py: Add optional support for descriptors' __get__ and __set__ methods.

Disabled by default.  Enabled on unix and windows ports.

py/mpconfig.h

@@ -364,6 +364,12 @@ typedef double mp_float_t;
 #define MICROPY_PY_FUNCTION_ATTRS (0)
 #endif
 
+// Whether to support descriptors (__get__ and __set__)
+// This costs some code size and makes all load attrs and store attrs slow
+#ifndef MICROPY_PY_DESCRIPTORS
+#define MICROPY_PY_DESCRIPTORS (0)
+#endif
+
 // Whether str object is proper unicode
 #ifndef MICROPY_PY_BUILTINS_STR_UNICODE
 #define MICROPY_PY_BUILTINS_STR_UNICODE (0)

py/objtype.c

@@ -463,16 +463,34 @@ void mp_obj_instance_load_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
     if (member != MP_OBJ_NULL) {
         #if MICROPY_PY_BUILTINS_PROPERTY
         if (MP_OBJ_IS_TYPE(member, &mp_type_property)) {
-            // object member is a property
-            // delegate the store to the property
-            // TODO should this be part of mp_convert_member_lookup?
+            // object member is a property; delegate the load to the property
+            // Note: This is an optimisation for code size and execution time.
+            // The proper way to do it is have the functionality just below
+            // in a __get__ method of the property object, and then it would
+            // be called by the descriptor code down below.  But that way
+            // requires overhead for the nested mp_call's and overhead for
+            // the code.
             const mp_obj_t *proxy = mp_obj_property_get(member);
             if (proxy[0] == mp_const_none) {
-                // TODO
+                nlr_raise(mp_obj_new_exception_msg(&mp_type_AttributeError, "unreadable attribute"));
             } else {
                 dest[0] = mp_call_function_n_kw(proxy[0], 1, 0, &self_in);
-                // TODO should we convert the returned value using mp_convert_member_lookup?
             }
+            return;
+        }
+        #endif
+
+        #if MICROPY_PY_DESCRIPTORS
+        // found a class attribute; if it has a __get__ method then call it with the
+        // class instance and class as arguments and return the result
+        // Note that this is functionally correct but very slow: each load_attr
+        // requires an extra mp_load_method_maybe to check for the __get__.
+        mp_obj_t attr_get_method[4];
+        mp_load_method_maybe(member, MP_QSTR___get__, attr_get_method);
+        if (attr_get_method[0] != MP_OBJ_NULL) {
+            attr_get_method[2] = self_in;
+            attr_get_method[3] = mp_obj_get_type(self_in);
+            dest[0] = mp_call_method_n_kw(2, 0, attr_get_method);
         }
         #endif
         return;
@@ -495,9 +513,11 @@ void mp_obj_instance_load_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
 bool mp_obj_instance_store_attr(mp_obj_t self_in, qstr attr, mp_obj_t value) {
     mp_obj_instance_t *self = self_in;
 
-#if MICROPY_PY_BUILTINS_PROPERTY
-    // for property, we need to do a lookup first in the class dict
-    // this makes all stores slow... how to fix?
+    #if MICROPY_PY_BUILTINS_PROPERTY || MICROPY_PY_DESCRIPTORS
+    // With property and/or descriptors enabled we need to do a lookup
+    // first in the class dict for the attribute to see if the store should
+    // be delegated.
+    // Note: this makes all stores slow... how to fix?
     mp_obj_t member[2] = {MP_OBJ_NULL};
     struct class_lookup_data lookup = {
         .obj = self,
@@ -507,12 +527,20 @@ bool mp_obj_instance_store_attr(mp_obj_t self_in, qstr attr, mp_obj_t value) {
         .is_type = false,
     };
     mp_obj_class_lookup(&lookup, self->base.type);
-    if (member[0] != MP_OBJ_NULL && MP_OBJ_IS_TYPE(member[0], &mp_type_property)) {
-        // attribute already exists and is a property
-        // delegate the store to the property
+
+    if (member[0] != MP_OBJ_NULL) {
+        #if MICROPY_PY_BUILTINS_PROPERTY
+        if (MP_OBJ_IS_TYPE(member[0], &mp_type_property)) {
+            // attribute exists and is a property; delegate the store
+            // Note: This is an optimisation for code size and execution time.
+            // The proper way to do it is have the functionality just below
+            // in a __set__ method of the property object, and then it would
+            // be called by the descriptor code down below.  But that way
+            // requires overhead for the nested mp_call's and overhead for
+            // the code.
             const mp_obj_t *proxy = mp_obj_property_get(member[0]);
             if (proxy[1] == mp_const_none) {
-            // TODO better error message
+                // TODO better error message?
                 return false;
             } else {
                 mp_obj_t dest[2] = {self_in, value};
@@ -522,6 +550,21 @@ bool mp_obj_instance_store_attr(mp_obj_t self_in, qstr attr, mp_obj_t value) {
         }
         #endif
 
+        #if MICROPY_PY_DESCRIPTORS
+        // found a class attribute; if it has a __set__ method then call it with the
+        // class instance and value as arguments
+        mp_obj_t attr_set_method[4];
+        mp_load_method_maybe(member[0], MP_QSTR___set__, attr_set_method);
+        if (attr_set_method[0] != MP_OBJ_NULL) {
+            attr_set_method[2] = self_in;
+            attr_set_method[3] = value;
+            mp_call_method_n_kw(2, 0, attr_set_method);
+            return true;
+        }
+        #endif
+    }
+    #endif
+
     if (value == MP_OBJ_NULL) {
         // delete attribute
         mp_map_elem_t *elem = mp_map_lookup(&self->members, MP_OBJ_NEW_QSTR(attr), MP_MAP_LOOKUP_REMOVE_IF_FOUND);

py/qstrdefs.h

@@ -66,6 +66,10 @@ Q(__add__)
 Q(__sub__)
 Q(__repr__)
 Q(__str__)
+#if MICROPY_PY_DESCRIPTORS
+Q(__get__)
+Q(__set__)
+#endif
 Q(__getattr__)
 Q(__del__)
 Q(__call__)

py/vm.c

@@ -393,8 +393,9 @@ dispatch_loop:
                     DISPATCH();
                 }
                 #else
-                // This caching code works with MICROPY_PY_BUILTINS_PROPERTY enabled because
-                // if the attr exists in self->members then it can't be a property.  A
+                // This caching code works with MICROPY_PY_BUILTINS_PROPERTY and/or
+                // MICROPY_PY_DESCRIPTORS enabled because if the attr exists in
+                // self->members then it can't be a property or have descriptors.  A
                 // consequence of this is that we can't use MP_MAP_LOOKUP_ADD_IF_NOT_FOUND
                 // in the fast-path below, because that store could override a property.
                 ENTRY(MP_BC_STORE_ATTR): {

tests/basics/class_descriptor.py

@@ -0,0 +1,20 @@
+class Descriptor:
+    def __get__(self, obj, cls):
+        print(type(obj) is Main)
+        print(cls is Main)
+        return 'result'
+
+    def __set__(self, obj, val):
+        print(type(obj) is Main)
+        print(val)
+
+class Main:
+    Forward = Descriptor()
+
+m = Main()
+r = m.Forward
+if 'Descriptor' in repr(r.__class__):
+    print('SKIP')
+else:
+    print(r)
+    m.Forward = 'a'

unix/mpconfigport.h

@@ -58,6 +58,7 @@
 #define MICROPY_OPT_CACHE_MAP_LOOKUP_IN_BYTECODE (1)
 #define MICROPY_CAN_OVERRIDE_BUILTINS (1)
 #define MICROPY_PY_FUNCTION_ATTRS   (1)
+#define MICROPY_PY_DESCRIPTORS      (1)
 #define MICROPY_PY_BUILTINS_STR_UNICODE (1)
 #define MICROPY_PY_BUILTINS_MEMORYVIEW (1)
 #define MICROPY_PY_BUILTINS_FROZENSET (1)

windows/mpconfigport.h

@@ -51,6 +51,7 @@
 #define MICROPY_STREAMS_NON_BLOCK   (1)
 #define MICROPY_OPT_COMPUTED_GOTO   (0)
 #define MICROPY_CAN_OVERRIDE_BUILTINS (1)
+#define MICROPY_PY_DESCRIPTORS      (1)
 #define MICROPY_PY_BUILTINS_STR_UNICODE (1)
 #define MICROPY_PY_BUILTINS_MEMORYVIEW (1)
 #define MICROPY_PY_BUILTINS_FROZENSET (1)