Thanks, Victor for awesome PEP I am a big +1 on this proposal since some of the core developers already need the evolution of C API. I believe this proposal is not only for alternative python compiler implementation but also gives a chance for enhancing CPython performance. And I love this proposal since the suggestion does not say break everything in a one-shot but pursuing incremental change. The one thing that we have to do for this PEP is that informing the change very well to the 3rd party library. With well-written changing documentation, I believe most of the impactful 3rd party library which sustains Python library community will have enough time for preparing change. I believe that if there is no change there will be no evolution. Let's make CPython more fastly with temporarily suffering Regards from Korea, Dong-hee 2020년 6월 22일 (월) 오후 9:13, Victor Stinner 님이 작성:

Hi,

PEP available at: https://www.python.org/dev/peps/pep-0620/

<introduction> This PEP is the result of 4 years of research work on the C API: https://pythoncapi.readthedocs.io/

It's the third version. The first version (2017) proposed to add a "new C API" and advised C extensions maintainers to opt-in for it: it was basically the same idea as PEP 384 limited C API but in a different color. Well, I had no idea of what I was doing :-) The second version (April 2020) proposed to add a new Python runtime built from the same code base as the regular Python runtime but in a different build mode, the regular Python would continue to be fully compatible.

I wrote the third version, the PEP 620, from scratch. It now gives an explicit and concrete list of incompatible C API changes, and has better motivation and rationale sections. The main PEP novelty is the new pythoncapi_compat.h header file distributed with Python to provide new C API functions to old Python versions, the second novelty is the process to reduce the number of broken C extensions.

Whereas PEPs are usually implemented in a single Python version, the implementation of this PEP is expected to be done carefully over multiple Python versions. The PEP lists many changes which are already implemented in Python 3.7, 3.8 and 3.9. It defines a process to reduce the number of broken C extensions when introducing the incompatible C API changes listed in the PEP. The process dictates the rhythm of these changes. </introduction>

PEP: 620 Title: Hide implementation details from the C API Author: Victor Stinner Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 19-June-2020 Python-Version: 3.10

Abstract ========

Introduce C API incompatible changes to hide implementation details.

Once most implementation details will be hidden, evolution of CPython internals would be less limited by C API backward compatibility issues. It will be way easier to add new features.

It becomes possible to experiment with more advanced optimizations in CPython than just micro-optimizations, like tagged pointers.

Define a process to reduce the number of broken C extensions.

The implementation of this PEP is expected to be done carefully over multiple Python versions. It already started in Python 3.7 and most changes are already completed. The `Process to reduce the number of broken C extensions`_ dictates the rhythm.

Motivation ==========

The C API blocks CPython evolutions -----------------------------------

Adding or removing members of C structures is causing multiple backward compatibility issues.

Adding a new member breaks the stable ABI (PEP 384), especially for types declared statically (e.g. ``static PyTypeObject MyType = {...};``). In Python 3.4, the PEP 442 "Safe object finalization" added the ``tp_finalize`` member at the end of the ``PyTypeObject`` structure. For ABI backward compatibility, a new ``Py_TPFLAGS_HAVE_FINALIZE`` type flag was required to announce if the type structure contains the ``tp_finalize`` member. The flag was removed in Python 3.8 (`bpo-32388 https://bugs.python.org/issue32388`_).

The ``PyTypeObject.tp_print`` member, deprecated since Python 3.0 released in 2009, has been removed in the Python 3.8 development cycle. But the change broke too many C extensions and had to be reverted before 3.8 final release. Finally, the member was removed again in Python 3.9.

C extensions rely on the ability to access directly structure members, indirectly through the C API, or even directly. Modifying structures like ``PyListObject`` cannot be even considered.

The ``PyTypeObject`` structure is the one which evolved the most, simply because there was no other way to evolve CPython than modifying it.

In the C API, all Python objects are passed as ``PyObject*``: a pointer to a ``PyObject`` structure. Experimenting tagged pointers in CPython is blocked by the fact that a C extension can technically dereference a ``PyObject*`` pointer and access ``PyObject`` members. Small "objects" can be stored as a tagged pointer with no concrete ``PyObject`` structure.

Replacing Python garbage collector with a tracing garbage collector would also need to remove ``PyObject.ob_refcnt`` reference counter, whereas currently ``Py_INCREF()`` and ``Py_DECREF()`` macros access directly to ``PyObject.ob_refcnt``.

Same CPython design since 1990: structures and reference counting -----------------------------------------------------------------

When the CPython project was created, it was written with one principle: keep the implementation simple enough so it can be maintained by a single developer. CPython complexity grew a lot and many micro-optimizations have been implemented, but CPython core design has not changed.

Members of ``PyObject`` and ``PyTupleObject`` structures have not changed since the "Initial revision" commit (1990)::

#define OB_HEAD \ unsigned int ob_refcnt; \ struct _typeobject *ob_type;

typedef struct _object { OB_HEAD } object;

typedef struct { OB_VARHEAD object *ob_item[1]; } tupleobject;

Only names changed: ``object`` was renamed to ``PyObject`` and ``tupleobject`` was renamed to ``PyTupleObject``.

CPython still tracks Python objects lifetime using reference counting internally and for third party C extensions (through the Python C API).

All Python objects must be allocated on the heap and cannot be moved.

Why is PyPy more efficient than CPython? ----------------------------------------

The PyPy project is a Python implementation which is 4.2x faster than CPython on average. PyPy developers chose to not fork CPython, but start from scratch to have more freedom in terms of optimization choices.

PyPy does not use reference counting, but a tracing garbage collector which moves objects. Objects can be allocated on the stack (or even not at all), rather than always having to be allocated on the heap.

Objects layouts are designed with performance in mind. For example, a list strategy stores integers directly as integers, rather than objects.

Moreover, PyPy also has a JIT compiler which emits fast code thanks to the efficient PyPy design.

PyPy bottleneck: the Python C API ---------------------------------

While PyPy is way more efficient than CPython to run pure Python code, it is as efficient or slower than CPython to run C extensions.

Since the C API requires ``PyObject*`` and allows to access directly structure members, PyPy has to associate a CPython object to PyPy objects and maintain both consistent. Converting a PyPy object to a CPython object is inefficient. Moreover, reference counting also has to be implemented on top of PyPy tracing garbage collector.

These conversions are required because the Python C API is too close to the CPython implementation: there is no high-level abstraction. For example, structures members are part of the public C API and nothing prevents a C extension to get or set directly ``PyTupleObject.ob_item[0]`` (the first item of a tuple).

See `Inside cpyext: Why emulating CPython C API is so Hard https://morepypy.blogspot.com/2018/09/inside-cpyext-why-emulating-cpython-c....`_ (Sept 2018) by Antonio Cuni for more details.

Rationale =========

Hide implementation details ---------------------------

Hiding implementation details from the C API has multiple advantages:

* It becomes possible to experiment with more advanced optimizations in CPython than just micro-optimizations. For example, tagged pointers, and replace the garbage collector with a tracing garbage collector which can move objects. * Adding new features in CPython becomes easier. * PyPy should be able to avoid conversions to CPython objects in more cases: keep efficient PyPy objects. * It becomes easier to implement the C API for a new Python implementation. * More C extensions will be compatible with Python implementations other than CPython.

Relationship with the limited C API -----------------------------------

The PEP 384 "Defining a Stable ABI" is in Python 3.4. It introduces the "limited C API": a subset of the C API. When the limited C API is used, it becomes possible to build a C extension only once and use it on multiple Python versions: that's the stable ABI.

The main limitation of the PEP 384 is that C extensions have to opt-in for the limited C API. Only very few projects made this choice, usually to ease distribution of binaries, especially on Windows.

This PEP moves the C API towards the limited C API.

Ideally, the C API will become the limited C API and all C extensions will use the stable ABI, but this is out of this PEP scope.

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

Summary -------

* (**Completed**) Reorganize the C API header files: create ``Include/cpython/`` and ``Include/internal/`` subdirectories. * (**Completed**) Move private functions exposing implementation details to the internal C API. * (**Completed**) Convert macros to static inline functions. * (**Completed**) Add new functions ``Py_SET_TYPE()``, ``Py_SET_REFCNT()`` and ``Py_SET_SIZE()``. The ``Py_TYPE()``, ``Py_REFCNT()`` and ``Py_SIZE()`` macros become functions which cannot be used as l-value. * (**Completed**) New C API functions must not return borrowed references. * (**In Progress**) Provide ``pythoncapi_compat.h`` header file. * (**In Progress**) Make structures opaque, add getter and setter functions. * (**Not Started**) Deprecate ``PySequence_Fast_ITEMS()``. * (**Not Started**) Convert ``PyTuple_GET_ITEM()`` and ``PyList_GET_ITEM()`` macros to static inline functions.

Reorganize the C API header files ---------------------------------

The first consumer of the C API was Python itself. There is no clear separation between APIs which must not be used outside Python, and API which are public on purpose.

Header files must be reorganized in 3 API:

* ``Include/`` directory is the limited C API: no implementation details, structures are opaque. C extensions using it get a stable ABI. * ``Include/cpython/`` directory is the CPython C API: less "portable" API, depends more on the Python version, expose some implementation details, few incompatible changes can happen. * ``Internal/internal/`` directory is the internal C API: implementation details, incompatible changes are likely at each Python release.

The creation of the ``Include/cpython/`` directory is fully backward compatible. ``Include/cpython/`` header files cannot be included directly and are included automatically by ``Include/`` header files when the ``Py_LIMITED_API`` macro is not defined.

The internal C API is installed and can be used for specific usage like debuggers and profilers which must access structures members without executing code. C extensions using the internal C API are tightly coupled to a Python version and must be recompiled at each Python version.

**STATUS**: Completed (in Python 3.8)

The reorganization of header files started in Python 3.7 and was completed in Python 3.8:

* `bpo-35134 https://bugs.python.org/issue35134`_: Add a new Include/cpython/ subdirectory for the "CPython API" with implementation details. * `bpo-35081 https://bugs.python.org/issue35081`_: Move internal headers to ``Include/internal/``

Move private functions to the internal C API --------------------------------------------

Private functions which expose implementation details must be moved to the internal C API.

If a C extension relies on a CPython private function which exposes CPython implementation details, other Python implementations have to re-implement this private function to support this C extension.

**STATUS**: Completed (in Python 3.9)

Private functions moved to the internal C API in Python 3.8:

* ``_PyObject_GC_TRACK()``, ``_PyObject_GC_UNTRACK()``

Macros and functions excluded from the limited C API in Python 3.9:

* ``_PyObject_SIZE()``, ``_PyObject_VAR_SIZE()`` * ``PyThreadState_DeleteCurrent()`` * ``PyFPE_START_PROTECT()``, ``PyFPE_END_PROTECT()`` * ``_Py_NewReference()``, ``_Py_ForgetReference()`` * ``_PyTraceMalloc_NewReference()`` * ``_Py_GetRefTotal()``

Private functions moved to the internal C API in Python 3.9:

* GC functions like ``_Py_AS_GC()``, ``_PyObject_GC_IS_TRACKED()`` and ``_PyGCHead_NEXT()`` * ``_Py_AddToAllObjects()`` (not exported) * ``_PyDebug_PrintTotalRefs()``, ``_Py_PrintReferences()``, ``_Py_PrintReferenceAddresses()`` (not exported)

Public "clear free list" functions moved to the internal C API an renamed to private functions and in Python 3.9:

* ``PyAsyncGen_ClearFreeLists()`` * ``PyContext_ClearFreeList()`` * ``PyDict_ClearFreeList()`` * ``PyFloat_ClearFreeList()`` * ``PyFrame_ClearFreeList()`` * ``PyList_ClearFreeList()`` * ``PyTuple_ClearFreeList()`` * Functions simply removed:

* ``PyMethod_ClearFreeList()`` and ``PyCFunction_ClearFreeList()``: bound method free list removed in Python 3.9. * ``PySet_ClearFreeList()``: set free list removed in Python 3.4. * ``PyUnicode_ClearFreeList()``: Unicode free list removed in Python 3.3.

Convert macros to static inline functions -----------------------------------------

Converting macros to static inline functions have multiple advantages:

* Functions have well defined parameter types and return type. * Functions can use variables with a well defined scope (the function). * Debugger can be put breakpoints on functions and profilers can display the function name in the call stacks. In most cases, it works even when a static inline function is inlined. * Functions don't have `macros pitfalls https://gcc.gnu.org/onlinedocs/cpp/Macro-Pitfalls.html`_.

Converting macros to static inline functions should only impact very few C extensions which use macros in unusual ways.

For backward compatibility, functions must continue to accept any type, not only ``PyObject*``, to avoid compiler warnings, since most macros cast their parameters to ``PyObject*``.

Python 3.6 requires C compilers to support static inline functions: the PEP 7 requires a subset of C99.

**STATUS**: Completed (in Python 3.9)

Macros converted to static inline functions in Python 3.8:

* ``Py_INCREF()``, ``Py_DECREF()`` * ``Py_XINCREF()``, ``Py_XDECREF()`` * ``PyObject_INIT()``, ``PyObject_INIT_VAR()`` * ``_PyObject_GC_TRACK()``, ``_PyObject_GC_UNTRACK()``, ``_Py_Dealloc()``

Macros converted to regular functions in Python 3.9:

* ``Py_EnterRecursiveCall()``, ``Py_LeaveRecursiveCall()`` (added to the limited C API) * ``PyObject_INIT()``, ``PyObject_INIT_VAR()`` * ``PyObject_GET_WEAKREFS_LISTPTR()`` * ``PyObject_CheckBuffer()`` * ``PyIndex_Check()`` * ``PyObject_IS_GC()`` * ``PyObject_NEW()`` (alias to ``PyObject_New()``), ``PyObject_NEW_VAR()`` (alias to ``PyObject_NewVar()``) * ``PyType_HasFeature()`` (always call ``PyType_GetFlags()``) * ``Py_TRASHCAN_BEGIN_CONDITION()`` and ``Py_TRASHCAN_END()`` macros now call functions which hide implementation details, rather than accessing directly members of the ``PyThreadState`` structure.

Make structures opaque ----------------------

All structures of the C API should become opaque: C extensions must use getter or setter functions to get or set structure members. For example, ``tuple->ob_item[0]`` must be replaced with ``PyTuple_GET_ITEM(tuple, 0)``.

To be able to move away from reference counting, ``PyObject`` must become opaque. Currently, the reference counter ``PyObject.ob_refcnt`` is exposed in the C API. All structures must become opaque, since they "inherit" from PyObject. For, ``PyFloatObject`` inherits from ``PyObject``::

typedef struct { PyObject ob_base; double ob_fval; } PyFloatObject;

Making ``PyObject`` fully opaque requires converting ``Py_INCREF()`` and ``Py_DECREF()`` macros to function calls. This change has an impact on performance. It is likely to be one of the very last changes when making structures opaque.

Making ``PyTypeObject`` structure opaque breaks C extensions declaring types statically (e.g. ``static PyTypeObject MyType = {...};``). C extensions must use ``PyType_FromSpec()`` to allocate types on the heap instead. Using heap types has other advantages like being compatible with subinterpreters. Combined with PEP 489 "Multi-phase extension module initialization", it makes a C extension behavior closer to a Python module, like allowing to create more than one module instance.

Making ``PyThreadState`` structure opaque requires adding getter and setter functions for members used by C extensions.

**STATUS**: In Progress (started in Python 3.8)

The ``PyInterpreterState`` structure was made opaque in Python 3.8 (`bpo-35886 https://bugs.python.org/issue35886`_) and the ``PyGC_Head`` structure (`bpo-40241 https://bugs.python.org/issue40241`_) was made opaque in Python 3.9.

Issues tracking the work to prepare the C API to make following structures opaque:

* ``PyObject``: `bpo-39573 https://bugs.python.org/issue39573`_ * ``PyTypeObject``: `bpo-40170 https://bugs.python.org/issue40170`_ * ``PyFrameObject``: `bpo-40421 https://bugs.python.org/issue40421`_

* Python 3.9 adds ``PyFrame_GetCode()`` and ``PyFrame_GetBack()`` getter functions, and moves ``PyFrame_GetLineNumber`` to the limited C API.

* ``PyThreadState``: `bpo-39947 https://bugs.python.org/issue39947`_

* Python 3.9 adds 3 getter functions: ``PyThreadState_GetFrame()``, ``PyThreadState_GetID()``, ``PyThreadState_GetInterpreter()``.

Disallow using Py_TYPE() as l-value -----------------------------------

The ``Py_TYPE()`` function gets an object type, its ``PyObject.ob_type`` member. It is implemented as a macro which can be used as an l-value to set the type: ``Py_TYPE(obj) = new_type``. This code relies on the assumption that ``PyObject.ob_type`` can be modified directly. It prevents making the ``PyObject`` structure opaque.

New setter functions ``Py_SET_TYPE()``, ``Py_SET_REFCNT()`` and ``Py_SET_SIZE()`` are added and must be used instead.

The ``Py_TYPE()``, ``Py_REFCNT()`` and ``Py_SIZE()`` macros must be converted to static inline functions which can not be used as l-value.

For example, the ``Py_TYPE()`` macro::

#define Py_TYPE(ob) (((PyObject*)(ob))->ob_type)

becomes::

#define _PyObject_CAST_CONST(op) ((const PyObject*)(op))

static inline PyTypeObject* _Py_TYPE(const PyObject *ob) { return ob->ob_type; }

#define Py_TYPE(ob) _Py_TYPE(_PyObject_CAST_CONST(ob))

**STATUS**: Completed (in Python 3.10)

New functions ``Py_SET_TYPE()``, ``Py_SET_REFCNT()`` and ``Py_SET_SIZE()`` were added to Python 3.9.

In Python 3.10, ``Py_TYPE()``, ``Py_REFCNT()`` and ``Py_SIZE()`` can no longer be used as l-value and the new setter functions must be used instead.

New C API functions must not return borrowed references -------------------------------------------------------

When a function returns a borrowed reference, Python cannot track when the caller stops using this reference.

For example, if the Python ``list`` type is specialized for small integers, store directly "raw" numbers rather than Python objects, ``PyList_GetItem()`` has to create a temporary Python object. The problem is to decide when it is safe to delete the temporary object.

The general guidelines is to avoid returning borrowed references for new C API functions.

No function returning borrowed functions is scheduled for removal by this PEP.

**STATUS**: Completed (in Python 3.9)

In Python 3.9, new C API functions returning Python objects only return strong references:

* ``PyFrame_GetBack()`` * ``PyFrame_GetCode()`` * ``PyObject_CallNoArgs()`` * ``PyObject_CallOneArg()`` * ``PyThreadState_GetFrame()``

Avoid functions returning PyObject** ------------------------------------

The ``PySequence_Fast_ITEMS()`` function gives a direct access to an array of ``PyObject*`` objects. The function is deprecated in favor of ``PyTuple_GetItem()`` and ``PyList_GetItem()``.

``PyTuple_GET_ITEM()`` can be abused to access directly the ``PyTupleObject.ob_item`` member::

PyObject **items = &PyTuple_GET_ITEM(0);

The ``PyTuple_GET_ITEM()`` and ``PyList_GET_ITEM()`` macros are converted to static inline functions to disallow that.

**STATUS**: Not Started

New pythoncapi_compat.h header file -----------------------------------

Making structures opaque requires modifying C extensions to use getter and setter functions. The practical issue is how to keep support for old Python versions which don't have these functions.

For example, in Python 3.10, it is no longer possible to use ``Py_TYPE()`` as an l-value. The new ``Py_SET_TYPE()`` function must be used instead::

#if PY_VERSION_HEX >= 0x030900A4 Py_SET_TYPE(&MyType, &PyType_Type); #else Py_TYPE(&MyType) = &PyType_Type; #endif

This code may ring a bell to developers who ported their Python code base from Python 2 to Python 3.

Python will distribute a new ``pythoncapi_compat.h`` header file which provides new C API functions to old Python versions. Example::

#if PY_VERSION_HEX < 0x030900A4 static inline void _Py_SET_TYPE(PyObject *ob, PyTypeObject *type) { ob->ob_type = type; } #define Py_SET_TYPE(ob, type) _Py_SET_TYPE((PyObject*)(ob), type) #endif // PY_VERSION_HEX < 0x030900A4

Using this header file, ``Py_SET_TYPE()`` can be used on old Python versions as well.

Developers can copy this file in their project, or even to only copy/paste the few functions needed by their C extension.

**STATUS**: In Progress (implemented but not distributed by CPython yet)

The ``pythoncapi_compat.h`` header file is currently developer at: https://github.com/pythoncapi/pythoncapi_compat

Process to reduce the number of broken C extensions ===================================================

Process to reduce the number of broken C extensions when introducing C API incompatible changes listed in this PEP:

* Estimate how many popular C extensions are affected by the incompatible change. * Coordinate with maintainers of broken C extensions to prepare their code for the future incompatible change. * Introduce the incompatible changes in Python. The documentation must explain how to port existing code. It is recommended to merge such changes at the beginning of a development cycle to have more time for tests. * Changes which are the most likely to break a large number of C extensions should be announced on the capi-sig mailing list to notify C extensions maintainers to prepare their project for the next Python. * If the change breaks too many projects, reverting the change should be discussed, taking in account the number of broken packages, their importance in the Python community, and the importance of the change.

The coordination usually means reporting issues to the projects, or even proposing changes. It does not require waiting for a new release including fixes for every broken project.

Since more and more C extensions are written using Cython, rather directly using the C API, it is important to ensure that Cython is prepared in advance for incompatible changes. It gives more time for C extension maintainers to release a new version with code generated with the updated Cython (for C extensions distributing the code generated by Cython).

Future incompatible changes can be announced by deprecating a function in the documentation and by annotating the function with ``Py_DEPRECATED()``. But making a structure opaque and preventing the usage of a macro as l-value cannot be deprecated with ``Py_DEPRECATED()``.

The important part is coordination and finding a balance between CPython evolutions and backward compatibility. For example, breaking a random, old, obscure and unmaintained C extension on PyPI is less severe than breaking numpy.

If a change is reverted, we move back to the coordination step to better prepare the change. Once more C extensions are ready, the incompatible change can be reconsidered.

Version History ===============

* Version 3, June 2020: PEP rewritten from scratch. Python now distributes a new ``pythoncapi_compat.h`` header and a process is defined to reduce the number of broken C extensions when introducing C API incompatible changes listed in this PEP. * Version 2, April 2020: `PEP: Modify the C API to hide implementation details https://mail.python.org/archives/list/python-dev@python.org/thread/HKM774XKU...`_. * Version 1, July 2017: `PEP: Hide implementation details in the C API https://mail.python.org/archives/list/python-ideas@python.org/thread/6XATDGW...`_ sent to python-ideas

Copyright =========

This document has been placed in the public domain.

-- Night gathers, and now my watch begins. It shall not end until my death. _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-leave@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/EV7F7Z6P... Code of Conduct: http://python.org/psf/codeofconduct/

-- Software Development Engineer at Kakao corp. Tel: +82 010-3353-9127 Email: donghee.na92@gmail.com | denny.i@kakaocorp.com Linkedin: https://www.linkedin.com/in/dong-hee-na-2b713b49/

Hi Victor, thanks for your continued work on improving the C-API. I'll comment on the PEP inline. Victor Stinner schrieb am 22.06.20 um 14:10:

PEP available at: https://www.python.org/dev/peps/pep-0620/ [...] Motivation ==========

The C API blocks CPython evolutions -----------------------------------

Adding or removing members of C structures is causing multiple backward compatibility issues.

Adding a new member breaks the stable ABI (PEP 384), especially for types declared statically (e.g. ``static PyTypeObject MyType = {...};``). In Python 3.4, the PEP 442 "Safe object finalization" added the ``tp_finalize`` member at the end of the ``PyTypeObject`` structure. For ABI backward compatibility, a new ``Py_TPFLAGS_HAVE_FINALIZE`` type flag was required to announce if the type structure contains the ``tp_finalize`` member. The flag was removed in Python 3.8 (`bpo-32388 https://bugs.python.org/issue32388`_).

Probably not the best example. I think this is pretty much normal API evolution. Changing the deallocation protocol for objects is going to impact any public API in one way or another. PyTypeObject is also not exposed with its struct fields in the limited API, so your point regarding "tp_print" is also not a strong one.

Same CPython design since 1990: structures and reference counting ----------------------------------------------------------------- Members of ``PyObject`` and ``PyTupleObject`` structures have not changed since the "Initial revision" commit (1990)

While I see an advantage in hiding the details of PyObject (specifically memory management internals), I would argue that there simply isn't much to improve in PyTupleObject, so these two don't fly at the same level for me.

Why is PyPy more efficient than CPython? ----------------------------------------

The PyPy project is a Python implementation which is 4.2x faster than CPython on average. PyPy developers chose to not fork CPython, but start from scratch to have more freedom in terms of optimization choices.

PyPy does not use reference counting, but a tracing garbage collector which moves objects. Objects can be allocated on the stack (or even not at all), rather than always having to be allocated on the heap.

Objects layouts are designed with performance in mind. For example, a list strategy stores integers directly as integers, rather than objects.

Moreover, PyPy also has a JIT compiler which emits fast code thanks to the efficient PyPy design.

I would be careful with presenting examples of PyPy optimisations here. Whichever you choose could easily give the impression that they are the most important changes that made PyPy faster and should therefore be followed in CPython. I doubt that there are any "top changes" that made the biggest difference for PyPy. Even large breakthroughs on their side stand on the shoulders of other important changes that may not have been visible by themselves in the performance graphs. CPython will not be rewritten from scratch, will continue to have its own infrastructure, and will therefore have its own specific tweaks that it will benefit from. Trying things out is fine, but there is no guarantee that following a specific change in PyPy will make a similar difference in CPython and its own ecosystem.

PyPy bottleneck: the Python C API --------------------------------- While PyPy is way more efficient than CPython to run pure Python code, it is as efficient or slower than CPython to run C extensions. [...] Hide implementation details ---------------------------

Hiding implementation details from the C API has multiple advantages:

* It becomes possible to experiment with more advanced optimizations in CPython than just micro-optimizations. For example, tagged pointers, and replace the garbage collector with a tracing garbage collector which can move objects. * Adding new features in CPython becomes easier. * PyPy should be able to avoid conversions to CPython objects in more cases: keep efficient PyPy objects. * It becomes easier to implement the C API for a new Python implementation. * More C extensions will be compatible with Python implementations other than CPython.

I understand the goal of experimenting with new optimisations and larger changes internally. If, however, the goal is to make it easier for other implementations to support (existing?) C extensions, then breaking all existing C extensions in CPython first does not strike me as a good way to get there. :) My feeling is that PyPy specifically is better served with the HPy API, which is different enough to consider it a mostly separate API, or an evolution of the limited API, if you want. Suggesting that extension authors support two different APIs is much, but forcing them to support the existing CPython C-API (for legacy reasons) and the changed CPython C-API (for future compatibility), and then asking them to support a separate C-API in addition (for platform independence, with performance penalties) seems stretching it a lot. If we want to make the life easier for PyPy, I think we should support their HPy effort. Creating additional churn on CPython side for extension authors will bind a lot of efforts on that side which will then not be available for them to try out and improve HPy while it's still early enough for major design choices. In the end, I think we shouldn't try to mix the two goals of "make it easier for other Python implementations" and "make it easier to optimise CPython", at least not from the start. The best overall solution is not necessarily the best for both goals independently, nor for all three sides (CPython, PyPy, extension authors).

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

Summary -------

* (**Completed**) Reorganize the C API header files: create ``Include/cpython/`` and ``Include/internal/`` subdirectories. * (**Completed**) Move private functions exposing implementation details to the internal C API. * (**Completed**) Convert macros to static inline functions.

Perfectly reasonable steps, IMHO.

* (**Completed**) Add new functions ``Py_SET_TYPE()``, ``Py_SET_REFCNT()`` and ``Py_SET_SIZE()``. The ``Py_TYPE()``, ``Py_REFCNT()`` and ``Py_SIZE()`` macros become functions which cannot be used as l-value. * (**Completed**) New C API functions must not return borrowed references. * (**In Progress**) Provide ``pythoncapi_compat.h`` header file. * (**In Progress**) Make structures opaque, add getter and setter functions. * (**Not Started**) Deprecate ``PySequence_Fast_ITEMS()``. * (**Not Started**) Convert ``PyTuple_GET_ITEM()`` and ``PyList_GET_ITEM()`` macros to static inline functions.

Most of these have the potential to break code, sometimes needlessly, AFAICT. Especially the efforts to block away the internal data structures annoy me. It's obviously ok if we don't require other implementations to provide this access, but CPython has these data structures and I think it should continue to expose them.

The internal C API is installed and can be used for specific usage like debuggers and profilers which must access structures members without executing code. C extensions using the internal C API are tightly coupled to a Python version and must be recompiled at each Python version. [...] Private functions which expose implementation details must be moved to the internal C API. If a C extension relies on a CPython private function which exposes CPython implementation details, other Python implementations have to re-implement this private function to support this C extension.

If we remove CPython specific features from the (de-facto) "official public Python C-API", then I think there should be a "public CPython 3.X C-API" that actively exposes the data structures natively, not just an "internal" one. That way, extension authors can take the usual decision between performance, maintenance effort and platform independence. I think it's perfectly ok to tell authors "if you use these, you may have to adapt your code for the next CPython release, which comes in a year's time". It's not so great to give them an unqualified "don't touch these!", because that will not help their decision process.

Make structures opaque ----------------------

All structures of the C API should become opaque: C extensions must use getter or setter functions to get or set structure members. For example, ``tuple->ob_item[0]`` must be replaced with ``PyTuple_GET_ITEM(tuple, 0)``.

To be able to move away from reference counting, ``PyObject`` must become opaque.

Careful with the wording. They don't have to be completely opaque. They can still be exposed in the "public CPython 3.X C-API" for those who want to use them, just not in the "public Python C-API". Changes to the ref-counting header obviously have a large impact on existing code, but ABI breakage here should be fine as long as we keep up API compatibility. Calling PyTuple_GET_ITEM() is perfectly ok, even if it's a straight macro (or inline function) that accesses the object struct, as long as that macro still works and does something reasonable in the next CPython release. That's exactly why extension code uses a macro and not a literal "tuple->ob_item[0]". For users who want ABI compatibility (and/or platform independence), we have the stable ABI and/or the limited API.

Currently, the reference counter ``PyObject.ob_refcnt`` is exposed in the C API. All structures must become opaque, since they "inherit" from PyObject. For, ``PyFloatObject`` inherits from ``PyObject``::

typedef struct { PyObject ob_base; double ob_fval; } PyFloatObject;

Please keep PyFloat_AS_DOUBLE() and friends do what they currently do.

Making ``PyObject`` fully opaque requires converting ``Py_INCREF()`` and ``Py_DECREF()`` macros to function calls. This change has an impact on performance. It is likely to be one of the very last changes when making structures opaque.

I like the HPy approach here of essentially replacing Py_INCREF(a); b = a; with a b = Py_NewRef(a); That gives a lot more flexibility in the underlying implementation than "Py_INCREF()", while trivially translating to b = (Py_INCREF(a), a); internally for existing CPython releases. I do not see why PyObject _must_ become opaque in CPython, as long as the access goes through macros. I'd reverse the order here: First add a "Py_NewRef()" macro, then see about replacing the internal GC implementation, and if that proves useful, think about how to re-implement the ref-counting macros based on it and change them under the hood, or break compatibility at that point if necessary. I don't see how making all PyObject structs opaque for user code helps here. The macros can continue to access any internals they like, they just don't have to be the same internals across CPython releases.

Making ``PyTypeObject`` structure opaque breaks C extensions declaring types statically (e.g. ``static PyTypeObject MyType = {...};``).

Not necessarily. There was an unimplemented feature proposed in PEP-3121, the PyType_Copy() function. https://www.python.org/dev/peps/pep-3121/#specification PyTypeObject does not have to be opaque. But it also doesn't have to be the same thing for defining and for using types. You could still define a type with a PyTypeObject struct and then copy it over into a heap type or other internal type structure from there. Whether that's better than using PyType_FromSpec(), maybe not, but at least it doesn't mean we have to break existing code that uses static extension type definitions.

Disallow using Py_TYPE() as l-value -----------------------------------

The ``Py_TYPE()`` function gets an object type, its ``PyObject.ob_type`` member. It is implemented as a macro which can be used as an l-value to set the type: ``Py_TYPE(obj) = new_type``. This code relies on the assumption that ``PyObject.ob_type`` can be modified directly. It prevents making the ``PyObject`` structure opaque.

New setter functions ``Py_SET_TYPE()``, ``Py_SET_REFCNT()`` and ``Py_SET_SIZE()`` are added and must be used instead.

Totally reasonable. In general, we should always allow C-API users to avoid hacks and write explicit code. That makes it easier for CPython to change its API implementation without changing the API. The above usages are very rare anyway and can just be part of the CPython-specfic API. Adapting the usages to other Python implementations is probably trivial as long as they provide similar features in some way. Speaking of which, "Py_SET_REFCNT()" is probably less explicit than "Py_INC_REFCNT()" and "Py_DEC_REFCNT()" macros would be, but the latter two seem more likely to a) fit the usual (or only?) use cases and b) be easily supportable by other Python implementations. I haven't come across a use case yet where I had to change a ref-count by more than 1, but allowing users to arbitrarily do that may require way more infrastructure under the hood than allowing them to create or remove a single reference to an object. I think explicit is really better than implicit here. The same does not seem to apply to "Py_SET_TYPE()" and "Py_SET_SIZE()", since any object or (applicable) container implementation would normally have to know its type and size, regardless of any implementation details.

Avoid functions returning PyObject** ------------------------------------

The ``PySequence_Fast_ITEMS()`` function gives a direct access to an array of ``PyObject*`` objects. The function is deprecated in favor of ``PyTuple_GetItem()`` and ``PyList_GetItem()``.

``PyTuple_GET_ITEM()`` can be abused to access directly the ``PyTupleObject.ob_item`` member::

PyObject **items = &PyTuple_GET_ITEM(0);

The ``PyTuple_GET_ITEM()`` and ``PyList_GET_ITEM()`` macros are converted to static inline functions to disallow that.

The same as I said above applies: CPython has these data structures. It should officially expose them, even if it does not guarantee them across minor releases. I would also ask that the usage PyObject **items = &PyTuple_GET_ITEM(0); should be replaced by an offical macro, e.g. "PyTuple_ITEMS()" (because the private version "_PyTuple_ITEMS()" of that already exists). This is yet another of those hacks where explicit would have been better than implicit. BTW, I think it is much less likely that the internal implementation of tuples changes than that of lists (which can benefit from native data type arrays à la "array.array()"). We should not take decisions about PyTuple based on arguments for PyList.

New pythoncapi_compat.h header file -----------------------------------

Making structures opaque requires modifying C extensions to use getter and setter functions. The practical issue is how to keep support for old Python versions which don't have these functions.

For example, in Python 3.10, it is no longer possible to use ``Py_TYPE()`` as an l-value. The new ``Py_SET_TYPE()`` function must be used instead::

#if PY_VERSION_HEX >= 0x030900A4 Py_SET_TYPE(&MyType, &PyType_Type); #else Py_TYPE(&MyType) = &PyType_Type; #endif

This code may ring a bell to developers who ported their Python code base from Python 2 to Python 3.

Python will distribute a new ``pythoncapi_compat.h`` header file which provides new C API functions to old Python versions. Example::

#if PY_VERSION_HEX < 0x030900A4 static inline void _Py_SET_TYPE(PyObject *ob, PyTypeObject *type) { ob->ob_type = type; } #define Py_SET_TYPE(ob, type) _Py_SET_TYPE((PyObject*)(ob), type) #endif // PY_VERSION_HEX < 0x030900A4

Using this header file, ``Py_SET_TYPE()`` can be used on old Python versions as well.

Developers can copy this file in their project, or even to only copy/paste the few functions needed by their C extension.

Yes, I think this is a good way to handle this. It keeps the final control over the implementation in CPython and gives a lot of freedom to extension developers.

Process to reduce the number of broken C extensions ===================================================

Process to reduce the number of broken C extensions when introducing C API incompatible changes listed in this PEP:

* Estimate how many popular C extensions are affected by the incompatible change. * Coordinate with maintainers of broken C extensions to prepare their code for the future incompatible change. * Introduce the incompatible changes in Python. The documentation must explain how to port existing code. It is recommended to merge such changes at the beginning of a development cycle to have more time for tests. * Changes which are the most likely to break a large number of C extensions should be announced on the capi-sig mailing list to notify C extensions maintainers to prepare their project for the next Python. * If the change breaks too many projects, reverting the change should be discussed, taking in account the number of broken packages, their importance in the Python community, and the importance of the change.

The coordination usually means reporting issues to the projects, or even proposing changes. It does not require waiting for a new release including fixes for every broken project.

Quite some effort, but yes, +1. This is a very fair way to communicate between both sides.

Since more and more C extensions are written using Cython, rather directly using the C API, it is important to ensure that Cython is prepared in advance for incompatible changes. It gives more time for C extension maintainers to release a new version with code generated with the updated Cython (for C extensions distributing the code generated by Cython).

Thank you! :) Cython isn't the only such tool, though. PyBind11 and a few others are probably also worth keeping in the loop. I think this can also be handled through the capi-sig mailing list. Any such project should naturally be interested in changes to the C-API and discussions about its evolution.

The important part is coordination and finding a balance between CPython evolutions and backward compatibility. For example, breaking a random, old, obscure and unmaintained C extension on PyPI is less severe than breaking numpy.

This sounds like a common CI testing infrastructure would help all sides. Currently, we have something like that mostly working by having different projects integrate with each other's master branch, e.g. Pandas, NumPy, Cython, and notifying each other of detected breakages. It's mostly every project setting up its own CI on travis&Co here, so a bit of duplicated work on all sides. Not sure if that's inherently bad, but there's definitely some room for generalisation and improvements. Again, thanks Victor for pushing these efforts. Even if me and others are giving you a hard time getting your proposals accepted, I appreciate the work that you put into improving the ecosystem(s). Stefan

On 2020-06-22 14:10, Victor Stinner wrote:

Hi,

PEP available at: https://www.python.org/dev/peps/pep-0620/

<introduction> This PEP is the result of 4 years of research work on the C API: https://pythoncapi.readthedocs.io/

It's the third version. The first version (2017) proposed to add a "new C API" and advised C extensions maintainers to opt-in for it: it was basically the same idea as PEP 384 limited C API but in a different color. Well, I had no idea of what I was doing :-) The second version (April 2020) proposed to add a new Python runtime built from the same code base as the regular Python runtime but in a different build mode, the regular Python would continue to be fully compatible.

I wrote the third version, the PEP 620, from scratch. It now gives an explicit and concrete list of incompatible C API changes, and has better motivation and rationale sections. The main PEP novelty is the new pythoncapi_compat.h header file distributed with Python to provide new C API functions to old Python versions, the second novelty is the process to reduce the number of broken C extensions.

Whereas PEPs are usually implemented in a single Python version, the implementation of this PEP is expected to be done carefully over multiple Python versions. The PEP lists many changes which are already implemented in Python 3.7, 3.8 and 3.9. It defines a process to reduce the number of broken C extensions when introducing the incompatible C API changes listed in the PEP. The process dictates the rhythm of these changes. </introduction>

PEP: 620 Title: Hide implementation details from the C API Author: Victor Stinner Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 19-June-2020 Python-Version: 3.10

Abstract ========

Introduce C API incompatible changes to hide implementation details.

Once most implementation details will be hidden, evolution of CPython internals would be less limited by C API backward compatibility issues. It will be way easier to add new features.

It becomes possible to experiment with more advanced optimizations in CPython than just micro-optimizations, like tagged pointers.

Define a process to reduce the number of broken C extensions.

The implementation of this PEP is expected to be done carefully over multiple Python versions. It already started in Python 3.7 and most changes are already completed. The `Process to reduce the number of broken C extensions`_ dictates the rhythm.

Motivation ==========

The C API blocks CPython evolutions -----------------------------------

Adding or removing members of C structures is causing multiple backward compatibility issues.

Adding a new member breaks the stable ABI (PEP 384), especially for types declared statically (e.g. ``static PyTypeObject MyType = {...};``).

PyTypeObject is explicitly not part of the stable ABI, see PEP 384: https://www.python.org/dev/peps/pep-0384/#structures I don't know why Py_TPFLAGS_HAVE_FINALIZE was added, but it wasn't for the PEP 384 stable ABI. Can you find a different example, so users are not misled?

In Python 3.4, the PEP 442 "Safe object finalization" added the ``tp_finalize`` member at the end of the ``PyTypeObject`` structure. For ABI backward compatibility, a new ``Py_TPFLAGS_HAVE_FINALIZE`` type flag was required to announce if the type structure contains the ``tp_finalize`` member. The flag was removed in Python 3.8 (`bpo-32388 https://bugs.python.org/issue32388`_).

The ``PyTypeObject.tp_print`` member, deprecated since Python 3.0 released in 2009, has been removed in the Python 3.8 development cycle. But the change broke too many C extensions and had to be reverted before 3.8 final release. Finally, the member was removed again in Python 3.9.

C extensions rely on the ability to access directly structure members, indirectly through the C API, or even directly.

I think you want to remove a "directly" from that sentence.

Modifying structures like ``PyListObject`` cannot be even considered.

The ``PyTypeObject`` structure is the one which evolved the most, simply because there was no other way to evolve CPython than modifying it.

In the C API, all Python objects are passed as ``PyObject*``: a pointer to a ``PyObject`` structure. Experimenting tagged pointers in CPython is blocked by the fact that a C extension can technically dereference a ``PyObject*`` pointer and access ``PyObject`` members. Small "objects" can be stored as a tagged pointer with no concrete ``PyObject`` structure.

I think this would be confusing to people who don't already know what you mean. May I suggest: A C extension can technically dereference a ``PyObject*`` pointer and access ``PyObject`` members. This prevents experiments like tagged pointers (storing small values as ``PyObject*`` which does not point to a valid ``PyObject`` structure).

Replacing Python garbage collector with a tracing garbage collector would also need to remove ``PyObject.ob_refcnt`` reference counter, whereas currently ``Py_INCREF()`` and ``Py_DECREF()`` macros access directly to ``PyObject.ob_refcnt``.

Same CPython design since 1990: structures and reference counting -----------------------------------------------------------------

When the CPython project was created, it was written with one principle: keep the implementation simple enough so it can be maintained by a single developer. CPython complexity grew a lot and many micro-optimizations have been implemented, but CPython core design has not changed.

Members of ``PyObject`` and ``PyTupleObject`` structures have not changed since the "Initial revision" commit (1990)::

#define OB_HEAD \ unsigned int ob_refcnt; \ struct _typeobject *ob_type;

typedef struct _object { OB_HEAD } object;

typedef struct { OB_VARHEAD object *ob_item[1]; } tupleobject;

Only names changed: ``object`` was renamed to ``PyObject`` and ``tupleobject`` was renamed to ``PyTupleObject``.

CPython still tracks Python objects lifetime using reference counting internally and for third party C extensions (through the Python C API).

All Python objects must be allocated on the heap and cannot be moved.

Why is PyPy more efficient than CPython? ----------------------------------------

The PyPy project is a Python implementation which is 4.2x faster than CPython on average. PyPy developers chose to not fork CPython, but start from scratch to have more freedom in terms of optimization choices.

PyPy does not use reference counting, but a tracing garbage collector which moves objects. Objects can be allocated on the stack (or even not at all), rather than always having to be allocated on the heap.

Objects layouts are designed with performance in mind. For example, a list strategy stores integers directly as integers, rather than objects.

Moreover, PyPy also has a JIT compiler which emits fast code thanks to the efficient PyPy design.

PyPy bottleneck: the Python C API ---------------------------------

While PyPy is way more efficient than CPython to run pure Python code, it is as efficient or slower than CPython to run C extensions.

Since the C API requires ``PyObject*`` and allows to access directly structure members, PyPy has to associate a CPython object to PyPy objects and maintain both consistent. Converting a PyPy object to a CPython object is inefficient. Moreover, reference counting also has to be implemented on top of PyPy tracing garbage collector.

These conversions are required because the Python C API is too close to the CPython implementation: there is no high-level abstraction. For example, structures members are part of the public C API and nothing prevents a C extension to get or set directly ``PyTupleObject.ob_item[0]`` (the first item of a tuple).

See `Inside cpyext: Why emulating CPython C API is so Hard https://morepypy.blogspot.com/2018/09/inside-cpyext-why-emulating-cpython-c....`_ (Sept 2018) by Antonio Cuni for more details.

Rationale =========

Hide implementation details ---------------------------

Hiding implementation details from the C API has multiple advantages:

* It becomes possible to experiment with more advanced optimizations in CPython than just micro-optimizations. For example, tagged pointers, and replace the garbage collector with a tracing garbage collector which can move objects. * Adding new features in CPython becomes easier. * PyPy should be able to avoid conversions to CPython objects in more cases: keep efficient PyPy objects. * It becomes easier to implement the C API for a new Python implementation. * More C extensions will be compatible with Python implementations other than CPython.

Relationship with the limited C API -----------------------------------

The PEP 384 "Defining a Stable ABI" is in Python 3.4. It introduces the "limited C API": a subset of the C API. When the limited C API is used, it becomes possible to build a C extension only once and use it on multiple Python versions: that's the stable ABI.

The main limitation of the PEP 384 is that C extensions have to opt-in for the limited C API. Only very few projects made this choice, usually to ease distribution of binaries, especially on Windows.

This PEP moves the C API towards the limited C API.

Ideally, the C API will become the limited C API and all C extensions will use the stable ABI, but this is out of this PEP scope.

And I would prefer to move the limited API closer to the regular C API. So we do agree on the goal :)

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

Summary -------

* (**Completed**) Reorganize the C API header files: create ``Include/cpython/`` and ``Include/internal/`` subdirectories. * (**Completed**) Move private functions exposing implementation details to the internal C API. * (**Completed**) Convert macros to static inline functions. * (**Completed**) Add new functions ``Py_SET_TYPE()``, ``Py_SET_REFCNT()`` and ``Py_SET_SIZE()``. The ``Py_TYPE()``, ``Py_REFCNT()`` and ``Py_SIZE()`` macros become functions which cannot be used as l-value. * (**Completed**) New C API functions must not return borrowed references. * (**In Progress**) Provide ``pythoncapi_compat.h`` header file. * (**In Progress**) Make structures opaque, add getter and setter functions. * (**Not Started**) Deprecate ``PySequence_Fast_ITEMS()``. * (**Not Started**) Convert ``PyTuple_GET_ITEM()`` and ``PyList_GET_ITEM()`` macros to static inline functions.

What does **Completed** mean? All work in that category is done, and any new changes in the category will require a new PEP (or a change to this PEP)? It seems that this PEP is largely asking for forgiveness, so trying to find better ways to solve the problems would be a waste of time at this point.

Reorganize the C API header files ---------------------------------

The first consumer of the C API was Python itself. There is no clear separation between APIs which must not be used outside Python, and API which are public on purpose.

Header files must be reorganized in 3 API:

* ``Include/`` directory is the limited C API: no implementation details, structures are opaque. C extensions using it get a stable ABI. * ``Include/cpython/`` directory is the CPython C API: less "portable" API, depends more on the Python version, expose some implementation details, few incompatible changes can happen. * ``Internal/internal/`` directory is the internal C API: implementation details, incompatible changes are likely at each Python release.

The creation of the ``Include/cpython/`` directory is fully backward compatible. ``Include/cpython/`` header files cannot be included directly and are included automatically by ``Include/`` header files when the ``Py_LIMITED_API`` macro is not defined.

The internal C API is installed and can be used for specific usage like debuggers and profilers which must access structures members without executing code. C extensions using the internal C API are tightly coupled to a Python version and must be recompiled at each Python version.

**STATUS**: Completed (in Python 3.8)

The reorganization of header files started in Python 3.7 and was completed in Python 3.8:

* `bpo-35134 https://bugs.python.org/issue35134`_: Add a new Include/cpython/ subdirectory for the "CPython API" with implementation details. * `bpo-35081 https://bugs.python.org/issue35081`_: Move internal headers to ``Include/internal/``

Move private functions to the internal C API --------------------------------------------

Private functions which expose implementation details must be moved to the internal C API.

If a C extension relies on a CPython private function which exposes CPython implementation details, other Python implementations have to re-implement this private function to support this C extension.

**STATUS**: Completed (in Python 3.9)

Private functions moved to the internal C API in Python 3.8:

* ``_PyObject_GC_TRACK()``, ``_PyObject_GC_UNTRACK()``

Macros and functions excluded from the limited C API in Python 3.9:

* ``_PyObject_SIZE()``, ``_PyObject_VAR_SIZE()`` * ``PyThreadState_DeleteCurrent()`` * ``PyFPE_START_PROTECT()``, ``PyFPE_END_PROTECT()`` * ``_Py_NewReference()``, ``_Py_ForgetReference()`` * ``_PyTraceMalloc_NewReference()`` * ``_Py_GetRefTotal()``

Private functions moved to the internal C API in Python 3.9:

* GC functions like ``_Py_AS_GC()``, ``_PyObject_GC_IS_TRACKED()`` and ``_PyGCHead_NEXT()`` * ``_Py_AddToAllObjects()`` (not exported) * ``_PyDebug_PrintTotalRefs()``, ``_Py_PrintReferences()``, ``_Py_PrintReferenceAddresses()`` (not exported)

Public "clear free list" functions moved to the internal C API an renamed to private functions and in Python 3.9:

* ``PyAsyncGen_ClearFreeLists()`` * ``PyContext_ClearFreeList()`` * ``PyDict_ClearFreeList()`` * ``PyFloat_ClearFreeList()`` * ``PyFrame_ClearFreeList()`` * ``PyList_ClearFreeList()`` * ``PyTuple_ClearFreeList()`` * Functions simply removed:

* ``PyMethod_ClearFreeList()`` and ``PyCFunction_ClearFreeList()``: bound method free list removed in Python 3.9. * ``PySet_ClearFreeList()``: set free list removed in Python 3.4. * ``PyUnicode_ClearFreeList()``: Unicode free list removed in Python 3.3.

Convert macros to static inline functions -----------------------------------------

Converting macros to static inline functions have multiple advantages:

* Functions have well defined parameter types and return type. * Functions can use variables with a well defined scope (the function). * Debugger can be put breakpoints on functions and profilers can display the function name in the call stacks. In most cases, it works even when a static inline function is inlined. * Functions don't have `macros pitfalls https://gcc.gnu.org/onlinedocs/cpp/Macro-Pitfalls.html`_.

Converting macros to static inline functions should only impact very few C extensions which use macros in unusual ways.

For backward compatibility, functions must continue to accept any type, not only ``PyObject*``, to avoid compiler warnings, since most macros cast their parameters to ``PyObject*``.

Python 3.6 requires C compilers to support static inline functions: the PEP 7 requires a subset of C99.

**STATUS**: Completed (in Python 3.9)

Macros converted to static inline functions in Python 3.8:

* ``Py_INCREF()``, ``Py_DECREF()`` * ``Py_XINCREF()``, ``Py_XDECREF()`` * ``PyObject_INIT()``, ``PyObject_INIT_VAR()`` * ``_PyObject_GC_TRACK()``, ``_PyObject_GC_UNTRACK()``, ``_Py_Dealloc()``

Macros converted to regular functions in Python 3.9:

* ``Py_EnterRecursiveCall()``, ``Py_LeaveRecursiveCall()`` (added to the limited C API) * ``PyObject_INIT()``, ``PyObject_INIT_VAR()`` * ``PyObject_GET_WEAKREFS_LISTPTR()`` * ``PyObject_CheckBuffer()`` * ``PyIndex_Check()`` * ``PyObject_IS_GC()`` * ``PyObject_NEW()`` (alias to ``PyObject_New()``), ``PyObject_NEW_VAR()`` (alias to ``PyObject_NewVar()``) * ``PyType_HasFeature()`` (always call ``PyType_GetFlags()``) * ``Py_TRASHCAN_BEGIN_CONDITION()`` and ``Py_TRASHCAN_END()`` macros now call functions which hide implementation details, rather than accessing directly members of the ``PyThreadState`` structure.

Make structures opaque ----------------------

All structures of the C API should become opaque: C extensions must use getter or setter functions to get or set structure members. For example, ``tuple->ob_item[0]`` must be replaced with ``PyTuple_GET_ITEM(tuple, 0)``.

All structures? I don't think we can make PyModuleDef opaque, for example. PEP 384 lists more structures, some of which I don't think should be opaque: https://www.python.org/dev/peps/pep-0384/#structures

To be able to move away from reference counting, ``PyObject`` must become opaque. Currently, the reference counter ``PyObject.ob_refcnt`` is exposed in the C API. All structures must become opaque, since they "inherit" from PyObject. For, ``PyFloatObject`` inherits from ``PyObject``::

typedef struct { PyObject ob_base; double ob_fval; } PyFloatObject;

Making ``PyObject`` fully opaque requires converting ``Py_INCREF()`` and ``Py_DECREF()`` macros to function calls. This change has an impact on performance. It is likely to be one of the very last changes when making structures opaque.

Making ``PyTypeObject`` structure opaque breaks C extensions declaring types statically (e.g. ``static PyTypeObject MyType = {...};``). C extensions must use ``PyType_FromSpec()`` to allocate types on the heap instead. Using heap types has other advantages like being compatible with subinterpreters. Combined with PEP 489 "Multi-phase extension module initialization", it makes a C extension behavior closer to a Python module, like allowing to create more than one module instance.

Making ``PyThreadState`` structure opaque requires adding getter and setter functions for members used by C extensions.

**STATUS**: In Progress (started in Python 3.8)

The ``PyInterpreterState`` structure was made opaque in Python 3.8 (`bpo-35886 https://bugs.python.org/issue35886`_) and the ``PyGC_Head`` structure (`bpo-40241 https://bugs.python.org/issue40241`_) was made opaque in Python 3.9.

Issues tracking the work to prepare the C API to make following structures opaque:

* ``PyObject``: `bpo-39573 https://bugs.python.org/issue39573`_ * ``PyTypeObject``: `bpo-40170 https://bugs.python.org/issue40170`_ * ``PyFrameObject``: `bpo-40421 https://bugs.python.org/issue40421`_

* Python 3.9 adds ``PyFrame_GetCode()`` and ``PyFrame_GetBack()`` getter functions, and moves ``PyFrame_GetLineNumber`` to the limited C API.

* ``PyThreadState``: `bpo-39947 https://bugs.python.org/issue39947`_

* Python 3.9 adds 3 getter functions: ``PyThreadState_GetFrame()``, ``PyThreadState_GetID()``, ``PyThreadState_GetInterpreter()``.

Disallow using Py_TYPE() as l-value -----------------------------------

The ``Py_TYPE()`` function gets an object type, its ``PyObject.ob_type`` member. It is implemented as a macro which can be used as an l-value to set the type: ``Py_TYPE(obj) = new_type``. This code relies on the assumption that ``PyObject.ob_type`` can be modified directly. It prevents making the ``PyObject`` structure opaque.

New setter functions ``Py_SET_TYPE()``, ``Py_SET_REFCNT()`` and ``Py_SET_SIZE()`` are added and must be used instead.

The ``Py_TYPE()``, ``Py_REFCNT()`` and ``Py_SIZE()`` macros must be converted to static inline functions which can not be used as l-value.

For example, the ``Py_TYPE()`` macro::

#define Py_TYPE(ob) (((PyObject*)(ob))->ob_type)

becomes::

#define _PyObject_CAST_CONST(op) ((const PyObject*)(op))

static inline PyTypeObject* _Py_TYPE(const PyObject *ob) { return ob->ob_type; }

#define Py_TYPE(ob) _Py_TYPE(_PyObject_CAST_CONST(ob))

**STATUS**: Completed (in Python 3.10)

New functions ``Py_SET_TYPE()``, ``Py_SET_REFCNT()`` and ``Py_SET_SIZE()`` were added to Python 3.9.

In Python 3.10, ``Py_TYPE()``, ``Py_REFCNT()`` and ``Py_SIZE()`` can no longer be used as l-value and the new setter functions must be used instead.

New C API functions must not return borrowed references -------------------------------------------------------

When a function returns a borrowed reference, Python cannot track when the caller stops using this reference.

For example, if the Python ``list`` type is specialized for small integers, store directly "raw" numbers rather than Python objects, ``PyList_GetItem()`` has to create a temporary Python object. The problem is to decide when it is safe to delete the temporary object.

The general guidelines is to avoid returning borrowed references for new C API functions.

No function returning borrowed functions is scheduled for removal by this PEP.

**STATUS**: Completed (in Python 3.9)

In Python 3.9, new C API functions returning Python objects only return strong references:

* ``PyFrame_GetBack()`` * ``PyFrame_GetCode()`` * ``PyObject_CallNoArgs()`` * ``PyObject_CallOneArg()`` * ``PyThreadState_GetFrame()``

Avoid functions returning PyObject** ------------------------------------

The ``PySequence_Fast_ITEMS()`` function gives a direct access to an array of ``PyObject*`` objects. The function is deprecated in favor of ``PyTuple_GetItem()`` and ``PyList_GetItem()``.

``PyTuple_GET_ITEM()`` can be abused to access directly the ``PyTupleObject.ob_item`` member::

PyObject **items = &PyTuple_GET_ITEM(0);

The ``PyTuple_GET_ITEM()`` and ``PyList_GET_ITEM()`` macros are converted to static inline functions to disallow that.

**STATUS**: Not Started

New pythoncapi_compat.h header file -----------------------------------

Making structures opaque requires modifying C extensions to use getter and setter functions. The practical issue is how to keep support for old Python versions which don't have these functions.

For example, in Python 3.10, it is no longer possible to use ``Py_TYPE()`` as an l-value. The new ``Py_SET_TYPE()`` function must be used instead::

#if PY_VERSION_HEX >= 0x030900A4 Py_SET_TYPE(&MyType, &PyType_Type); #else Py_TYPE(&MyType) = &PyType_Type; #endif

This code may ring a bell to developers who ported their Python code base from Python 2 to Python 3.

Python will distribute a new ``pythoncapi_compat.h`` header file which provides new C API functions to old Python versions. Example::

#if PY_VERSION_HEX < 0x030900A4 static inline void _Py_SET_TYPE(PyObject *ob, PyTypeObject *type) { ob->ob_type = type; } #define Py_SET_TYPE(ob, type) _Py_SET_TYPE((PyObject*)(ob), type) #endif // PY_VERSION_HEX < 0x030900A4

Using this header file, ``Py_SET_TYPE()`` can be used on old Python versions as well.

Developers can copy this file in their project, or even to only copy/paste the few functions needed by their C extension.

**STATUS**: In Progress (implemented but not distributed by CPython yet)

The ``pythoncapi_compat.h`` header file is currently developer at: https://github.com/pythoncapi/pythoncapi_compat

Process to reduce the number of broken C extensions ===================================================

Process to reduce the number of broken C extensions when introducing C API incompatible changes listed in this PEP:

* Estimate how many popular C extensions are affected by the incompatible change. * Coordinate with maintainers of broken C extensions to prepare their code for the future incompatible change. * Introduce the incompatible changes in Python. The documentation must explain how to port existing code. It is recommended to merge such changes at the beginning of a development cycle to have more time for tests. * Changes which are the most likely to break a large number of C extensions should be announced on the capi-sig mailing list to notify C extensions maintainers to prepare their project for the next Python. * If the change breaks too many projects, reverting the change should be discussed, taking in account the number of broken packages, their importance in the Python community, and the importance of the change.

What is "popular"? What is "too many"? Who decides?

The coordination usually means reporting issues to the projects, or even proposing changes. It does not require waiting for a new release including fixes for every broken project.

Since more and more C extensions are written using Cython, rather directly using the C API, it is important to ensure that Cython is prepared in advance for incompatible changes. It gives more time for C extension maintainers to release a new version with code generated with the updated Cython (for C extensions distributing the code generated by Cython).

Future incompatible changes can be announced by deprecating a function in the documentation and by annotating the function with ``Py_DEPRECATED()``.

How long should functions be deprecated before being removed?

But making a structure opaque and preventing the usage of a macro as l-value cannot be deprecated with ``Py_DEPRECATED()``.

So, how should things like this be deprecated? And for how long?

The important part is coordination and finding a balance between CPython evolutions and backward compatibility. For example, breaking a random, old, obscure and unmaintained C extension on PyPI is less severe than breaking numpy.

How will this balance be found?

If a change is reverted, we move back to the coordination step to better prepare the change. Once more C extensions are ready, the incompatible change can be reconsidered.

When in the process should it be reverted?

Version History ===============

* Version 3, June 2020: PEP rewritten from scratch. Python now distributes a new ``pythoncapi_compat.h`` header and a process is defined to reduce the number of broken C extensions when introducing C API incompatible changes listed in this PEP. * Version 2, April 2020: `PEP: Modify the C API to hide implementation details https://mail.python.org/archives/list/python-dev@python.org/thread/HKM774XKU...`_. * Version 1, July 2017: `PEP: Hide implementation details in the C API https://mail.python.org/archives/list/python-ideas@python.org/thread/6XATDGW...`_ sent to python-ideas

Copyright =========

This document has been placed in the public domain.