How Python resolves type attribute access

type_getattr.c

/* This is similar to PyObject_GenericGetAttr(),
   but uses _PyType_Lookup() instead of just looking in type->tp_dict. */
static PyObject *
type_getattro(PyTypeObject *type, PyObject *name)
{
    PyTypeObject *metatype = Py_TYPE(type);
    PyObject *meta_attribute, *attribute;
    descrgetfunc meta_get;

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

    /* Initialize this type (we'll assume the metatype is initialized) */
    if (type->tp_dict == NULL) {
        if (PyType_Ready(type) < 0)
            return NULL;
    }

    /* No readable descriptor found yet */
    meta_get = NULL;

    /* Look for the attribute in the metatype */
    meta_attribute = _PyType_Lookup(metatype, name);

    if (meta_attribute != NULL) {
        meta_get = Py_TYPE(meta_attribute)->tp_descr_get;

        if (meta_get != NULL && PyDescr_IsData(meta_attribute)) {
            /* Data descriptors implement tp_descr_set to intercept
             * writes. Assume the attribute is not overridden in
             * type's tp_dict (and bases): call the descriptor now.
             */
            return meta_get(meta_attribute, (PyObject *)type,
                            (PyObject *)metatype);
        }
        Py_INCREF(meta_attribute);
    }

    /* No data descriptor found on metatype. Look in tp_dict of this
     * type and its bases */
    attribute = _PyType_Lookup(type, name);
    if (attribute != NULL) {
        /* Implement descriptor functionality, if any */
        descrgetfunc local_get = Py_TYPE(attribute)->tp_descr_get;

        Py_XDECREF(meta_attribute);

        if (local_get != NULL) {
            /* NULL 2nd argument indicates the descriptor was
             * found on the target object itself (or a base)  */
            return local_get(attribute, (PyObject *)NULL,
                             (PyObject *)type);
        }

        Py_INCREF(attribute);
        return attribute;
    }

    /* No attribute found in local __dict__ (or bases): use the
     * descriptor from the metatype, if any */
    if (meta_get != NULL) {
        PyObject *res;
        res = meta_get(meta_attribute, (PyObject *)type,
                       (PyObject *)metatype);
        Py_DECREF(meta_attribute);
        return res;
    }

    /* If an ordinary attribute was found on the metatype, return it now */
    if (meta_attribute != NULL) {
        return meta_attribute;
    }

    /* Give up */
    PyErr_Format(PyExc_AttributeError,
                 "type object '%.50s' has no attribute '%U'",
                 type->tp_name, name);
    return NULL;
}