micropython · AJMansfield · Jul 19, 2024 · Jul 19, 2024 · Jul 21, 2025
diff --git a/docs/differences/python_36.rst b/docs/differences/python_36.rst
@@ -25,7 +25,8 @@ Python 3.6 beta 1 was released on 12 Sep 2016, and a summary of the new features
   +--------------------------------------------------------+--------------------------------------------------+-----------------+
   | `PEP 468 <https://www.python.org/dev/peps/pep-0468/>`_ | Preserving the order of *kwargs* in a function   |                 |
   +--------------------------------------------------------+--------------------------------------------------+-----------------+
-  | `PEP 487 <https://www.python.org/dev/peps/pep-0487/>`_ | Simpler customization of class creation          |                 |
+  | `PEP 487 <https://www.python.org/dev/peps/pep-0487/>`_ | Simpler customization of class creation          | Partial         |
+  |                                                        |                                                  | [#setname]_     |
   +--------------------------------------------------------+--------------------------------------------------+-----------------+
   | `PEP 520 <https://www.python.org/dev/peps/pep-0520/>`_ | Preserving Class Attribute Definition Order      |                 |
   +--------------------------------------------------------+--------------------------------------------------+-----------------+
@@ -198,3 +199,7 @@ Changes to built-in modules:
   +--------------------------------------------------------------------------------------------------------------+----------------+
   | The *compress()* and *decompress()* functions now accept keyword arguments                                   |                |
   +--------------------------------------------------------------------------------------------------------------+----------------+
+
+.. rubric:: Notes
+
+.. [#setname] Only :func:`__set_name__` is implemented.
diff --git a/py/mpconfig.h b/py/mpconfig.h
@@ -1124,7 +1124,7 @@ typedef time_t mp_timestamp_t;
 #define MICROPY_PY_FUNCTION_ATTRS_CODE (MICROPY_CONFIG_ROM_LEVEL_AT_LEAST_FULL_FEATURES)
 #endif
 
-// Whether to support the descriptors __get__, __set__, __delete__
+// Whether to support the descriptors __get__, __set__, __delete__, __set_name__
 // This costs some code size and makes load/store/delete of instance
 // attributes slower for the classes that use this feature
 #ifndef MICROPY_PY_DESCRIPTORS

diff --git a/py/objtype.c b/py/objtype.c
@@ -661,8 +661,8 @@ static void mp_obj_instance_load_attr(mp_obj_t self_in, qstr attr, mp_obj_t *des
     // try __getattr__
     if (attr != MP_QSTR___getattr__) {
         #if MICROPY_PY_DESCRIPTORS
-        // With descriptors enabled, don't delegate lookups of __get__/__set__/__delete__.
-        if (attr == MP_QSTR___get__ || attr == MP_QSTR___set__ || attr == MP_QSTR___delete__) {
+        // With descriptors enabled, don't delegate lookups of __get__/__set__/__delete__/__set_name__.
+        if (attr == MP_QSTR___get__ || attr == MP_QSTR___set__ || attr == MP_QSTR___delete__ || attr == MP_QSTR___set_name__) {
             return;
         }
         #endif
@@ -960,7 +960,7 @@ static bool check_for_special_accessors(mp_obj_t key, mp_obj_t value) {
     #endif
     #if MICROPY_PY_DESCRIPTORS
     static const uint8_t to_check[] = {
-        MP_QSTR___get__, MP_QSTR___set__, MP_QSTR___delete__,
+        MP_QSTR___get__, MP_QSTR___set__, MP_QSTR___delete__, // not needed for MP_QSTR___set_name__ tho
     };
     for (size_t i = 0; i < MP_ARRAY_SIZE(to_check); ++i) {
         mp_obj_t dest_temp[2];
@@ -974,6 +974,51 @@ static bool check_for_special_accessors(mp_obj_t key, mp_obj_t value) {
 }
 #endif
 
+#if MICROPY_PY_DESCRIPTORS
+// Shared data layout for the __set_name__ call and a linked list of calls to be made.
+typedef union _setname_list_t setname_list_t;
+union _setname_list_t {
+    mp_obj_t call[4];
+    struct {
+        mp_obj_t _meth;
+        mp_obj_t _self;
+        setname_list_t *next; // can use the "owner" argument position temporarily for the linked list
+        mp_obj_t _name;
+    };
+};
+
+// Append any `__set_name__` method on `value` to the setname list, with its per-attr args
+static setname_list_t *setname_maybe_bind_append(setname_list_t *tail, mp_obj_t name, mp_obj_t value) {
+    // make certain our type-punning is safe:
+    MP_STATIC_ASSERT_NONCONSTEXPR(offsetof(setname_list_t, next) == offsetof(setname_list_t, call[2]));
+
+    // tail is a blank list entry
+    mp_load_method_maybe(value, MP_QSTR___set_name__, tail->call);
+    if (tail->call[1] != MP_OBJ_NULL) {
+        // Each time a __set_name__ is found, leave it in-place in the former tail and allocate a new tail
+        tail->next = m_new_obj(setname_list_t);
+        tail->next->next = NULL;
+        tail->call[3] = name;
+        return tail->next;
+    } else {
+        return tail;
+    }
+}
+
+// Execute the captured `__set_name__` calls, destroying the setname list in the process.
+static void setname_consume_call_all(setname_list_t head, mp_obj_t owner) {
+    setname_list_t *iter = &head, *next;
+    while ((next = iter->next) != NULL) {
+        iter->call[2] = owner;
+        mp_call_method_n_kw(2, 0, iter->call);
+        if (iter != &head) { // head is stack, just mark blank
+            m_del_obj(setname_list_t, iter); // optimistic free, help gc not take too many passes to unwind
+        }
+        iter = next;
+    }
+}
+#endif
+
 static void type_print(const mp_print_t *print, mp_obj_t self_in, mp_print_kind_t kind) {
     (void)kind;
     mp_obj_type_t *self = MP_OBJ_TO_PTR(self_in);
@@ -1210,20 +1255,38 @@ mp_obj_t mp_obj_new_type(qstr name, mp_obj_t bases_tuple, mp_obj_t locals_dict)
         }
     }
 
+    #if MICROPY_PY_DESCRIPTORS
+    // To avoid any dynamic allocations when no __set_name__ exists,
+    // the head of this list is kept on the stack (marked blank with `next = NULL`).
+    setname_list_t setname_list = { .next = NULL };
+    setname_list_t *setname_tail = &setname_list;
+    #endif
+
     #if ENABLE_SPECIAL_ACCESSORS
-    // Check if the class has any special accessor methods
-    if (!(o->flags & MP_TYPE_FLAG_HAS_SPECIAL_ACCESSORS)) {
-        for (size_t i = 0; i < locals_ptr->map.alloc; i++) {
-            if (mp_map_slot_is_filled(&locals_ptr->map, i)) {
-                const mp_map_elem_t *elem = &locals_ptr->map.table[i];
-                if (check_for_special_accessors(elem->key, elem->value)) {
-                    o->flags |= MP_TYPE_FLAG_HAS_SPECIAL_ACCESSORS;
-                    break;
-                }
+    // Check if the class has any special accessor methods,
+    // and accumulate bound __set_name__ methods that need to be called
+    for (size_t i = 0; i < locals_ptr->map.alloc; i++) {
+        #if !MICROPY_PY_DESCRIPTORS
+        // __set_name__ needs to scan the entire locals map, can't early-terminate
+        if (o->flags & MP_TYPE_FLAG_HAS_SPECIAL_ACCESSORS) {
+            break;
+        }
+        #endif
+
+        if (mp_map_slot_is_filled(&locals_ptr->map, i)) {
+            const mp_map_elem_t *elem = &locals_ptr->map.table[i];
+
+            if (!(o->flags & MP_TYPE_FLAG_HAS_SPECIAL_ACCESSORS) // elidable when the early-termination check is enabled
+                && check_for_special_accessors(elem->key, elem->value)) {
+                o->flags |= MP_TYPE_FLAG_HAS_SPECIAL_ACCESSORS;
             }
+
+            #if MICROPY_PY_DESCRIPTORS
+            setname_tail = setname_maybe_bind_append(setname_tail, elem->key, elem->value);
+            #endif
         }
     }
-    #endif
+    #endif // ENABLE_SPECIAL_ACCESSORS
 
     const mp_obj_type_t *native_base;
     size_t num_native_bases = instance_count_native_bases(o, &native_base);
@@ -1241,6 +1304,10 @@ mp_obj_t mp_obj_new_type(qstr name, mp_obj_t bases_tuple, mp_obj_t locals_dict)
         }
     }
 
+    #if MICROPY_PY_DESCRIPTORS
+    setname_consume_call_all(setname_list, MP_OBJ_FROM_PTR(o));
+    #endif
+
     return MP_OBJ_FROM_PTR(o);
 }
 

diff --git a/tests/basics/class_descriptor.py b/tests/basics/class_descriptor.py
@@ -1,22 +1,28 @@
 class Descriptor:
     def __get__(self, obj, cls):
-        print('get')
+        print("get")
         print(type(obj) is Main)
         print(cls is Main)
-        return 'result'
+        return "result"
 
     def __set__(self, obj, val):
-        print('set')
+        print("set")
         print(type(obj) is Main)
         print(val)
 
     def __delete__(self, obj):
-        print('delete')
+        print("delete")
         print(type(obj) is Main)
 
+    def __set_name__(self, owner, name):
+        print("set_name", name)
+        print(owner.__name__ == "Main")
+
+
 class Main:
     Forward = Descriptor()
 
+
 m = Main()
 try:
     m.__class__
@@ -26,15 +32,15 @@ class Main:
     raise SystemExit
 
 r = m.Forward
-if 'Descriptor' in repr(r.__class__):
+if "Descriptor" in repr(r.__class__):
     # Target doesn't support descriptors.
-    print('SKIP')
+    print("SKIP")
     raise SystemExit
 
 # Test assignment and deletion.
 
 print(r)
-m.Forward = 'a'
+m.Forward = "a"
 del m.Forward
 
 # Test that lookup of descriptors like __get__ are not passed into __getattr__.

diff --git a/tests/basics/class_setname_hazard.py b/tests/basics/class_setname_hazard.py
@@ -0,0 +1,179 @@
+def skip_if_no_descriptors():
+    class Descriptor:
+        def __get__(self, obj, cls):
+            return
+
+    class TestClass:
+        Forward = Descriptor()
+
+    a = TestClass()
+    try:
+        a.__class__
+    except AttributeError:
+        # Target doesn't support __class__.
+        print("SKIP")
+        raise SystemExit
+
+    b = a.Forward
+    if "Descriptor" in repr(b.__class__):
+        # Target doesn't support descriptors.
+        print("SKIP")
+        raise SystemExit
+
+
+skip_if_no_descriptors()
+
+
+# Test basic hazard-free mutations of the enclosing class.
+
+
+class GetSibling:
+    def __set_name__(self, owner, name):
+        print(getattr(owner, name + "_sib"))
+
+
+class GetSiblingTest:
+    desc = GetSibling()
+    desc_sib = 111
+
+
+t110 = GetSiblingTest()
+
+
+class SetSibling:
+    def __set_name__(self, owner, name):
+        setattr(owner, name + "_sib", 121)
+
+
+class SetSiblingTest:
+    desc = SetSibling()
+
+
+t120 = SetSiblingTest()
+
+print(t120.desc_sib)
+
+
+class DelSibling:
+    def __set_name__(self, owner, name):
+        delattr(owner, name + "_sib")
+
+
+class DelSiblingTest:
+    desc = DelSibling()
+    desc_sib = 131
+
+
+t130 = DelSiblingTest()
+
+try:
+    print(t130.desc_sib)
+except AttributeError:
+    print("AttributeError")
+
+
+class GetSelf:
+    x = 211
+
+    def __set_name__(self, owner, name):
+        print(getattr(owner, name).x)
+
+
+class GetSelfTest:
+    desc = GetSelf()
+
+
+t210 = GetSelfTest()
+
+
+class SetSelf:
+    def __set_name__(self, owner, name):
+        setattr(owner, name, 221)
+
+
+class SetSelfTest:
+    desc = SetSelf()
+
+
+t220 = SetSelfTest()
+
+print(t220.desc)
+
+
+class DelSelf:
+    def __set_name__(self, owner, name):
+        delattr(owner, name)
+
+
+class DelSelfTest:
+    desc = DelSelf()
+
+
+t230 = DelSelfTest()
+
+try:
+    print(t230.desc)
+except AttributeError:
+    print("AttributeError")
+
+
+# Test exception behavior
+
+
+class Raise:
+    def __set_name__(self, owner, name):
+        raise Exception()
+
+
+try:
+
+    class RaiseTest:
+        desc = Raise()
+except Exception as e:
+    print("Exception")
+
+
+# Test simple hazards: whether other class attributes still get __set_name__ even if removed before being run
+
+
+class SetSpecific:
+    def __init__(self, sib_name, sib_replace):
+        self.sib_name = sib_name
+        self.sib_replace = sib_replace
+
+    def __set_name__(self, owner, name):
+        setattr(owner, self.sib_name, self.sib_replace)
+
+
+class SetReplaceTest:
+    a = SetSpecific("b", 312)  # one of these is changed first
+    b = SetSpecific("a", 311)
+
+
+t310 = SetReplaceTest()
+print(t310.a)
+print(t310.b)
+
+
+class DelSpecific:
+    def __init__(self, sib_name):
+        self.sib_name = sib_name
+
+    def __set_name__(self, owner, name):
+        delattr(owner, self.sib_name)
+
+
+class DelReplaceTest:
+    a = DelSpecific("b")  # one of these is removed first
+    b = DelSpecific("a")
+
+
+t320 = DelReplaceTest()
+try:
+    print(t320.a)
+except AttributeError:
+    print("AttributeError")
+try:
+    print(t320.b)
+except AttributeError:
+    print("AttributeError")