Addressed PR comments.

python/map.c

@@ -89,6 +89,9 @@ PyTypeObject* PyUpb_MapContainer_GetClass(const upb_fielddef* f) {
 
 PyObject* PyUpb_MapContainer_NewStub(PyObject* parent, const upb_fielddef* f,
                                      PyObject* arena) {
+  // We only create stubs when the parent is reified, by convention.  However
+  // this is not an invariant: the parent could become reified at any time.
+  assert(PyUpb_CMessage_GetIfReified(parent) == NULL);
   PyTypeObject* cls = PyUpb_MapContainer_GetClass(f);
   PyUpb_MapContainer* map = (void*)PyType_GenericAlloc(cls, 0);
   map->arena = arena;
@@ -196,7 +199,6 @@ PyObject* PyUpb_MapContainer_Contains(PyObject* _self, PyObject* key) {
 }
 
 PyObject* PyUpb_MapContainer_Clear(PyObject* _self, PyObject* key) {
-  PyUpb_MapContainer* self = (PyUpb_MapContainer*)_self;
   upb_map* map = PyUpb_MapContainer_EnsureReified(_self);
   upb_map_clear(map);
   Py_RETURN_NONE;

python/map.h

@@ -34,6 +34,7 @@
 #include "upb/def.h"
 
 // Creates a new repeated field stub for field `f` of message object `parent`.
+// Precondition: `parent` must be a stub.
 PyObject* PyUpb_MapContainer_NewStub(PyObject* parent, const upb_fielddef* f,
                                      PyObject* arena);
 

python/message.c

@@ -715,12 +715,30 @@ PyObject* PyUpb_CMessage_Get(upb_msg* u_msg, const upb_msgdef* m,
   return ret;
 }
 
-PyObject* PyUpb_CMessage_GetUnsetWrapper(PyUpb_CMessage* self,
-                                         const upb_fielddef* field) {
+/* PyUpb_CMessage_GetStub()
+ *
+ * Non-present messages return "stub" objects that point to their parent, but
+ * will materialize into real upb objects if they are mutated.
+ * 
+ * Note: we do *not* create stubs for repeated/map fields unless the parent
+ * is a stub:
+ * 
+ *    msg = TestMessage()
+ *    msg.submessage                # (A) Creates a stub
+ *    msg.repeated_foo              # (B) Does *not* create a stub
+ *    msg.submessage.repeated_bar   # (C) Creates a stub
+ *    
+ * In case (B) we have some freedom: we could either create a stub, or create
+ * a reified object with underlying data.  It appears that either could work
+ * equally well, with no observable change to users.  There isn't a clear
+ * advantage to either choice.  We choose to follow the behavior of the
+ * pre-existing C++ behavior for consistency, but if it becomes apparent that
+ * there would be some benefit to reversing this decision, it should be totally
+ * within the realm of possibility.
+ */
+PyObject* PyUpb_CMessage_GetStub(PyUpb_CMessage* self,
+                                 const upb_fielddef* field) {
   PyObject* _self = (void*)self;
-  // Non-present messages return magical "empty" messages that point to their
-  // parent, but will materialize into real messages if any fields are
-  // assigned.
   if (!self->unset_subobj_map) {
     self->unset_subobj_map = PyUpb_WeakMap_New();
   }
@@ -786,8 +804,8 @@ PyObject* PyUpb_CMessage_GetFieldValue(PyObject* _self,
   bool seq = upb_fielddef_isseq(field);
 
   if ((PyUpb_CMessage_IsStub(self) && (submsg || seq)) ||
-      (submsg && !upb_msg_has(self->ptr.msg, field))) {
-    return PyUpb_CMessage_GetUnsetWrapper(self, field);
+      (submsg && !seq && !upb_msg_has(self->ptr.msg, field))) {
+    return PyUpb_CMessage_GetStub(self, field);
   } else if (seq) {
     return PyUpb_CMessage_GetPresentWrapper(self, field);
   } else {
@@ -1053,7 +1071,7 @@ void PyUpb_CMessage_DoClearField(PyObject* _self, const upb_fielddef* f) {
     PyUpb_MapContainer_Invalidate(sub);
   } else if (upb_fielddef_isseq(f)) {
     if (sub) {
-      //PyUpb_RepeatedContainer_EnsureReified(sub);
+      PyUpb_RepeatedContainer_EnsureReified(sub);
     }
   } else if (upb_fielddef_issubmsg(f)) {
     if (sub) {

python/minimal_test.py

@@ -82,11 +82,11 @@ class TestMessageExtension(unittest.TestCase):
         self.assertRaises(AttributeError, getattr, msg, 'Extensions')
     
     def testClearStubMapField(self):
-        msg = map_unittest_pb2.TestMap()
-        int32_map = msg.map_int32_int32
-        msg.ClearField("map_int32_int32")
+        msg = map_unittest_pb2.TestMapSubmessage()
+        int32_map = msg.test_map.map_int32_int32
+        msg.test_map.ClearField("map_int32_int32")
         int32_map[123] = 456
-        self.assertEqual(0, msg.ByteSize())
+        self.assertEqual(0, msg.test_map.ByteSize())
 
     def testClearReifiedMapField(self):
         msg = map_unittest_pb2.TestMap()
@@ -97,11 +97,11 @@ class TestMessageExtension(unittest.TestCase):
         self.assertEqual(0, msg.ByteSize())
 
     def testClearStubRepeatedField(self):
-        msg = unittest_pb2.TestAllTypes()
-        int32_array = msg.repeated_int32
-        msg.ClearField("repeated_int32")
+        msg = unittest_pb2.NestedTestAllTypes()
+        int32_array = msg.payload.repeated_int32
+        msg.payload.ClearField("repeated_int32")
         int32_array.append(123)
-        self.assertEqual(0, msg.ByteSize())
+        self.assertEqual(0, msg.payload.ByteSize())
 
     def testClearReifiedRepeatdField(self):
         msg = unittest_pb2.TestAllTypes()

python/repeated.c

@@ -160,6 +160,9 @@ static Py_ssize_t PyUpb_RepeatedContainer_Length(PyObject* self) {
 PyObject* PyUpb_RepeatedContainer_NewStub(PyObject* parent,
                                           const upb_fielddef* f,
                                           PyObject* arena) {
+  // We only create stubs when the parent is reified, by convention.  However
+  // this is not an invariant: the parent could become reified at any time.
+  assert(PyUpb_CMessage_GetIfReified(parent) == NULL);
   PyTypeObject* cls = PyUpb_RepeatedContainer_GetClass(f);
   PyUpb_RepeatedContainer* repeated = (void*)PyType_GenericAlloc(cls, 0);
   repeated->arena = arena;
@@ -193,8 +196,8 @@ static PyObject* PyUpb_RepeatedContainer_MergeFrom(PyObject* _self,
 PyObject* PyUpb_RepeatedContainer_DeepCopy(PyObject* _self, PyObject* value) {
   PyUpb_RepeatedContainer* self = (PyUpb_RepeatedContainer*)_self;
   PyUpb_RepeatedContainer* clone = (void*)PyType_GenericAlloc(Py_TYPE(_self), 0);
-  const upb_fielddef* f = PyUpb_RepeatedContainer_GetField(self);
   if (clone == NULL) return NULL;
+  const upb_fielddef* f = PyUpb_RepeatedContainer_GetField(self);
   clone->arena = PyUpb_Arena_New();
   clone->field = (uintptr_t)PyUpb_FieldDescriptor_Get(f);
   clone->ptr.arr =

python/repeated.h

@@ -34,6 +34,7 @@
 #include "upb/def.h"
 
 // Creates a new repeated field stub for field `f` of message object `parent`.
+// Precondition: `parent` must be a stub.
 PyObject* PyUpb_RepeatedContainer_NewStub(PyObject* parent,
                                           const upb_fielddef* f,
                                           PyObject* arena);

upb/reflection.c

@@ -108,6 +108,7 @@ static upb_msgval _upb_msg_getraw(const upb_msg *msg, const upb_fielddef *f) {
 }
 
 bool upb_msg_has(const upb_msg *msg, const upb_fielddef *f) {
+  assert(upb_fielddef_haspresence(f));
   if (upb_fielddef_isextension(f)) {
     const upb_msglayout_ext *ext = _upb_fielddef_extlayout(f);
     return _upb_msg_getext(msg, ext) != NULL;