A blocker issue for many :ref:`optimization ideas <optim-ideas>` is that the PyObject structure fields are exposed in the public C API. Example:

PyObject *
PyUnicode_FromObject(PyObject *obj)
{
    ...
    PyErr_Format(PyExc_TypeError,
                 "Can't convert '%.100s' object to str implicitly",
                 Py_TYPE(obj)->tp_name);
    return NULL;
}

with:

#define Py_TYPE(ob)           (_PyObject_CAST(ob)->ob_type)
#define _PyObject_CAST(op)    ((PyObject*)(op))

The issue is that obj->ob_type is accessed directly. It prevents to implement :ref:`Tagged pointers <tagged-pointer>` for example.

By the way, Py_TYPE() returns a :ref:`borrowed reference <borrowed-ref>` which is another kind of problem. See :ref:`Py_TYPE() corner case <py-type>`.

In the long term, PyObject structure should be opaque. Accessing ob_refcnt and ob_type fields should always go through functions.

XXX which functions?

XXX how to convert old code to these new functions?