> In case of malloc, every memory allocating code need to use malloc/calloc/realloc. This is official and the only way to allocate a memory. But we do not guarantee that Python core uses only public C API like PyErr_Occurred(). It can use more low-level and efficient but less safer C API internally. It can replace the function with a macro which access internal structures directly (for compiling the core only). And this is actually the case. Overridding the public C API functions not always has an effect on the core.

To confirm what you said: if we take the specific example of PyErr_Occurred(), I recently added a new _PyErr_Occurred() function which is declared as a static inline function. _PyErr_Occurred() cannot be overriden.

static inline PyObject* _PyErr_Occurred(PyThreadState *tstate)
{
    assert(tstate != NULL);
    return tstate->curexc_type;
}