gh-137210: Add a struct, slot & function for checking an extension's ABI #137212

encukou · 2025-07-29T16:21:09Z

See the issue.

Issue: Add a struct, slot & function for checking an extension's ABI #137210

📚 Documentation preview 📚: https://cpython-previews--137212.org.readthedocs.build/

Doc/c-api/stable.rst

zooba · 2025-07-29T18:48:50Z

Doc/c-api/stable.rst

+      (except if :c:macro:`Py_LIMITED_API` is ``3``; use
+      :c:expr:`Py_PACK_VERSION(3, 2)` in that case).


Seems easier for us to just check this on our side than to expect users to get it right?

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

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.)

zooba · 2025-07-29T18:50:58Z

Doc/c-api/stable.rst

+            Specifies ABI specific to a particular build of CPython.
+            Internal use only.
+
+      Free-threading compatibility -- one of:


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.

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

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.

zooba · 2025-07-29T18:51:49Z

Include/modsupport.h

@@ -56,55 +56,14 @@ PyAPI_FUNC(int) PyModule_ExecDef(PyObject *module, PyModuleDef *def);

 #define Py_CLEANUP_SUPPORTED 0x20000

+/* The API and ABI versions are left for backwards compatibility.


Can't we just... update them?

To what? Py_VERSION_HEX? Or tell the RM to increment on release?

I don't think it particularly matters, it could be inferred from Py_LIMITED_API if that's set.

It's just an arbitrary value with a purpose that isn't really going away, so deprecating them seems unnecessary when we could just keep using them for their intended purposes (and ignore that they didn't get updated for a large number of ABI changes).

Oh, do you want to use an int instead of the PyABIInfo struct?

I don't think reusing these would buy us much.
These version numbers are currently used in PyModule_Create and PyModule_FromDefAndSpec, i.e. single-phase init (legacy) and manual module object creation (where version checking isn't that useful).

Or just use it as one of the values that goes into the struct. We can include more information if needed, it just seems like a waste to declare these names as irrelevant when we could just as easily make them relevant.

We can make them relevant, but what relevance should they have?
We switched to a versioning scheme where Py_VERSION_HEX gives you all the necessary info. Dropping these extra numbers is a win.

If we can find a use case in the future, that's what the versioning in PyABIInfo is for.

zooba · 2025-07-29T18:52:45Z

Include/modsupport.h

@@ -134,6 +93,72 @@ PyAPI_FUNC(PyObject *) PyModule_FromDefAndSpec2(PyModuleDef *def,

 #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)


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

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

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.

Co-authored-by: Steve Dower <steve.dower@microsoft.com>

encukou added 9 commits July 29, 2025 17:13

Add _PyUnicodeWriter_FormatV

894ef3d

Add capi test file for modsupport

72349ba

Add PyABIInfo

3b6aa96

Add _Py_INTERNAL_ABI_SLOT (to _datetime for now)

c015606

Add docs

c5740af

Add to limited API manifest

da293e3

Un-document PyABIInfo_FREETHREADING_AGNOSTIC for now

44df416

Blurb & What's New

feb428d

Fix docs

aa1eb4a

encukou requested review from a team, pganssle, abalkin, ericsnowcurrently, erlend-aasland and corona10 as code owners July 29, 2025 16:21

bedevere-app bot added the awaiting core review label Jul 29, 2025

bedevere-app bot mentioned this pull request Jul 29, 2025

Add a struct, slot & function for checking an extension's ABI #137210

Open

Remove duplicated test

b628d7d

encukou mentioned this pull request Jul 29, 2025

Add a struct, slot & function for checking an extension's ABI capi-workgroup/decisions#72

Open

zooba reviewed Jul 29, 2025

View reviewed changes

encukou and others added 4 commits July 30, 2025 09:37

Apply suggestions from code review

4800a5d

Co-authored-by: Steve Dower <steve.dower@microsoft.com>

Casts to avoid data loss warnings

4c9e918

Use hex literal for 3.15

1b18761

Merge in the main branch

814ac60

encukou requested a review from AA-Turner as a code owner August 7, 2025 11:52

		(except if :c:macro:`Py_LIMITED_API` is ``3``; use
		:c:expr:`Py_PACK_VERSION(3, 2)` in that case).

		@@ -56,55 +56,14 @@ PyAPI_FUNC(int) PyModule_ExecDef(PyObject module, PyModuleDef def);

		#define Py_CLEANUP_SUPPORTED 0x20000

		/* The API and ABI versions are left for backwards compatibility.

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gh-137210: Add a struct, slot & function for checking an extension's ABI #137212

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encukou commented Jul 29, 2025 •

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