PEP: 743
Title: Add Py_COMPAT_API_VERSION to the Python C API
Author: Victor Stinner <vstinner@python.org>,
        Petr Viktorin <encukou@gmail.com>,
PEP-Delegate: C API Working Group
Status: Draft
Type: Standards Track
Created: 11-Mar-2024
Python-Version: 3.14

.. highlight:: c


Abstract
========

Add ``Py_COMPAT_API_VERSION`` C macro that hides some deprecated and
soft-deprecated symbols, allowing users to opt out of using API with known
issues that other API solves.
The macro is versioned, allowing users to update (or not) on their own pace.

Also, add namespaced alternatives for API without the ``Py_`` prefix,
and soft-deprecate the original names.


Motivation
==========

Some of Python's C API has flaws that are only obvious in hindsight.

If an API prevents adding features or optimizations, or presents a serious
security risk or maintenance burden, we can deprecate and remove it as
described in :pep:`387`.

However, this leaves us with some API that has “sharp edges” -- it works fine
for its current users, but should be avoided in new code.
For example:

- API that cannot signal an exception, so failures are either ignored or
  exit the process with a fatal error. For example ``PyObject_HasAttr``.
- API that is not thread-safe, for example by borrowing references from
  mutable objects, or exposing unfinished mutable objects. For example
  ``PyDict_GetItemWithError``.
- API with names that don't use the ``Py``/``_Py`` prefix, and so can clash
  with other code. For example: ``setter``.

It is important to note that despite such flaws, it's usually possible
to use the API correctly. For example, in a single-threaded environment,
thread safety is not an issue.
We do not want to break working code, even if it uses API that would be wrong
in some -- or even *most* -- other contexts.

On the other hand, we want to steer users away from such “undesirable” API
in *new* code, especially if a safer alternative exists.


Adding the ``Py`` prefix
------------------------

Some names defined in CPython headers is not namespaced: it that lacks the
``Py`` prefix (or a variant: ``_Py``, and alternative capitalizations).
For example, we declare a function type named simply ``setter``.

While such names are not exported in the ABI (as checked by ``make smelly``),
they can clash with user code and, more importantly, with libraries linked
to third-party extensions.

While it would be possible to provide namespaced aliases and (soft-)deprecate
these names, the only way to make them not clash with third-party code is to
not define them in Python headers at all.


Rationale
=========

We want to allow an easy way for users to avoid “undesirable” API if they
choose to do so.

It might be be sufficient to leave this to third-party linters.
For that we'd need a good way to expose a list of (soft-)deprecated
API to such linters.
While adding that, we can -- rather easily -- do the linter's job directly
in CPython headers, avoiding the neel for an extra tool.
Unlike Python, C makes it rather easy to limit available API -- for a whole
project or for each individual source file -- by having users define
an “opt-in” macro.

We already do something similar with ``Py_LIMITED_API``, which limits the
available API to a subset that compiles to stable ABI. (In hindsight, we should
have used a different macro name for that particular kind of limiting, but it's
too late to change that now.)

To prevent working code from breaking as we identify more “undesirable” API
and add safer alternatives to it, the opt-in macro should be *versioned*.
Users can choose a version they need based on their compatibility requirements,
and update it at their own pace.

To be clear, this mechanism is *not* a replacement for deprecation.
Deprecation is for API that prevents new features or optimizations, or
presents a security risk or maintenance burden.
This mechanism, on the other hand, is meant for cases where “we found
a slightly better way of doing things” -- perhaps one that's harder to misuse,
or just has a less misleading name.
(On a lighter note: many people configure a code quality checker to shout at
them about the number of blank lines between functions. Let's help them
identify more substantial “code smells”!)

The proposed macro does not *change* any API definitions; it only *hides* them.
So, if code compiles with the macro, it'll also compile without it, with
identical behaviour.
This has implications for core devs: to deal with undesirable behaviour,
we'll need to introduce new, better API, and *then* discourage the old one.
In turn, this implies that we should look at an individual API and fix all its
known issues at once, rather than do codebase-wide sweeps for a single kind of
issue, so that we avoid multiple renames of the same function.


Adding the ``Py`` prefix
------------------------

An opt-in macro allows us to omit definitions that could clash with
third-party libraries.


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

We introduce a ``Py_COMPAT_API_VERSION`` macro.
If this macro is defined before ``#include <Python.h>``, some API definitions
-- as described below -- will be omitted from the Python header files.

The macro only omits complete top-level definitions exposed from ``<Python.h>``.
Other things (the ABI, structure definitions, macro expansions, static inline
function bodies, etc.) are not affected.

The C API working group (:pep:`731`) has authority over the set of omitted
definitions.

The set of omitted definitions will be tied to a particular feature release
of CPython, and is finalized in each 3.x.0 Beta 1 release.
In rare cases, entries can be removed (i.e. made available for use) at any
time.

The macro should be defined to a version in the format used by
``PY_VERSION_HEX``, with the “micro”, “release” and “serial” fields
set to zero.
For example, to omit API deemed undesirable in 3.14.0b1, users should define
``Py_COMPAT_API_VERSION`` to ``0x030e0000``.


Requirements for omitted API
----------------------------

An API that is omitted with ``Py_COMPAT_API_VERSION`` must:

- be soft-deprecated (see :pep:`387`);
- for all known use cases of the API, have a documented alternative
  or workaround;
- have tests to ensure it keeps working (except for 1:1 renames using
  ``#define`` or ``typedef``);
- be documented (except if it was never mentioned in previous versions of the
  documentation); and
- be approved by the C API working group. (The WG may give blanket approvals
  for groups of related API; see *Initial set* below for examples.)

Note that ``Py_COMPAT_API_VERSION`` is meant for API that can be trivially
replaced by a better alternative.
API without a replacement should generally be deprecated instead.


Location
--------

All API definitions omitted by ``Py_COMPAT_API_VERSION`` will be moved to
a new header, ``Include/legacy.h``.

This is meant to help linter authors compile lists, so they can flag the API
with warnings rather than errors.

Note that for simple renaming of source-only constructs (macros, types), we
expect names to be omitted in the same version -- or the same PR -- that adds
a replacement.
This means that the original definition will be renamed, and a ``typedef``
or ``#define`` for the old name added to ``Include/legacy.h``.


Documentation
-------------

Documentation for omitted API should generally:

- appear after the recommended replacement,
- reference the replacement (e.g. “Similar to X, but…”), and
- focus on differences from the replacement and migration advice.

Exceptions are possible if there is a good reason for them.


Initial set
-----------

The following API will be omitted with ``Py_COMPAT_API_VERSION`` set to
``0x030e0000`` (3.14) or greater:

- Omit API returning borrowed references:

  ====================================  ==============================
  Omitted API                           Replacement
  ====================================  ==============================
  ``PyDict_GetItem()``                  ``PyDict_GetItemRef()``
  ``PyDict_GetItemString()``            ``PyDict_GetItemStringRef()``
  ``PyImport_AddModule()``              ``PyImport_AddModuleRef()``
  ``PyList_GetItem()``                  ``PyList_GetItemRef()``
  ====================================  ==============================

- Omit deprecated APIs:

  ====================================  ==============================
  Omitted Deprecated API                Replacement
  ====================================  ==============================
  ``PY_FORMAT_SIZE_T``                  ``"z"``
  ``PY_UNICODE_TYPE``                   ``wchar_t``
  ``PyCode_GetFirstFree()``             ``PyUnstable_Code_GetFirstFree()``
  ``PyCode_New()``                      ``PyUnstable_Code_New()``
  ``PyCode_NewWithPosOnlyArgs()``       ``PyUnstable_Code_NewWithPosOnlyArgs()``
  ``PyImport_ImportModuleNoBlock()``    ``PyImport_ImportModule()``
  ``PyMem_DEL()``                       ``PyMem_Free()``
  ``PyMem_Del()``                       ``PyMem_Free()``
  ``PyMem_FREE()``                      ``PyMem_Free()``
  ``PyMem_MALLOC()``                    ``PyMem_Malloc()``
  ``PyMem_NEW()``                       ``PyMem_New()``
  ``PyMem_REALLOC()``                   ``PyMem_Realloc()``
  ``PyMem_RESIZE()``                    ``PyMem_Resize()``
  ``PyModule_GetFilename()``            ``PyModule_GetFilenameObject()``
  ``PyOS_AfterFork()``                  ``PyOS_AfterFork_Child()``
  ``PyObject_DEL()``                    ``PyObject_Free()``
  ``PyObject_Del()``                    ``PyObject_Free()``
  ``PyObject_FREE()``                   ``PyObject_Free()``
  ``PyObject_MALLOC()``                 ``PyObject_Malloc()``
  ``PyObject_REALLOC()``                ``PyObject_Realloc()``
  ``PySlice_GetIndicesEx()``            (two calls; see current docs)
  ``PyThread_ReInitTLS()``              (no longer needed)
  ``PyThread_create_key()``             ``PyThread_tss_alloc()``
  ``PyThread_delete_key()``             ``PyThread_tss_free()``
  ``PyThread_delete_key_value()``       ``PyThread_tss_delete()``
  ``PyThread_get_key_value()``          ``PyThread_tss_get()``
  ``PyThread_set_key_value()``          ``PyThread_tss_set()``
  ``PyUnicode_AsDecodedObject()``       ``PyUnicode_Decode()``
  ``PyUnicode_AsDecodedUnicode()``      ``PyUnicode_Decode()``
  ``PyUnicode_AsEncodedObject()``       ``PyUnicode_AsEncodedString()``
  ``PyUnicode_AsEncodedUnicode()``      ``PyUnicode_AsEncodedString()``
  ``PyUnicode_IS_READY()``              (no longer needed)
  ``PyUnicode_READY()``                 (no longer needed)
  ``PyWeakref_GET_OBJECT()``            ``PyWeakref_GetRef()``
  ``PyWeakref_GetObject()``             ``PyWeakref_GetRef()``
  ``Py_UNICODE``                        ``wchar_t``
  ``_PyCode_GetExtra()``                ``PyUnstable_Code_GetExtra()``
  ``_PyCode_SetExtra()``                ``PyUnstable_Code_SetExtra()``
  ``_PyDict_GetItemStringWithError()``  ``PyDict_GetItemStringRef()``
  ``_PyEval_RequestCodeExtraIndex()``   ``PyUnstable_Eval_RequestCodeExtraIndex()``
  ``_PyHASH_BITS``                      ``PyHASH_BITS``
  ``_PyHASH_IMAG``                      ``PyHASH_IMAG``
  ``_PyHASH_INF``                       ``PyHASH_INF``
  ``_PyHASH_MODULUS``                   ``PyHASH_MODULUS``
  ``_PyHASH_MULTIPLIER``                ``PyHASH_MULTIPLIER``
  ``_PyObject_EXTRA_INIT``              (no longer needed)
  ``_PyThreadState_UncheckedGet()``     ``PyThreadState_GetUnchecked()``
  ``_PyUnicode_AsString()``             ``PyUnicode_AsUTF8()``
  ``_Py_HashPointer()``                 ``Py_HashPointer()``
  ``_Py_T_OBJECT``                      ``Py_T_OBJECT_EX``
  ``_Py_WRITE_RESTRICTED``              (no longer needed)
  ====================================  ==============================

- Soft-deprecate and omit APIs:

  ====================================  ==============================
  Omitted Deprecated API                Replacement
  ====================================  ==============================
  ``PyDict_GetItemWithError()``         ``PyDict_GetItemRef()``
  ``PyDict_SetDefault()``               ``PyDict_SetDefaultRef()``
  ``PyMapping_HasKey()``                ``PyMapping_HasKeyWithError()``
  ``PyMapping_HasKeyString()``          ``PyMapping_HasKeyStringWithError()``
  ``PyObject_HasAttr()``                ``PyObject_HasAttrWithError()``
  ``PyObject_HasAttrString()``          ``PyObject_HasAttrStringWithError()``
  ====================================  ==============================

- Omit ``<structmember.h>`` legacy API:

  The header file ``structmember.h``, which is not included from ``<Python.h>``
  and must be included separately, will ``#error`` if
  ``Py_COMPAT_API_VERSION`` is defined.
  This affects the following API:

  ====================================  ==============================
  Omitted Deprecated API                Replacement
  ====================================  ==============================
  ``T_SHORT``                           ``Py_T_SHORT``
  ``T_INT``                             ``Py_T_INT``
  ``T_LONG``                            ``Py_T_LONG``
  ``T_FLOAT``                           ``Py_T_FLOAT``
  ``T_DOUBLE``                          ``Py_T_DOUBLE``
  ``T_STRING``                          ``Py_T_STRING``
  ``T_OBJECT``                          (``tp_getset``; docs to be written)
  ``T_CHAR``                            ``Py_T_CHAR``
  ``T_BYTE``                            ``Py_T_BYTE``
  ``T_UBYTE``                           ``Py_T_UBYTE``
  ``T_USHORT``                          ``Py_T_USHORT``
  ``T_UINT``                            ``Py_T_UINT``
  ``T_ULONG``                           ``Py_T_ULONG``
  ``T_STRING_INPLACE``                  ``Py_T_STRING_INPLACE``
  ``T_BOOL``                            ``Py_T_BOOL``
  ``T_OBJECT_EX``                       ``Py_T_OBJECT_EX``
  ``T_LONGLONG``                        ``Py_T_LONGLONG``
  ``T_ULONGLONG``                       ``Py_T_ULONGLONG``
  ``T_PYSSIZET``                        ``Py_T_PYSSIZET``
  ``T_NONE``                            (``tp_getset``; docs to be written)
  ``READONLY``                          ``Py_READONLY``
  ``PY_AUDIT_READ``                     ``Py_AUDIT_READ``
  ``READ_RESTRICTED``                   ``Py_AUDIT_READ``
  ``PY_WRITE_RESTRICTED``               (no longer needed)
  ``RESTRICTED``                        ``Py_AUDIT_READ``
  ====================================  ==============================

- Omit soft deprecated macros:

  ======================  =====================================
  Omitted Macros          Replacement
  ======================  =====================================
  ``Py_IS_NAN()``         ``isnan()`` (C99+ ``<math.h>``)
  ``Py_IS_INFINITY()``    ``isinf(X)`` (C99+ ``<math.h>``)
  ``Py_IS_FINITE()``      ``isfinite(X)`` (C99+ ``<math.h>``)
  ``Py_MEMCPY()``         ``memcpy()`` (C ``<string.h>``)
  ======================  =====================================

- Soft-deprecate and omit typedefs without the ``Py``/``_Py`` prefix
  (``getter``, ``setter``, ``allocfunc``, …), in favour of *new* ones
  that add the prefix (``Py_getter`` , etc.)

- Soft-deprecate and omit macros without the ``Py``/``_Py`` prefix
  (``METH_O``, ``CO_COROUTINE``, ``FUTURE_ANNOTATIONS``, ``WAIT_LOCK``, …),
  favour of *new* ones that add the prefix  (``Py_METH_O`` , etc.).

- Any others approved by the C API workgroup


If any of these proposed replacements, or associated documentation,
are not added in time for 3.14.0b1, they'll be omitted with later versions
of ``Py_COMPAT_API_VERSION``.
(We expect this for macros generated by ``configure``: ``HAVE_*``, ``WITH_*``,
``ALIGNOF_*``, ``SIZEOF_*``, and several without a common prefix.)


Implementation
==============

TBD


Open issues
===========

The name ``Py_COMPAT_API_VERSION`` was taken from the earlier PEP;
it doesn't fit this version.


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

The macro is backwards compatible.
Developers can introduce and update the macro on their own pace, potentially
for one source file at a time.


Discussions
===========

* C API Evolutions: `Macro to hide deprecated functions
  <https://github.com/capi-workgroup/api-evolution/issues/24>`_
  (October 2023)
* C API Problems: `Opt-in macro for a new clean API? Subset of functions
  with no known issues
  <https://github.com/capi-workgroup/problems/issues/54>`_
  (June 2023)
* `Finishing the Great Renaming
  <https://discuss.python.org/t/finishing-the-great-renaming/54082>`_
  (May 2024)


Prior Art
=========

* ``Py_LIMITED_API`` macro of :pep:`384` "Defining a Stable ABI".
* Rejected :pep:`606` "Python Compatibility Version" which has a global
  scope.


Copyright
=========

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