The macro isdocumented as:

Declare an extension module initialization function. This macro:

• specifies thePyObject* 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 withPy_ENABLE_SHARED -- that is, whenPython itself is built with a shared library.

According toMSVC docs:

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

1. __declspec(dllexport) in the source code
2. AnEXPORTS statement in a .def file
3. An /EXPORT specification in a LINK command
4. Acomment 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 thePEP-793PyModExport 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