1685. Descriptor and method

What we are porting

Four files, ~5000 lines:

• Objects/descrobject.c: descriptor objects (property, member_descriptor, getset_descriptor, method_descriptor, wrapper_descriptor, slot_wrapper). The descriptor protocol (__get__, __set__, __delete__).
• Objects/methodobject.c: PyCFunction (built-in functions and bound methods of C-implemented methods). v0.4 lands the surface; v0.6 wires it into the VM call path.
• Objects/classobject.c: bound-method (method) for Python functions called on instances. Same shape as the C-method bound, different underlying callable.
• Objects/funcobject.c: Python function object. Carries the code object, defaults, closure, globals, name, qualname, annotations, type-params (PEP 695). v0.6 once frame/gen exist.

Go shape

// Function mirrors PyFunctionObject.
type Function struct {
    Header
    Code         *Code
    Globals      *Dict
    Defaults     *Tuple   // positional defaults
    KwDefaults   *Dict    // keyword-only defaults
    Closure      *Tuple   // cell objects
    Name         *Str
    Qualname     *Str
    Doc          Object   // str or None
    Dict         *Dict    // function attributes
    Module       Object
    Annotations  Object   // dict or function (lazy)
    TypeParams   *Tuple   // PEP 695
    Vectorcall   func(args []Object, kwnames *Tuple) (Object, error)
    Version      uint32   // bumped on Defaults / Globals change
}

// Method (bound method) mirrors PyMethodObject.
type Method struct {
    Header
    Func Object  // the function (or any callable)
    Self Object  // the bound instance
}

// CFunction mirrors PyCFunctionObject (C-implemented builtin).
type CFunction struct {
    Header
    Def    *MethodDef
    Self   Object   // module or class
    Module *Module
}

Descriptor protocol

// LookupDescriptor walks the type MRO and applies the
// data-descriptor / non-data-descriptor rule. Mirrors
// _PyObject_GenericGetAttrWithDict from object.c.
func LookupDescriptor(obj Object, name *Str) (Object, error)

Order: data descriptors on the type (those with __set__ or __delete__) > instance __dict__ > non-data descriptors > TypeError.

This ordering is observable through user code; getting it wrong breaks every metaclass, every property, every classmethod.

Property

property(fget, fset, fdel, doc) is a data descriptor. The four slots wrap callables. prop.setter, prop.getter, prop.deleter return new property objects with one slot replaced.

File mapping

Checklist

Status legend: [x] shipped, [ ] pending, [~] partial / scaffold, [n] deferred / not in scope this phase.

Files (v0.4 descriptor protocol)

• objects/descr.go: descriptor base, the __get__ / __set__ / __delete__ slot wrappers, the descriptor-walk helper used by type_getattro.
• objects/property.go: property type with fget/fset/fdel, setter / getter / deleter returning replacement properties, __set_name__ hook.
• objects/descr_member.go: member_descriptor and getset_descriptor for slot-backed instance attributes.
• objects/cfunction.go: CFunction type, MethodDef table, the dispatch table for METH_O / METH_NOARGS / METH_VARARGS / METH_FASTCALL / METH_FASTCALL|METH_KEYWORDS / METH_METHOD.
• objects/method.go: bound method type, __call__ that prepends self, __func__ / __self__ accessors.
• objects/descr_test.go: data-vs-non-data ordering panel, property setter chaining, MRO walk.

Files (v0.6 function object)

• objects/function.go: Function struct, MakeFunction (used by VM MAKE_FUNCTION), defaults handling, closure cell binding, vectorcall implementation that builds a frame.
• objects/function_versions.go: version tag bumped on defaults/globals/code change (used by the VM specializer).

Files (v0.7 tp_new / tp_init)

• objects/descr_classmethod.go, descr_staticmethod.go: classmethod / staticmethod descriptors. Land alongside user type creation.

Surface guarantees

• Data descriptor beats instance dict beats non-data descriptor. Pinned by compat/descriptor_test.go.
• property raises AttributeError with the CPython text when the missing slot is accessed.
• method.__self__ is obj, method.__func__ is f.
• CFunction calls preserve the right self for module functions vs unbound class methods.
• __set_name__ fires at class-creation time.
• Function __defaults__ is shared (mutable surface; same object across calls until reassigned).
• __closure__ cells round-trip with cell_contents.

Cross-references

• Type slots: 1672.
• Code object: 1687.
• Cell object: 1687 (cellobject.c).
• Call dispatch: 1684.

Out of scope

• PEP 558 frame f_locals semantics; lives in 1687.