses it
is pinned to a specific build of Python.

However, in practice, even the internal API usually happens to be stable
in patch releases:

- Some CPython core developers take this as an an unwritten rule.
- Patch releases only contain bugfixes, which are unlikely to
  change the API.

Unstable API will make the stability expectations more explicit.

It will also hopefully encourage existing users of the private API to
reach out to python-dev, so we can expose, standardize and test an API
for some of their use cases.


Reserving underscores for Private API
-------------------------------------

:pep:`523` introduced functions for use by debuggers and JIT compilers,
which are stable only across minor releases.
The functions names have leading underscores to suggest their limited
stability.

However, leading underscores usually mark *fully private* API.
CPython developers familiar with the “underscore means internal”
convention are unlikely to check if underscored functions they are
changing are documented and used outside CPython itself.

This proposal brings us a bit closer to reserving underscores
only for truly internal, private, hands-off API.


Warning about API that is changed often
---------------------------------------

The ``PyCode_New()`` family is an example of functions that are
documented as unstable (“Calling [it] directly can bind you to a precise
Python version”), and also often change in practice.

Moving it to the unstable tier will make its status obvious even
to people who don't read the docs carefully enough, and will make it
hard to use accidentally.


Changes during the Beta period
------------------------------

Since the API itself can change continuously up until Beta 1 (feature freeze)
of a minor version, major users of this API are unlikely to test
Alpha releases and provide feedback.
It is very difficult to determine what needs to be exposed as unstable.

Additions to the unstable tier will count as *stabilization*,
and will be allowed up to Release Candidate 1.


Specification
=============

Several functions and types (“APIs”) will be moved to a new *unstable* tier.

They will be expected to stay stable across patch releases,
but may change or be removed without warning in minor releases (3.x.0),
including Alpha and Beta releases of 3.x.0.

When they change significantly, code that uses them should no longer compile
(e.g. arguments should be added/removed, or a function should be renamed,
but the semantic meaning of an argument should not change).

Their definitions will be moved to a new directory, ``Include/unstable/``,
and will be included from ``Python.h``.

From Python 3.12 on, these APIs will only be usable when the
``Py_USING_UNSTABLE_API`` macro is defined.
CPython will only define the macro for building CPython itself
(``Py_BUILD_CORE``).

To make transition to unstable API easier,
in Python 3.11 the APIs will be available without ``Py_USING_UNSTABLE_API``
defined. In this case, using them will generate a deprecation warning on
compilers that support ``Py_DEPRECATED``.

A similar deprecation period will be used when making more APIs unstable
in the future:

- When moving from public API, the deprecation period should follow Python's
  backwards compatibility policy (currently, it should last at least
  two releases).
- When moving from public API that is documented as unstable,
  the deprecation period can only last one release.
- When moving from private API or adding new API, no deprecation period
  is necessary.

Leading underscores will be removed from the names of the moved APIs.
The old underscored name of a renamed API will be available (as an alias
using ``#define``) at least until that API changes.

The unstable C-API tier and ``Py_USING_UNSTABLE_API`` will be documented,
and documentation of each unstable API will be updated.


Adjustments during Beta periods
-------------------------------

New APIs can be added to the unstable tier, and private APIs can be moved
to it, up to the first release candidate of a new minor version.
Consensus on the ``capi-sig`` or ``python-dev`` is needed in the Beta period.

In the Beta period, no API may be moved to more private tier, e.g.
what is public in Beta 1 must stay public until the final release.


Initial unstable API
--------------------

The following API will initially be unstable.
The set may be adjusted for 3.11.

Code object constructors:

- ``PyCode_New()``
- ``PyCode_NewWithPosOnlyArgs()``

Frame evaluation API (PEP 523):

- ``_PyFrameEvalFunction``
- ``_PyInterpreterState_GetEvalFrameFunc()``
- ``_PyInterpreterState_SetEvalFrameFunc()``
- ``_PyEval_RequestCodeExtraIndex()``
- ``_PyCode_GetExtra()``
- ``_PyCode_SetExtra()``
- ``struct _PyInterpreterFrame`` (as an incomplete, opaque struct)
- ``_PyFrame_GetFrameObject``
- ``PyEval_EvalFrameDefault``
  (new function that calls ``_PyEval_EvalFrameDefault``, but takes
  ``PyFrameObject`` rather than ``_PyInterpreterFrame``)

(Leading underscores will be removed as mentioned above.)


Backwards Compatibility
=======================

The C API backwards compatibility story will be made clearer.


How to Teach This
=================

The changes affect advanced C programmers, who should consult the
updated reference documentation, devguide and/or What's New document·.


Reference Implementation
========================

https://github.com/python/cpython/issues/91744


Rejected Ideas
==============

It might be good to add a similar tier in the Python (not C) API,
e.g. for ``types.CodeType``.
However, the opt-in mechanism would need to be different (if any).
This is outside the scope of the PEP.


Open Issues
===========

The exact set of exposed API may change.


Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.