PyMODINIT_FUNC does not apply __declspec(dllexport) on Windows

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:

__declspec(dllexport) in the source code

An EXPORTS statement in a .def file

An /EXPORT specification in a LINK command

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

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

I filed a setuptools issue: pypa/setuptools#5118

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.

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

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.

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).

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

Navigation Menu

Repository navigation

`PyMODINIT_FUNC` does not apply `__declspec(dllexport)` on Windows #141671

Linked PRs

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

PyMODINIT_FUNC does not apply __declspec(dllexport) on Windows #141671

Description

Linked PRs

Activity

encukou commented on Nov 18, 2025

AraHaan commented on Dec 29, 2025

encukou commented on Jan 7, 2026

rgommers commented on Jan 13, 2026

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions

`PyMODINIT_FUNC` does not apply `__declspec(dllexport)` on Windows #141671