How Python resolves object attribute access

object_getattr.c

/* Generic GetAttr functions - put these in your tp_[gs]etattro slot */

PyObject *
_PyObject_GenericGetAttrWithDict(PyObject *obj, PyObject *name, PyObject *dict)
{
    PyTypeObject *tp = Py_TYPE(obj);
    PyObject *descr = NULL;
    PyObject *res = NULL;
    descrgetfunc f;
    Py_ssize_t dictoffset;
    PyObject **dictptr;

    if (!PyUnicode_Check(name)){
        PyErr_Format(PyExc_TypeError,
                     "attribute name must be string, not '%.200s'",
                     name->ob_type->tp_name);
        return NULL;
    }
    Py_INCREF(name);

    if (tp->tp_dict == NULL) {
        if (PyType_Ready(tp) < 0)
            goto done;
    }

    descr = _PyType_Lookup(tp, name);

    f = NULL;
    if (descr != NULL) {
        Py_INCREF(descr);
        f = descr->ob_type->tp_descr_get;
        if (f != NULL && PyDescr_IsData(descr)) {
            res = f(descr, obj, (PyObject *)obj->ob_type);
            goto done;
        }
    }

    if (dict == NULL) {
        /* Inline _PyObject_GetDictPtr */
        dictoffset = tp->tp_dictoffset;
        if (dictoffset != 0) {
            if (dictoffset < 0) {
                Py_ssize_t tsize;
                size_t size;

                tsize = ((PyVarObject *)obj)->ob_size;
                if (tsize < 0)
                    tsize = -tsize;
                size = _PyObject_VAR_SIZE(tp, tsize);
                assert(size <= PY_SSIZE_T_MAX);

                dictoffset += (Py_ssize_t)size;
                assert(dictoffset > 0);
                assert(dictoffset % SIZEOF_VOID_P == 0);
            }
            dictptr = (PyObject **) ((char *)obj + dictoffset);
            dict = *dictptr;
        }
    }
    if (dict != NULL) {
        Py_INCREF(dict);
        res = PyDict_GetItem(dict, name);
        if (res != NULL) {
            Py_INCREF(res);
            Py_DECREF(dict);
            goto done;
        }
        Py_DECREF(dict);
    }

    if (f != NULL) {
        res = f(descr, obj, (PyObject *)Py_TYPE(obj));
        goto done;
    }

    if (descr != NULL) {
        res = descr;
        descr = NULL;
        goto done;
    }

    PyErr_Format(PyExc_AttributeError,
                 "'%.50s' object has no attribute '%U'",
                 tp->tp_name, name);
  done:
    Py_XDECREF(descr);
    Py_DECREF(name);
    return res;
}