descrobject.c

Objects/descrobject.c implements the five built-in descriptor types that back Python's method resolution, slot access, and property-like get/set on C-defined types. All five share the descriptor protocol: a __get__ slot that returns a bound object when accessed through an instance.

Map

Lines	Symbol	Role
1–120	PyMethodDescrObject / method_get	Wraps a PyCFunction; __get__ returns a bound method
121–280	PyClassMethodDescrObject / classmethod_get	Like method but binds to the type, not the instance
281–480	PyMemberDescrObject / member_get / member_set	Reads and writes C struct fields via PyMemberDef
481–700	PyGetSetDescrObject / getset_get / getset_set	Calls C getter/setter functions from PyGetSetDef
701–900	PyWrapperDescrObject / wrapperdescr_get	Wraps a type slot function (e.g. tp_richcompare)
901–1050	slot_tp_descr_get	Default __get__ for types that have none; produces method-wrapper
1051–1200	PyDescr_NewMethod etc.	Factory functions used by type_ready to populate tp_dict

Reading

method_get and bound method creation

When Python evaluates obj.method, the attribute lookup finds a PyMethodDescrObject in the type's tp_dict and calls its tp_descr_get slot. If obj is not NULL (instance access rather than class access), it allocates a PyCFunctionObject that captures both the C function pointer and obj as self.

// CPython: Objects/descrobject.c:90 method_get
static PyObject *
method_get(PyObject *self, PyObject *obj, PyObject *type)
{
    PyMethodDescrObject *descr = (PyMethodDescrObject *)self;
    if (obj == Py_None || obj == NULL)
        return Py_NewRef(self);
    if (!descr_check((PyDescrObject *)descr, obj))
        return NULL;
    return PyCFunction_NewEx(descr->d_method, obj, NULL);
}

member_get and struct field access

PyMemberDescrObject stores a PyMemberDef that records the byte offset and type code of a C struct field. member_get calls PyMember_GetOne, which adds the offset to the object pointer and casts according to type code (T_INT, T_DOUBLE, T_OBJECT_EX, etc.).

// CPython: Objects/descrobject.c:350 member_get
static PyObject *
member_get(PyObject *self, PyObject *obj, PyObject *type)
{
    PyMemberDescrObject *descr = (PyMemberDescrObject *)self;
    if (descr_check((PyDescrObject *)descr, obj) < 0)
        return NULL;
    if (descr->d_member->flags & READ_RESTRICTED)
        if (PySys_Audit("object.__getattr__", "Os",
                        obj, descr->d_member->name) < 0)
            return NULL;
    return PyMember_GetOne((char *)obj, descr->d_member);
}

getset_get and C-level property

PyGetSetDescrObject holds a PyGetSetDef with a get function pointer and an optional set function pointer plus a closure void pointer passed to both. This is the mechanism used by property on C extension types.

// CPython: Objects/descrobject.c:550 getset_get
static PyObject *
getset_get(PyObject *self, PyObject *obj, PyObject *type)
{
    PyGetSetDescrObject *descr = (PyGetSetDescrObject *)self;
    if (obj == NULL || obj == Py_None)
        return Py_NewRef(self);
    if (descr->d_getset->get == NULL) {
        PyErr_SetString(PyExc_AttributeError, "unreadable attribute");
        return NULL;
    }
    return descr->d_getset->get(obj, descr->d_getset->closure);
}

slot_tp_descr_get as fallback binding

Types that do not define a custom tp_descr_get inherit this function from PyBaseObject_Type. It produces a method-wrapper object that binds the slot function to the instance, enabling Python-level calls like obj.__add__(other) to work even for C slot functions.

// CPython: Objects/descrobject.c:970 slot_tp_descr_get
static PyObject *
slot_tp_descr_get(PyObject *self, PyObject *obj, PyObject *type)
{
    if (obj == NULL || obj == Py_None)
        return Py_NewRef(self);
    return PyWrapper_New(self, obj);
}

gopy notes

• method_get maps to objects/method.go BoundMethod; the Go port stores fn and self and implements __call__ by forwarding to the underlying objects/function.go callable.
• member_get / member_set are not needed for pure-Python types; they become relevant when exposing Go struct fields to Python via PyGetSetDef equivalents in objects/descr.go.
• getset_get is the backing model for objects/property.go; the closure pointer maps to a Go any carried on the GetSetDescriptor struct.
• wrapperdescr_get (slot wrappers) currently surfaces as objects/protocol.go helper stubs; full binding is tracked separately.

CPython 3.14 changes

• The audit event object.__getattr__ on restricted members was added in 3.10 and is unchanged in 3.14.
• PyDescr_NewGetSet and PyDescr_NewMember gained Py_TPFLAGS_IMMUTABLETYPE guards in 3.12 to prevent mutation of descriptors on immutable types.
• No structural changes to the five descriptor types between 3.13 and 3.14; the file received only minor const-correctness and annotation patches.