At commit cb15afcccffc6c42cbfb7456ce8db89cd2f77512, I am able to rename PyObject members (to make sure that the structure is not accessed directly), I only had to modify header files:

* Py_REFCNT(), Py_SET_REFCNT()
* Py_INCREF(), Py_DECREF()
* Py_TYPE(), Py_SET_TYPE()
* Py_IS_TYPE()

And just two more C files which corner cases:

* 1 line in Python/specialize.c
* 1 line in Modules/_testcapimodule.c: check_pyobject_forbidden_bytes_is_freed()

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I did the same with PyVarObject, rename the ob_size member. I had to modify header files:

* Py_SIZE(), Py_SET_SIZE()

But I had to modify the following function of the array module:

static int
array_buffer_getbuf(arrayobject *self, Py_buffer *view, int flags)
{
    ...
    if ((flags & PyBUF_ND)==PyBUF_ND) {
        view->shape = &((PyVarObject*)self)->ob_size;
    }
    ...
    return 0;
}

I'm not sure how to patch this function.

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This experience doesn't check usage of sizeof(PyObject) and sizeof(PyVarObject) which would break if these structures become opaque. sizeof() issues are listed in previous comments.