Objects/descrobject.c (part 6)

Source:

cpython 3.14 @ ab2d84fe1023/Objects/descrobject.c ↗

This annotation covers built-in descriptor types. See objects_descrobject5_detail for wrapper_descriptor, method_wrapper, slot_wrapper, and getset_descriptor basics.

Map

Lines	Symbol	Role
1-80	classmethod_descriptor	classmethod built-in: binds to the class, not instance
81-180	staticmethod_descriptor	staticmethod built-in: no binding
181-300	member_descriptor	Access C-level struct members (e.g., int.numerator)
301-420	getset_descriptor	C PyGetSetDef: getter/setter pair
421-500	__doc__ on descriptors	Per-descriptor docstring storage

Reading

classmethod_descriptor

// CPython: Objects/descrobject.c:1380 classmethod_get
static PyObject *
classmethod_get(PyObject *self, PyObject *obj, PyObject *type)
{
    classmethodobject *cm = (classmethodobject *)self;
    if (type == NULL) type = (PyObject *)Py_TYPE(obj);
    /* Return a bound method with the class as first arg */
    return PyMethod_New(cm->cm_callable, type);
}

@classmethod stores the original function in cm_callable. When accessed via __get__, it returns a method bound to the class (not the instance). MyClass.my_classmethod and obj.my_classmethod both bind to MyClass.

staticmethod_descriptor

// CPython: Objects/descrobject.c:1460 staticmethod_get
static PyObject *
staticmethod_get(PyObject *self, PyObject *obj, PyObject *type)
{
    /* Return the original callable unchanged — no binding. */
    staticmethodobject *sm = (staticmethodobject *)self;
    return Py_NewRef(sm->sm_callable);
}

@staticmethod is a no-op wrapper: __get__ returns the original function directly. The descriptor protocol is called for attribute lookup, but no self or cls is prepended.

member_descriptor

// CPython: Objects/descrobject.c:1580 member_get
static PyObject *
member_get(PyMemberDescrObject *descr, PyObject *obj, PyObject *type)
{
    /* Read a C struct member at a fixed offset from the object pointer */
    char *addr = (char *)obj + descr->d_member->offset;
    switch (descr->d_member->type) {
    case T_INT:    return PyLong_FromLong(*(int *)addr);
    case T_DOUBLE: return PyFloat_FromDouble(*(double *)addr);
    case T_OBJECT: return Py_NewRef(*(PyObject **)addr);
    ...
    }
}

(1.5).real and (1.5).imag are member_descriptor attributes reading the ob_fval (or the real/imag parts of PyComplexObject.cval) at a fixed C struct offset. This avoids any hash lookup.

getset_descriptor

// CPython: Objects/descrobject.c:1680 getset_get
static PyObject *
getset_get(PyGetSetDescrObject *descr, PyObject *obj, PyObject *type)
{
    if (descr->d_getset->get == NULL) {
        PyErr_SetString(PyExc_AttributeError, "unreadable attribute");
        return NULL;
    }
    return descr->d_getset->get(obj, descr->d_getset->closure);
}

getset_descriptor wraps a PyGetSetDef (a C struct with get and set function pointers). Used for computed attributes: list.__len__, function.__code__, type.__mro__. The closure pointer lets the getter be parameterized.

gopy notes

classmethod is objects.ClassMethod in objects/method.go. staticmethod is objects.StaticMethod. member_descriptor reads Go struct fields via unsafe.Pointer at a fixed offset. getset_descriptor wraps a Go function pair (getter, setter func(obj) Object).