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merged 18 commits into from
Sep 5, 2025
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gh-137210: Add a struct, slot & function for checking an extension's ABI #137212

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894ef3d
Add _PyUnicodeWriter_FormatV
encukou Jul 24, 2025
72349ba
Add capi test file for modsupport
encukou Jul 24, 2025
3b6aa96
Add PyABIInfo
encukou Jul 24, 2025
c015606
Add _Py_INTERNAL_ABI_SLOT (to _datetime for now)
encukou Jul 24, 2025
c5740af
Add docs
encukou Jul 29, 2025
da293e3
Add to limited API manifest
encukou Jul 29, 2025
44df416
Un-document PyABIInfo_FREETHREADING_AGNOSTIC for now
encukou Jul 29, 2025
feb428d
Blurb & What's New
encukou Jul 29, 2025
aa1eb4a
Fix docs
encukou Jul 29, 2025
b628d7d
Remove duplicated test
encukou Jul 29, 2025
4800a5d
Apply suggestions from code review
encukou Jul 30, 2025
4c9e918
Casts to avoid data loss warnings
encukou Jul 30, 2025
1b18761
Use hex literal for 3.15
encukou Aug 7, 2025
814ac60
Merge in the main branch
encukou Aug 7, 2025
f6f5b1d
Merge in the main branch
encukou Aug 26, 2025
2644080
Add example of intended usage before the API documentation
encukou Aug 26, 2025
cb0309f
Restore changelog / comments for PYTHON_API_VERSION
encukou Aug 26, 2025
7492e16
Merge branch 'main' into abiinfo-slot
encukou Sep 5, 2025
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22 changes: 22 additions & 0 deletions Doc/c-api/module.rst
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Expand Up @@ -388,6 +388,28 @@ The available slot types are:

.. versionadded:: 3.13

.. c:macro:: Py_mod_abi

A pointer to a :c:struct:`PyABIInfo` structure that describes the ABI that
the extension is using.

When the module is loaded, the :c:struct:`!PyABIInfo` in this slot is checked
using :c:func:`PyABIInfo_Check`.

A suitable :c:struct:`!PyABIInfo` variable can be defined using the
:c:macro:`PyABIInfo_VAR` macro, as in:

.. code-block:: c

PyABIInfo_VAR(abi_info);

static PyModuleDef_Slot mymodule_slots[] = {
{Py_mod_abi, &abi_info},
...
};

.. versionadded:: 3.15


.. _moduledef-dynamic:

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151 changes: 148 additions & 3 deletions Doc/c-api/stable.rst
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Expand Up @@ -2,9 +2,9 @@

.. _stable:

***************
C API Stability
***************
***********************
C API and ABI Stability
***********************

Unless documented otherwise, Python's C API is covered by the Backwards
Compatibility Policy, :pep:`387`.
Expand Down Expand Up @@ -199,6 +199,151 @@
third-party distributors.


ABI Checking
============

.. versionadded:: next

Python includes a rudimentary check for ABI compatibility.

This check is not comprehensive.
It only guards against common cases of incompatible modules being
installed for the wrong interpreter.
It also does not take :ref:`platform incompatibilities <stable-abi-platform>`
into account.
It can only be done after an extension is successfully loaded.

Despite these limitations, it is recommended that extension modules use this
mechanism, so that detectable incompatibilities raise exceptions rather than
crash.

Most modules can use this check via the :c:data:`Py_mod_abi`
slot and the :c:macro:`PyABIInfo_VAR` macro.
The full API is described below for advanced use cases.

.. c:function:: int PyABIInfo_Check(PyABIInfo *info, const char *module_name)
Verify that the given *info* is compatible with the currently running
interpreter.
Return 0 on success. On failure, raise an exception and return -1.
If the ABI is incompatible, the raised exception will be :py:exc:`ImportError`.
The *module_name* argument can be ``NULL``, or point to a NUL-terminated
UTF-8-encoded string used for error messages.
Note that if *info* describes the ABI taht the current code uses (as defined
by :c:macro:`PyABIInfo_VAR`, for example), using any other Python C API
may lead to crashes.
In particular, it is not safe to examine the raised exception.
.. versionadded:: next
.. c:macro:: PyABIInfo_VAR(NAME)
Define a static :c:struct:`PyABIInfo` variable with the given *NAME* that
describes the ABI that the current code will use.
This macro expands to:
.. code-block:: c
static PyABIInfo NAME = {
1, 0,
PyABIInfo_DEFAULT_FLAGS,
PY_VERSION_HEX,
PyABIInfo_DEFAULT_ABI_VERSION
}
.. versionadded:: next

.. c:type:: PyABIInfo
.. c:member:: uint8_t abiinfo_major_version
The major version of :c:struct:`PyABIInfo`. Can be set to:

* ``0`` to skip all checking, or
* ``1`` to specify this version of :c:struct:`!PyABIInfo`.

.. c:member:: uint8_t abiinfo_minor_version
The major version of :c:struct:`PyABIInfo`.
Must be set to ``0``; larger values are reserved for backwards-compatible
future versions of :c:struct:`!PyABIInfo`.

.. c:member:: uint16_t flags
.. c:namespace:: NULL
This field is usually set to the following macro:

.. c:macro:: PyABIInfo_DEFAULT_FLAGS
Default flags, based on current values of macros such as
:c:macro:`Py_LIMITED_API` and :c:macro:`Py_GIL_DISABLED`.

Alternately, the field can be set to to the following flags, combined

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by bitwise OR.
Unused bits must be set to zero.

ABI variant -- one of:

.. c:macro:: PyABIInfo_STABLE
Specifies that the stable ABI is used.

.. c:macro:: PyABIInfo_INTERNAL
Specifies ABI specific to a particular build of CPython.
Internal use only.

Free-threading compatibility -- one of:
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We're still planning to get rid of these options once FT is merged, right? Maybe we ought to use abiflags here (i.e. a string) instead, for future extensibility? This isn't going to be checked anywhere that it needs to be bitflags.

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How would you encode being compatible with both GIL and FT?

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IMO, we can remove them when there are no GIL-only extensions around any more. That would be a lot of time after FT is the only option.


.. c:macro:: PyABIInfo_FREETHREADED
Specifies ABI compatible with free-threading builds of CPython.
(That is, ones compiled with :option:`--disable-gil`; with ``t``

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.. c:macro:: PyABIInfo_GIL
Specifies ABI compatible with non-free-threading builds of CPython
(ones compiled *without* :option:`--disable-gil`).

.. c:member:: uint32_t build_version
The version of the Python headers used to build the code, in the format
used by :c:macro:`PY_VERSION_HEX`.

This can be set to ``0`` to skip any checks related to this field.
This option is meant mainly for projects that do not use the CPython
headers directly, and do not emulate a specific version of them.

.. c:member:: uint32_t abi_version
The ABI version.

For the Stable ABI, this field should be the value of
:c:macro:`Py_LIMITED_API`
(except if :c:macro:`Py_LIMITED_API` is ``3``; use
:c:expr:`Py_PACK_VERSION(3, 2)` in that case).
Comment on lines +340 to +341
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Seems easier for us to just check this on our side than to expect users to get it right?

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And isn't the legacy value 1 rather than 3?

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Well, users should use PyABIInfo_DEFAULT_ABI_VERSION or PyABIInfo_VAR -- those are our side too.
I prefer the data to be simpler, but if you want, we can keep the exception in.

The legacy value is 3. (Not that it matters that much; it's always compared to a Python version.)


Otherwise, it should be set to :c:macro:`PY_VERSION_HEX`.

It can also be set to ``0`` to skip any checks related to this field.

.. c:namespace:: NULL
.. c:macro:: PyABIInfo_DEFAULT_ABI_VERSION
The value that should be used for this field, based on current
values of macros such as :c:macro:`Py_LIMITED_API`,

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.. versionadded:: next


.. _limited-api-list:

Contents of Limited API
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3 changes: 3 additions & 0 deletions Doc/data/stable_abi.dat
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3 changes: 3 additions & 0 deletions Doc/using/configure.rst
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Expand Up @@ -293,6 +293,9 @@ General Options

.. option:: --disable-gil

.. c:macro:: Py_GIL_DISABLED
:no-typesetting:

Enables support for running Python without the :term:`global interpreter
lock` (GIL): free threading build.

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5 changes: 5 additions & 0 deletions Doc/whatsnew/3.15.rst
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Expand Up @@ -516,6 +516,11 @@ New features
a string. See the documentation for caveats.
(Contributed by Petr Viktorin in :gh:`131510`)

* Add API for checking an extension module's ABI compatibility:
:c:data:`Py_mod_abi`, :c:func:`PyABIInfo_Check`, :c:macro:`PyABIInfo_VAR`
and :c:data:`Py_mod_abi`.
(Contributed by Petr Viktorin in :gh:`137210`)


Porting to Python 3.15
----------------------
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12 changes: 12 additions & 0 deletions Include/cpython/modsupport.h
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Expand Up @@ -24,3 +24,15 @@ typedef struct _PyArg_Parser {

PyAPI_FUNC(int) _PyArg_ParseTupleAndKeywordsFast(PyObject *, PyObject *,
struct _PyArg_Parser *, ...);

#ifdef Py_BUILD_CORE
// Internal; defined here to avoid explicitly including pycore_modsupport.h
#define _Py_INTERNAL_ABI_SLOT \
{Py_mod_abi, (void*) &(PyABIInfo) { \
.abiinfo_major_version = 1, \
.abiinfo_minor_version = 0, \
.flags = PyABIInfo_INTERNAL, \
.build_version = PY_VERSION_HEX, \
.abi_version = PY_VERSION_HEX }} \
///////////////////////////////////////////////////////
#endif
6 changes: 6 additions & 0 deletions Include/internal/pycore_unicodeobject.h
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Expand Up @@ -82,6 +82,12 @@ extern int _PyUnicode_FormatAdvancedWriter(
Py_ssize_t start,
Py_ssize_t end);

/* PyUnicodeWriter_Format, with va_list instead of `...` */
extern int _PyUnicodeWriter_FormatV(
PyUnicodeWriter *writer,
const char *format,
va_list vargs);

/* --- UTF-7 Codecs ------------------------------------------------------- */

extern PyObject* _PyUnicode_EncodeUTF7(
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119 changes: 72 additions & 47 deletions Include/modsupport.h
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Expand Up @@ -56,55 +56,14 @@

#define Py_CLEANUP_SUPPORTED 0x20000

/* The API and ABI versions are left for backwards compatibility.
They've not been updated since 2006 and 2010, respectively.
API/ABI versioning is now tied to the CPython version.
The *_VERSION and *_STRING symbols should define the same value; as
number and string literal respectively. Make sure the definitions match.
*/
#define PYTHON_API_VERSION 1013
#define PYTHON_API_STRING "1013"
/* The API version is maintained (independently from the Python version)
so we can detect mismatches between the interpreter and dynamically
loaded modules. These are diagnosed by an error message but
the module is still loaded (because the mismatch can only be tested
after loading the module). The error message is intended to
explain the core dump a few seconds later.
The symbol PYTHON_API_STRING defines the same value as a string
literal. *** PLEASE MAKE SURE THE DEFINITIONS MATCH. ***
Please add a line or two to the top of this log for each API
version change:
22-Feb-2006 MvL 1013 PEP 353 - long indices for sequence lengths
19-Aug-2002 GvR 1012 Changes to string object struct for
interning changes, saving 3 bytes.
17-Jul-2001 GvR 1011 Descr-branch, just to be on the safe side
25-Jan-2001 FLD 1010 Parameters added to PyCode_New() and
PyFrame_New(); Python 2.1a2
14-Mar-2000 GvR 1009 Unicode API added
3-Jan-1999 GvR 1007 Decided to change back! (Don't reuse 1008!)
3-Dec-1998 GvR 1008 Python 1.5.2b1
18-Jan-1997 GvR 1007 string interning and other speedups
11-Oct-1996 GvR renamed Py_Ellipses to Py_Ellipsis :-(
30-Jul-1996 GvR Slice and ellipses syntax added
23-Jul-1996 GvR For 1.4 -- better safe than sorry this time :-)
7-Nov-1995 GvR Keyword arguments (should've been done at 1.3 :-( )
10-Jan-1995 GvR Renamed globals to new naming scheme
9-Jan-1995 GvR Initial version (incompatible with older API)
*/

/* The PYTHON_ABI_VERSION is introduced in PEP 384. For the lifetime of
Python 3, it will stay at the value of 3; changes to the limited API
must be performed in a strictly backwards-compatible manner. */
#define PYTHON_ABI_VERSION 3
#define PYTHON_ABI_STRING "3"

Expand Down Expand Up @@ -134,6 +93,72 @@

#endif /* New in 3.5 */

/* ABI info & checking (new in 3.15) */
#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= _Py_PACK_VERSION(3, 15)

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What are all of these warnings about? Does _Py_PACK_VERSION do something funky?

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No idea. I'll need to check on a Windows box.

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I wasn't able to figure it out. Something seems to be compiling modsupport.h outside of Python.h -- that is, without pymacro.h being included first: I get an error if I add this before the line:

#ifndef Py_PYMACRO_H
#error this should not happen
#endif

The error is currently quite impenetrable to me:

       ResourceCompile:
         C:\Program Files (x86)\Windows Kits\10\bin\10.0.26100.0\x86\rc.exe /D "ORIGINAL_FILENAME=\\\"python315_d.dll\\\"" /D FIELD3=100 /D _DEBUG /l"0x0409" /IC:\Users\encukou\dev\cpython\PC /IC:\Users\encu 
         kou\dev\cpython\Include /IC:\Users\encukou\dev\cpython\PCbuild\obj\315amd64_Debug\pythoncore\ /nologo /fo"C:\Users\encukou\dev\cpython\PCbuild\obj\315amd64_Debug\pythoncore\python_nt.res" ..\PC\pyth 
         on_nt.rc
     3>C:\Users\encukou\dev\cpython\Include\modsupport.h(98): error RC2188: C:\Users\encukou\dev\cpython\PCbuild\obj\315amd64_Debug\pythoncore\RCa08340(52) : fatal error RC1116: RC terminating after preproce
       ssor errors [C:\Users\encukou\dev\cpython\PCbuild\pythoncore.vcxproj]

     3>Done Building Project "C:\Users\encukou\dev\cpython\PCbuild\pythoncore.vcxproj" (Build target(s)) -- FAILED.

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Ah, it's being referenced from PC\python_ver_rc.h, which runs under the resource compiler rather than the C compiler. It's pretty much entirely compatible, but occasionally has some obscure/subtle differences.

I think it's just bypassing a lot of unnecessary includes here, assuming that modsupport.h stands alone. If that's not true, then the header needs updating and whatever constant it's referencing will have to move elsewhere.

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I switched this PR to use 0x030f0000; I plan to switch to _Py_PACK_VERSION(3, 15) in a follow-up PR.

typedef struct PyABIInfo {
uint8_t abiinfo_major_version;
uint8_t abiinfo_minor_version;
uint16_t flags;
uint32_t build_version;
uint32_t abi_version;
} PyABIInfo;
#define PyABIInfo_STABLE 0x0001
#define PyABIInfo_GIL 0x0002
#define PyABIInfo_FREETHREADED 0x0004
#define PyABIInfo_INTERNAL 0x0008

#define PyABIInfo_FREETHREADING_AGNOSTIC (PyABIInfo_GIL|PyABIInfo_FREETHREADED)

PyAPI_FUNC(int) PyABIInfo_Check(PyABIInfo *info, const char *module_name);

// Define the defaults
#ifdef Py_LIMITED_API
#define _PyABIInfo_DEFAULT_FLAG_STABLE PyABIInfo_STABLE
#if Py_LIMITED_API == 3
#define PyABIInfo_DEFAULT_ABI_VERSION _Py_PACK_VERSION(3, 2)
#else
#define PyABIInfo_DEFAULT_ABI_VERSION Py_LIMITED_API
#endif
#else
#define _PyABIInfo_DEFAULT_FLAG_STABLE 0
#define PyABIInfo_DEFAULT_ABI_VERSION PY_VERSION_HEX
#endif
#if defined(Py_LIMITED_API) && defined(_Py_OPAQUE_PYOBJECT)
#define _PyABIInfo_DEFAULT_FLAG_FT PyABIInfo_FREETHREADING_AGNOSTIC
#elif defined(Py_GIL_DISABLED)
#define _PyABIInfo_DEFAULT_FLAG_FT PyABIInfo_FREETHREADED
#else
#define _PyABIInfo_DEFAULT_FLAG_FT PyABIInfo_GIL
#endif
#if defined(Py_BUILD_CORE)
#define _PyABIInfo_DEFAULT_FLAG_INTERNAL PyABIInfo_INTERNAL
#else
#define _PyABIInfo_DEFAULT_FLAG_INTERNAL 0
#endif

#define PyABIInfo_DEFAULT_FLAGS ( \
_PyABIInfo_DEFAULT_FLAG_STABLE \
| _PyABIInfo_DEFAULT_FLAG_FT \
| _PyABIInfo_DEFAULT_FLAG_INTERNAL \
) \
/////////////////////////////////////////////////////////

#define _PyABIInfo_DEFAULT() { \
1, 0, \
PyABIInfo_DEFAULT_FLAGS, \
PY_VERSION_HEX, \
PyABIInfo_DEFAULT_ABI_VERSION } \
/////////////////////////////////////////////////////////

#define PyABIInfo_VAR(NAME) \
static PyABIInfo NAME = _PyABIInfo_DEFAULT;

#undef _PyABIInfo_DEFAULT_STABLE
#undef _PyABIInfo_DEFAULT_FT
#undef _PyABIInfo_DEFAULT_INTERNAL

#endif /* ABI info (new in 3.15) */

#ifndef Py_LIMITED_API
# define Py_CPYTHON_MODSUPPORT_H
# include "cpython/modsupport.h"
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