py/objtype: Don't delegate lookup of descriptor methods to __getattr__.

When descriptors are enabled, lookup of the `__get__`, `__set__` and
`__delete__` descriptor methods should not be delegated to `__getattr__`.
That follows CPython behaviour.

Signed-off-by: Damien George <damien@micropython.org>

py/objtype.c

@@ -660,6 +660,13 @@ 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__) {
+            return;
+        }
+        #endif
+
         #if MICROPY_PY_DELATTR_SETATTR
         // If the requested attr is __setattr__/__delattr__ then don't delegate the lookup
         // to __getattr__.  If we followed CPython's behaviour then __setattr__/__delattr__

tests/basics/class_descriptor.py

@@ -21,14 +21,41 @@ m = Main()
 try:
     m.__class__
 except AttributeError:
+    # Target doesn't support __class__.
     print("SKIP")
     raise SystemExit
 
 r = m.Forward
 if 'Descriptor' in repr(r.__class__):
+    # Target doesn't support descriptors.
     print('SKIP')
     raise SystemExit
 
+# Test assignment and deletion.
+
 print(r)
 m.Forward = 'a'
 del m.Forward
+
+# Test that lookup of descriptors like __get__ are not passed into __getattr__.
+
+
+class NonDescriptor:
+    def __getattr__(self, attr):
+        print("getattr", attr)
+
+
+class TestClass:
+    non_descriptor = NonDescriptor()
+
+
+print(isinstance(TestClass().non_descriptor, NonDescriptor))
+
+t = TestClass()
+t.non_descriptor = 123
+print(t.non_descriptor)
+
+try:
+    del TestClass().non_descriptor
+except AttributeError:
+    print("AttributeError")