> Actually, interp->none shared _Py_NoneStruct variable.

My PR 18301 is a draft to check if we can solve the issue without breaking the C API compatibility. You're right that it doesn't solve the issue, it only checks the C API issue. IMO the PR 18301 proves that the "#define Py_None Py_GetNone()" trick works.

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By the way, when I worked on a tagged pointer experiment:
https://github.com/vstinner/cpython/pull/6

I had to introduce Py_IS_NONE(op) function, since it was no longer possible to compare directly "op == Py_None".

static inline int Py_IS_NONE(PyObject *op) {
    return (op == &_Py_NoneStruct || op == _Py_TAGPTR_NONE);
}

But this is not needed to solve this issue.