Description

encukou

opened on Nov 17, 2025

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The macro is documented as:

Declare an extension module initialization function. This macro:

• specifies the PyObject* return type,
• adds any special linkage declarations required by the platform, and
• for C++, declares the function as extern "C".

Specifically, the special linkage declarations should be __attribute__ ((visibility ("default"))) on GCC/clang, and __declspec(dllexport) on Windows.
However, the definition assumes we're building CPython itself, and only adds a linkage declaration with Py_ENABLE_SHARED -- that is, when Python itself is built with a shared library.

According to MSVC docs:

There are four methods for exporting a definition, listed in recommended order of use:

1. __declspec(dllexport) in the source code
2. An EXPORTS statement in a .def file
3. An /EXPORT specification in a LINK command
4. A comment directive in the source code, of the form #pragma comment(linker, "/export: definition ").

Setuptools, and historically distutils, does (2) -- and so does, presumably, any other build system for native extensions on Windows.
However, this breaks for modules that define the PEP-793 PyModExport hook.

It would be great to switch to the preferred (1), and then tell Setuptools to avoid /EXPORT for 3.15+.

Linked PRs

• gh-141671: PyMODINIT_FUNC: apply __declspec(dllexport) on Windows #141672

Activity

1. encukou

   self-assigned this

   on Nov 17, 2025
2. encukou

   added

   OS-windows

   topic-C-API

   on Nov 17, 2025
3. encukou

   added a commit that references this issue on Nov 17, 2025

   pythongh-141671: Use __declspec(dllexport) when not building core

   39d18d1
4. StanFromIreland

   added

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   on Nov 17, 2025
5. encukou commented on Nov 18, 2025

   

   encukou

   on Nov 18, 2025

   MemberAuthor

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   I filed a setuptools issue: pypa/setuptools#5118
6. AraHaan commented on Dec 29, 2025

   

   AraHaan

   on Dec 29, 2025

   Contributor

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   I personally think this would be safe to be backported to 3.12, 3.13, 3.14 as adding __declspec(dllexport) to PyMODINIT_FUNC does not break anything.

   Also then the setuptools patch would also apply to those versions of python as well when they upgrade the package.

   This is because currently what I do to work around this was to

   #undef PyMODINIT_FUNC
   #if defined(__cplusplus)
   #define PyMODINIT_FUNC extern "C" Py_EXPORTED_SYMBOL PyObject*
   #else /* __cplusplus */
   #define PyMODINIT_FUNC Py_EXPORTED_SYMBOL PyObject*
   #endif /* __cplusplus */

   in my C extension code which I feel could have been avoided entirely.
7. encukou

   added a commit that references this issue on Jan 7, 2026

   gh-141671: PyMODINIT_FUNC: apply `__declspec(dllexport)` on Windows (G…

   8565ddd
8. encukou commented on Jan 7, 2026

   

   encukou

   on Jan 7, 2026

   MemberAuthor

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   I'd rather let this change have the full alpha/beta period before it hits production. It's old, platform-specific, widely used, and tied to build systems.

   Reacted by Ralf Gommers
9. rgommers commented on Jan 13, 2026

   

   rgommers

   on Jan 13, 2026

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   Setuptools, and historically distutils, does (2) -- and so does, presumably, any other build system for native extensions on Windows.

   For clarity: I think you meant that setuptools/distutils does (3), not (2). And other build systems/backends do (2).
10. thunder-coding

    added a commit that references this issue on Feb 15, 2026

    pythongh-141671: PyMODINIT_FUNC: apply `__declspec(dllexport)` on Win…

    417f14e
11. encukou

    closed this as completedon Jul 30, 2026

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