Integrate internal changes

12 files changed, 533 insertions, 375 deletions

diff --git a/python/google/protobuf/pyext/descriptor.cc b/python/google/protobuf/pyext/descriptor.cc
index e6ef5ef5..924ae0b9 100644
--- a/python/google/protobuf/pyext/descriptor.cc
+++ b/python/google/protobuf/pyext/descriptor.cc
@@ -41,6 +41,7 @@
 #include <google/protobuf/pyext/descriptor_containers.h>
 #include <google/protobuf/pyext/descriptor_pool.h>
 #include <google/protobuf/pyext/message.h>
+#include <google/protobuf/pyext/message_factory.h>
 #include <google/protobuf/pyext/scoped_pyobject_ptr.h>
 
 #if PY_MAJOR_VERSION >= 3
@@ -204,8 +205,9 @@ static PyObject* GetOrBuildOptions(const DescriptorClass *descriptor) {
   // read-only instance.
   const Message& options(descriptor->options());
   const Descriptor *message_type = options.GetDescriptor();
-  CMessageClass* message_class(
-      cdescriptor_pool::GetMessageClass(pool, message_type));
+  PyMessageFactory* message_factory = pool->py_message_factory;
+  CMessageClass* message_class = message_factory::GetMessageClass(
+      message_factory, message_type);
   if (message_class == NULL) {
     // The Options message was not found in the current DescriptorPool.
     // This means that the pool cannot contain any extensions to the Options
@@ -213,7 +215,9 @@ static PyObject* GetOrBuildOptions(const DescriptorClass *descriptor) {
     // the chances of successfully parsing the options.
     PyErr_Clear();
     pool = GetDefaultDescriptorPool();
-    message_class = cdescriptor_pool::GetMessageClass(pool, message_type);
+    message_factory = pool->py_message_factory;
+    message_class = message_factory::GetMessageClass(
+      message_factory, message_type);
   }
   if (message_class == NULL) {
     PyErr_Format(PyExc_TypeError, "Could not retrieve class for Options: %s",
@@ -243,7 +247,7 @@ static PyObject* GetOrBuildOptions(const DescriptorClass *descriptor) {
     options.SerializeToString(&serialized);
     io::CodedInputStream input(
         reinterpret_cast<const uint8*>(serialized.c_str()), serialized.size());
-    input.SetExtensionRegistry(pool->pool, pool->message_factory);
+    input.SetExtensionRegistry(pool->pool, message_factory->message_factory);
     bool success = cmsg->message->MergePartialFromCodedStream(&input);
     if (!success) {
       PyErr_Format(PyExc_ValueError, "Error parsing Options message");
@@ -439,8 +443,9 @@ static PyObject* GetConcreteClass(PyBaseDescriptor* self, void *closure) {
   // which contains this descriptor.
   // This might not be the one you expect! For example the returned object does
   // not know about extensions defined in a custom pool.
-  CMessageClass* concrete_class(cdescriptor_pool::GetMessageClass(
-      GetDescriptorPool_FromPool(_GetDescriptor(self)->file()->pool()),
+  CMessageClass* concrete_class(message_factory::GetMessageClass(
+      GetDescriptorPool_FromPool(
+          _GetDescriptor(self)->file()->pool())->py_message_factory,
       _GetDescriptor(self)));
   Py_XINCREF(concrete_class);
   return concrete_class->AsPyObject();
@@ -699,6 +704,10 @@ static PyObject* GetCamelcaseName(PyBaseDescriptor* self, void *closure) {
   return PyString_FromCppString(_GetDescriptor(self)->camelcase_name());
 }
 
+static PyObject* GetJsonName(PyBaseDescriptor* self, void *closure) {
+  return PyString_FromCppString(_GetDescriptor(self)->json_name());
+}
+
 static PyObject* GetType(PyBaseDescriptor *self, void *closure) {
   return PyInt_FromLong(_GetDescriptor(self)->type());
 }
@@ -888,6 +897,7 @@ static PyGetSetDef Getters[] = {
   { "full_name", (getter)GetFullName, NULL, "Full name"},
   { "name", (getter)GetName, NULL, "Unqualified name"},
   { "camelcase_name", (getter)GetCamelcaseName, NULL, "Camelcase name"},
+  { "json_name", (getter)GetJsonName, NULL, "Json name"},
   { "type", (getter)GetType, NULL, "C++ Type"},
   { "cpp_type", (getter)GetCppType, NULL, "C++ Type"},
   { "label", (getter)GetLabel, NULL, "Label"},
diff --git a/python/google/protobuf/pyext/descriptor_pool.cc b/python/google/protobuf/pyext/descriptor_pool.cc
index cfd98690..a42e5431 100644
--- a/python/google/protobuf/pyext/descriptor_pool.cc
+++ b/python/google/protobuf/pyext/descriptor_pool.cc
@@ -33,11 +33,11 @@
 #include <Python.h>
 
 #include <google/protobuf/descriptor.pb.h>
-#include <google/protobuf/dynamic_message.h>
 #include <google/protobuf/pyext/descriptor.h>
 #include <google/protobuf/pyext/descriptor_database.h>
 #include <google/protobuf/pyext/descriptor_pool.h>
 #include <google/protobuf/pyext/message.h>
+#include <google/protobuf/pyext/message_factory.h>
 #include <google/protobuf/pyext/scoped_pyobject_ptr.h>
 
 #if PY_MAJOR_VERSION >= 3
@@ -73,18 +73,16 @@ static PyDescriptorPool* _CreateDescriptorPool() {
   cpool->underlay = NULL;
   cpool->database = NULL;
 
-  DynamicMessageFactory* message_factory = new DynamicMessageFactory();
-  // This option might be the default some day.
-  message_factory->SetDelegateToGeneratedFactory(true);
-  cpool->message_factory = message_factory;
-
-  // TODO(amauryfa): Rewrite the SymbolDatabase in C so that it uses the same
-  // storage.
-  cpool->classes_by_descriptor =
-      new PyDescriptorPool::ClassesByMessageMap();
   cpool->descriptor_options =
       new hash_map<const void*, PyObject *>();
 
+  cpool->py_message_factory = message_factory::NewMessageFactory(
+      &PyMessageFactory_Type, cpool);
+  if (cpool->py_message_factory == NULL) {
+    Py_DECREF(cpool);
+    return NULL;
+  }
+
   return cpool;
 }
 
@@ -151,20 +149,14 @@ static PyObject* New(PyTypeObject* type,
 }
 
 static void Dealloc(PyDescriptorPool* self) {
-  typedef PyDescriptorPool::ClassesByMessageMap::iterator iterator;
   descriptor_pool_map.erase(self->pool);
-  for (iterator it = self->classes_by_descriptor->begin();
-       it != self->classes_by_descriptor->end(); ++it) {
-    Py_DECREF(it->second);
-  }
-  delete self->classes_by_descriptor;
+  Py_CLEAR(self->py_message_factory);
   for (hash_map<const void*, PyObject*>::iterator it =
            self->descriptor_options->begin();
        it != self->descriptor_options->end(); ++it) {
     Py_DECREF(it->second);
   }
   delete self->descriptor_options;
-  delete self->message_factory;
   delete self->database;
   delete self->pool;
   Py_TYPE(self)->tp_free(reinterpret_cast<PyObject*>(self));
@@ -188,35 +180,8 @@ PyObject* FindMessageByName(PyDescriptorPool* self, PyObject* arg) {
   return PyMessageDescriptor_FromDescriptor(message_descriptor);
 }
 
-// Add a message class to our database.
-int RegisterMessageClass(PyDescriptorPool* self,
-                         const Descriptor* message_descriptor,
-                         CMessageClass* message_class) {
-  Py_INCREF(message_class);
-  typedef PyDescriptorPool::ClassesByMessageMap::iterator iterator;
-  std::pair<iterator, bool> ret = self->classes_by_descriptor->insert(
-      std::make_pair(message_descriptor, message_class));
-  if (!ret.second) {
-    // Update case: DECREF the previous value.
-    Py_DECREF(ret.first->second);
-    ret.first->second = message_class;
-  }
-  return 0;
-}
 
-// Retrieve the message class added to our database.
-CMessageClass* GetMessageClass(PyDescriptorPool* self,
-                               const Descriptor* message_descriptor) {
-  typedef PyDescriptorPool::ClassesByMessageMap::iterator iterator;
-  iterator ret = self->classes_by_descriptor->find(message_descriptor);
-  if (ret == self->classes_by_descriptor->end()) {
-    PyErr_Format(PyExc_TypeError, "No message class registered for '%s'",
-                 message_descriptor->full_name().c_str());
-    return NULL;
-  } else {
-    return ret->second;
-  }
-}
+
 
 PyObject* FindFileByName(PyDescriptorPool* self, PyObject* arg) {
   Py_ssize_t name_size;
@@ -228,11 +193,9 @@ PyObject* FindFileByName(PyDescriptorPool* self, PyObject* arg) {
   const FileDescriptor* file_descriptor =
       self->pool->FindFileByName(string(name, name_size));
   if (file_descriptor == NULL) {
-    PyErr_Format(PyExc_KeyError, "Couldn't find file %.200s",
-                 name);
+    PyErr_Format(PyExc_KeyError, "Couldn't find file %.200s", name);
     return NULL;
   }
-
   return PyFileDescriptor_FromDescriptor(file_descriptor);
 }
 
diff --git a/python/google/protobuf/pyext/descriptor_pool.h b/python/google/protobuf/pyext/descriptor_pool.h
index 2a42c112..8de6c60b 100644
--- a/python/google/protobuf/pyext/descriptor_pool.h
+++ b/python/google/protobuf/pyext/descriptor_pool.h
@@ -38,10 +38,10 @@
 
 namespace google {
 namespace protobuf {
-class MessageFactory;
-
 namespace python {
 
+class PyMessageFactory;
+
 // The (meta) type of all Messages classes.
 struct CMessageClass;
 
@@ -69,20 +69,10 @@ typedef struct PyDescriptorPool {
   // This pointer is owned.
   const DescriptorDatabase* database;
 
-  // DynamicMessageFactory used to create C++ instances of messages.
-  // This object cache the descriptors that were used, so the DescriptorPool
-  // needs to get rid of it before it can delete itself.
-  //
-  // Note: A C++ MessageFactory is different from the Python MessageFactory.
-  // The C++ one creates messages, when the Python one creates classes.
-  MessageFactory* message_factory;
-
-  // Make our own mapping to retrieve Python classes from C++ descriptors.
-  //
-  // Descriptor pointers stored here are owned by the DescriptorPool above.
-  // Python references to classes are owned by this PyDescriptorPool.
-  typedef hash_map<const Descriptor*, CMessageClass*> ClassesByMessageMap;
-  ClassesByMessageMap* classes_by_descriptor;
+  // The preferred MessageFactory to be used by descriptors.
+  // TODO(amauryfa): Don't create the Factory from the DescriptorPool, but
+  // use the one passed while creating message classes. And remove this member.
+  PyMessageFactory* py_message_factory;
 
   // Cache the options for any kind of descriptor.
   // Descriptor pointers are owned by the DescriptorPool above.
@@ -100,19 +90,6 @@ namespace cdescriptor_pool {
 const Descriptor* FindMessageTypeByName(PyDescriptorPool* self,
                                         const string& name);
 
-// Registers a new Python class for the given message descriptor.
-// On error, returns -1 with a Python exception set.
-int RegisterMessageClass(PyDescriptorPool* self,
-                         const Descriptor* message_descriptor,
-                         CMessageClass* message_class);
-
-// Retrieves the Python class registered with the given message descriptor.
-//
-// Returns a *borrowed* reference if found, otherwise returns NULL with an
-// exception set.
-CMessageClass* GetMessageClass(PyDescriptorPool* self,
-                               const Descriptor* message_descriptor);
-
 // The functions below are also exposed as methods of the DescriptorPool type.
 
 // Looks up a message by name. Returns a PyMessageDescriptor corresponding to
diff --git a/python/google/protobuf/pyext/extension_dict.cc b/python/google/protobuf/pyext/extension_dict.cc
index 21bbb8c2..dbb7bca0 100644
--- a/python/google/protobuf/pyext/extension_dict.cc
+++ b/python/google/protobuf/pyext/extension_dict.cc
@@ -39,8 +39,8 @@
 #include <google/protobuf/dynamic_message.h>
 #include <google/protobuf/message.h>
 #include <google/protobuf/pyext/descriptor.h>
-#include <google/protobuf/pyext/descriptor_pool.h>
 #include <google/protobuf/pyext/message.h>
+#include <google/protobuf/pyext/message_factory.h>
 #include <google/protobuf/pyext/repeated_composite_container.h>
 #include <google/protobuf/pyext/repeated_scalar_container.h>
 #include <google/protobuf/pyext/scoped_pyobject_ptr.h>
@@ -60,35 +60,6 @@ PyObject* len(ExtensionDict* self) {
 #endif
 }
 
-// TODO(tibell): Use VisitCompositeField.
-int ReleaseExtension(ExtensionDict* self,
-                     PyObject* extension,
-                     const FieldDescriptor* descriptor) {
-  if (descriptor->label() == FieldDescriptor::LABEL_REPEATED) {
-    if (descriptor->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
-      if (repeated_composite_container::Release(
-              reinterpret_cast<RepeatedCompositeContainer*>(
-                  extension)) < 0) {
-        return -1;
-      }
-    } else {
-      if (repeated_scalar_container::Release(
-              reinterpret_cast<RepeatedScalarContainer*>(
-                  extension)) < 0) {
-        return -1;
-      }
-    }
-  } else if (descriptor->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
-    if (cmessage::ReleaseSubMessage(
-            self->parent, descriptor,
-            reinterpret_cast<CMessage*>(extension)) < 0) {
-      return -1;
-    }
-  }
-
-  return 0;
-}
-
 PyObject* subscript(ExtensionDict* self, PyObject* key) {
   const FieldDescriptor* descriptor = cmessage::GetExtensionDescriptor(key);
   if (descriptor == NULL) {
@@ -130,8 +101,8 @@ PyObject* subscript(ExtensionDict* self, PyObject* key) {
 
   if (descriptor->label() == FieldDescriptor::LABEL_REPEATED) {
     if (descriptor->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
-      CMessageClass* message_class = cdescriptor_pool::GetMessageClass(
-          cmessage::GetDescriptorPoolForMessage(self->parent),
+      CMessageClass* message_class = message_factory::GetMessageClass(
+          cmessage::GetFactoryForMessage(self->parent),
           descriptor->message_type());
       if (message_class == NULL) {
         return NULL;
@@ -183,47 +154,6 @@ int ass_subscript(ExtensionDict* self, PyObject* key, PyObject* value) {
   return 0;
 }
 
-PyObject* ClearExtension(ExtensionDict* self, PyObject* extension) {
-  const FieldDescriptor* descriptor =
-      cmessage::GetExtensionDescriptor(extension);
-  if (descriptor == NULL) {
-    return NULL;
-  }
-  PyObject* value = PyDict_GetItem(self->values, extension);
-  if (self->parent) {
-    if (value != NULL) {
-      if (ReleaseExtension(self, value, descriptor) < 0) {
-        return NULL;
-      }
-    }
-    if (ScopedPyObjectPtr(cmessage::ClearFieldByDescriptor(
-            self->parent, descriptor)) == NULL) {
-      return NULL;
-    }
-  }
-  if (PyDict_DelItem(self->values, extension) < 0) {
-    PyErr_Clear();
-  }
-  Py_RETURN_NONE;
-}
-
-PyObject* HasExtension(ExtensionDict* self, PyObject* extension) {
-  const FieldDescriptor* descriptor =
-      cmessage::GetExtensionDescriptor(extension);
-  if (descriptor == NULL) {
-    return NULL;
-  }
-  if (self->parent) {
-    return cmessage::HasFieldByDescriptor(self->parent, descriptor);
-  } else {
-    int exists = PyDict_Contains(self->values, extension);
-    if (exists < 0) {
-      return NULL;
-    }
-    return PyBool_FromLong(exists);
-  }
-}
-
 PyObject* _FindExtensionByName(ExtensionDict* self, PyObject* name) {
   ScopedPyObjectPtr extensions_by_name(PyObject_GetAttrString(
       reinterpret_cast<PyObject*>(self->parent), "_extensions_by_name"));
@@ -282,8 +212,6 @@ static PyMappingMethods MpMethods = {
 
 #define EDMETHOD(name, args, doc) { #name, (PyCFunction)name, args, doc }
 static PyMethodDef Methods[] = {
-  EDMETHOD(ClearExtension, METH_O, "Clears an extension from the object."),
-  EDMETHOD(HasExtension, METH_O, "Checks if the object has an extension."),
   EDMETHOD(_FindExtensionByName, METH_O,
            "Finds an extension by name."),
   EDMETHOD(_FindExtensionByNumber, METH_O,
diff --git a/python/google/protobuf/pyext/extension_dict.h b/python/google/protobuf/pyext/extension_dict.h
index 2456eda1..65b87862 100644
--- a/python/google/protobuf/pyext/extension_dict.h
+++ b/python/google/protobuf/pyext/extension_dict.h
@@ -86,49 +86,6 @@ namespace extension_dict {
 // Builds an Extensions dict for a specific message.
 ExtensionDict* NewExtensionDict(CMessage *parent);
 
-// Gets the number of extension values in this ExtensionDict as a python object.
-//
-// Returns a new reference.
-PyObject* len(ExtensionDict* self);
-
-// Releases extensions referenced outside this dictionary to keep outside
-// references alive.
-//
-// Returns 0 on success, -1 on failure.
-int ReleaseExtension(ExtensionDict* self,
-                     PyObject* extension,
-                     const FieldDescriptor* descriptor);
-
-// Gets an extension from the dict for the given extension descriptor.
-//
-// Returns a new reference.
-PyObject* subscript(ExtensionDict* self, PyObject* key);
-
-// Assigns a value to an extension in the dict. Can only be used for singular
-// simple types.
-//
-// Returns 0 on success, -1 on failure.
-int ass_subscript(ExtensionDict* self, PyObject* key, PyObject* value);
-
-// Clears an extension from the dict. Will release the extension if there
-// is still an external reference left to it.
-//
-// Returns None on success.
-PyObject* ClearExtension(ExtensionDict* self,
-                                       PyObject* extension);
-
-// Gets an extension from the dict given the extension name as opposed to
-// descriptor.
-//
-// Returns a new reference.
-PyObject* _FindExtensionByName(ExtensionDict* self, PyObject* name);
-
-// Gets an extension from the dict given the extension field number as
-// opposed to descriptor.
-//
-// Returns a new reference.
-PyObject* _FindExtensionByNumber(ExtensionDict* self, PyObject* number);
-
 }  // namespace extension_dict
 }  // namespace python
 }  // namespace protobuf
diff --git a/python/google/protobuf/pyext/map_container.cc b/python/google/protobuf/pyext/map_container.cc
index 0987b898..318c2e7c 100644
--- a/python/google/protobuf/pyext/map_container.cc
+++ b/python/google/protobuf/pyext/map_container.cc
@@ -42,7 +42,9 @@
 #include <google/protobuf/map_field.h>
 #include <google/protobuf/map.h>
 #include <google/protobuf/message.h>
+#include <google/protobuf/pyext/message_factory.h>
 #include <google/protobuf/pyext/message.h>
+#include <google/protobuf/pyext/repeated_composite_container.h>
 #include <google/protobuf/pyext/scoped_pyobject_ptr.h>
 
 #if PY_MAJOR_VERSION >= 3
@@ -328,6 +330,15 @@ PyObject* Clear(PyObject* _self) {
   Py_RETURN_NONE;
 }
 
+PyObject* GetEntryClass(PyObject* _self) {
+  MapContainer* self = GetMap(_self);
+  CMessageClass* message_class = message_factory::GetMessageClass(
+      cmessage::GetFactoryForMessage(self->parent),
+      self->parent_field_descriptor->message_type());
+  Py_XINCREF(message_class);
+  return reinterpret_cast<PyObject*>(message_class);
+}
+
 PyObject* MapReflectionFriend::Contains(PyObject* _self, PyObject* key) {
   MapContainer* self = GetMap(_self);
 
@@ -400,12 +411,7 @@ PyObject *NewScalarMapContainer(
     return NULL;
   }
 
-#if PY_MAJOR_VERSION >= 3
-  ScopedPyObjectPtr obj(PyType_GenericAlloc(
-        reinterpret_cast<PyTypeObject *>(ScalarMapContainer_Type), 0));
-#else
-  ScopedPyObjectPtr obj(PyType_GenericAlloc(&ScalarMapContainer_Type, 0));
-#endif
+  ScopedPyObjectPtr obj(PyType_GenericAlloc(ScalarMapContainer_Type, 0));
   if (obj.get() == NULL) {
     return PyErr_Format(PyExc_RuntimeError,
                         "Could not allocate new container.");
@@ -527,6 +533,8 @@ static PyMethodDef ScalarMapMethods[] = {
     "Removes all elements from the map." },
   { "get", ScalarMapGet, METH_VARARGS,
     "Gets the value for the given key if present, or otherwise a default" },
+  { "GetEntryClass", (PyCFunction)GetEntryClass, METH_NOARGS,
+    "Return the class used to build Entries of (key, value) pairs." },
   /*
   { "__deepcopy__", (PyCFunction)DeepCopy, METH_VARARGS,
     "Makes a deep copy of the class." },
@@ -536,6 +544,7 @@ static PyMethodDef ScalarMapMethods[] = {
   {NULL, NULL},
 };
 
+PyTypeObject *ScalarMapContainer_Type;
 #if PY_MAJOR_VERSION >= 3
   static PyType_Slot ScalarMapContainer_Type_slots[] = {
       {Py_tp_dealloc, (void *)ScalarMapDealloc},
@@ -554,7 +563,6 @@ static PyMethodDef ScalarMapMethods[] = {
       Py_TPFLAGS_DEFAULT,
       ScalarMapContainer_Type_slots
   };
-  PyObject *ScalarMapContainer_Type;
 #else
   static PyMappingMethods ScalarMapMappingMethods = {
     MapReflectionFriend::Length,             // mp_length
@@ -562,7 +570,7 @@ static PyMethodDef ScalarMapMethods[] = {
     MapReflectionFriend::ScalarMapSetItem,   // mp_ass_subscript
   };
 
-  PyTypeObject ScalarMapContainer_Type = {
+  PyTypeObject _ScalarMapContainer_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
     FULL_MODULE_NAME ".ScalarMapContainer",  //  tp_name
     sizeof(MapContainer),                //  tp_basicsize
@@ -643,12 +651,7 @@ PyObject* NewMessageMapContainer(
     return NULL;
   }
 
-#if PY_MAJOR_VERSION >= 3
-  PyObject* obj = PyType_GenericAlloc(
-        reinterpret_cast<PyTypeObject *>(MessageMapContainer_Type), 0);
-#else
-  PyObject* obj = PyType_GenericAlloc(&MessageMapContainer_Type, 0);
-#endif
+  PyObject* obj = PyType_GenericAlloc(MessageMapContainer_Type, 0);
   if (obj == NULL) {
     return PyErr_Format(PyExc_RuntimeError,
                         "Could not allocate new container.");
@@ -780,6 +783,8 @@ static PyMethodDef MessageMapMethods[] = {
     "Gets the value for the given key if present, or otherwise a default" },
   { "get_or_create", MapReflectionFriend::MessageMapGetItem, METH_O,
     "Alias for getitem, useful to make explicit that the map is mutated." },
+  { "GetEntryClass", (PyCFunction)GetEntryClass, METH_NOARGS,
+    "Return the class used to build Entries of (key, value) pairs." },
   /*
   { "__deepcopy__", (PyCFunction)DeepCopy, METH_VARARGS,
     "Makes a deep copy of the class." },
@@ -789,6 +794,7 @@ static PyMethodDef MessageMapMethods[] = {
   {NULL, NULL},
 };
 
+PyTypeObject *MessageMapContainer_Type;
 #if PY_MAJOR_VERSION >= 3
   static PyType_Slot MessageMapContainer_Type_slots[] = {
       {Py_tp_dealloc, (void *)MessageMapDealloc},
@@ -807,8 +813,6 @@ static PyMethodDef MessageMapMethods[] = {
       Py_TPFLAGS_DEFAULT,
       MessageMapContainer_Type_slots
   };
-
-  PyObject *MessageMapContainer_Type;
 #else
   static PyMappingMethods MessageMapMappingMethods = {
     MapReflectionFriend::Length,              // mp_length
@@ -816,7 +820,7 @@ static PyMethodDef MessageMapMethods[] = {
     MapReflectionFriend::MessageMapSetItem,   // mp_ass_subscript
   };
 
-  PyTypeObject MessageMapContainer_Type = {
+  PyTypeObject _MessageMapContainer_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
     FULL_MODULE_NAME ".MessageMapContainer",  //  tp_name
     sizeof(MessageMapContainer),         //  tp_basicsize
@@ -965,6 +969,63 @@ PyTypeObject MapIterator_Type = {
   0,                                   //  tp_init
 };
 
+bool InitMapContainers() {
+  // ScalarMapContainer_Type derives from our MutableMapping type.
+  ScopedPyObjectPtr containers(PyImport_ImportModule(
+      "google.protobuf.internal.containers"));
+  if (containers == NULL) {
+    return false;
+  }
+
+  ScopedPyObjectPtr mutable_mapping(
+      PyObject_GetAttrString(containers.get(), "MutableMapping"));
+  if (mutable_mapping == NULL) {
+    return false;
+  }
+
+  if (!PyObject_TypeCheck(mutable_mapping.get(), &PyType_Type)) {
+    return false;
+  }
+
+  Py_INCREF(mutable_mapping.get());
+#if PY_MAJOR_VERSION >= 3
+  PyObject* bases = PyTuple_New(1);
+  PyTuple_SET_ITEM(bases, 0, mutable_mapping.get());
+
+  ScalarMapContainer_Type = reinterpret_cast<PyTypeObject*>(
+      PyType_FromSpecWithBases(&ScalarMapContainer_Type_spec, bases));
+#else
+  _ScalarMapContainer_Type.tp_base =
+      reinterpret_cast<PyTypeObject*>(mutable_mapping.get());
+
+  if (PyType_Ready(&_ScalarMapContainer_Type) < 0) {
+    return false;
+  }
+
+  ScalarMapContainer_Type = &_ScalarMapContainer_Type;
+#endif
+
+  if (PyType_Ready(&MapIterator_Type) < 0) {
+    return false;
+  }
+
+#if PY_MAJOR_VERSION >= 3
+  MessageMapContainer_Type = reinterpret_cast<PyTypeObject*>(
+      PyType_FromSpecWithBases(&MessageMapContainer_Type_spec, bases));
+#else
+  Py_INCREF(mutable_mapping.get());
+  _MessageMapContainer_Type.tp_base =
+      reinterpret_cast<PyTypeObject*>(mutable_mapping.get());
+
+  if (PyType_Ready(&_MessageMapContainer_Type) < 0) {
+    return false;
+  }
+
+  MessageMapContainer_Type = &_MessageMapContainer_Type;
+#endif
+  return true;
+}
+
 }  // namespace python
 }  // namespace protobuf
 }  // namespace google
diff --git a/python/google/protobuf/pyext/map_container.h b/python/google/protobuf/pyext/map_container.h
index fbd6713f..615657b0 100644
--- a/python/google/protobuf/pyext/map_container.h
+++ b/python/google/protobuf/pyext/map_container.h
@@ -112,16 +112,10 @@ struct MessageMapContainer : public MapContainer {
   PyObject* message_dict;
 };
 
-#if PY_MAJOR_VERSION >= 3
-  extern PyObject *MessageMapContainer_Type;
-  extern PyType_Spec MessageMapContainer_Type_spec;
-  extern PyObject *ScalarMapContainer_Type;
-  extern PyType_Spec ScalarMapContainer_Type_spec;
-#else
-  extern PyTypeObject MessageMapContainer_Type;
-  extern PyTypeObject ScalarMapContainer_Type;
-#endif
+bool InitMapContainers();
 
+extern PyTypeObject* MessageMapContainer_Type;
+extern PyTypeObject* ScalarMapContainer_Type;
 extern PyTypeObject MapIterator_Type;  // Both map types use the same iterator.
 
 // Builds a MapContainer object, from a parent message and a
diff --git a/python/google/protobuf/pyext/message.cc b/python/google/protobuf/pyext/message.cc
index 5535338d..1b325469 100644
--- a/python/google/protobuf/pyext/message.cc
+++ b/python/google/protobuf/pyext/message.cc
@@ -63,6 +63,7 @@
 #include <google/protobuf/pyext/repeated_composite_container.h>
 #include <google/protobuf/pyext/repeated_scalar_container.h>
 #include <google/protobuf/pyext/map_container.h>
+#include <google/protobuf/pyext/message_factory.h>
 #include <google/protobuf/pyext/scoped_pyobject_ptr.h>
 #include <google/protobuf/stubs/strutil.h>
 
@@ -244,6 +245,12 @@ static PyObject* New(PyTypeObject* type,
     return NULL;
   }
 
+  // Messages have no __dict__
+  ScopedPyObjectPtr slots(PyTuple_New(0));
+  if (PyDict_SetItemString(dict, "__slots__", slots.get()) < 0) {
+    return NULL;
+  }
+
   // Build the arguments to the base metaclass.
   // We change the __bases__ classes.
   ScopedPyObjectPtr new_args;
@@ -300,16 +307,19 @@ static PyObject* New(PyTypeObject* type,
   newtype->message_descriptor = descriptor;
   // TODO(amauryfa): Don't always use the canonical pool of the descriptor,
   // use the MessageFactory optionally passed in the class dict.
-  newtype->py_descriptor_pool = GetDescriptorPool_FromPool(
-      descriptor->file()->pool());
-  if (newtype->py_descriptor_pool == NULL) {
+  PyDescriptorPool* py_descriptor_pool =
+      GetDescriptorPool_FromPool(descriptor->file()->pool());
+  if (py_descriptor_pool == NULL) {
     return NULL;
   }
-  Py_INCREF(newtype->py_descriptor_pool);
+  newtype->py_message_factory = py_descriptor_pool->py_message_factory;
+  Py_INCREF(newtype->py_message_factory);
 
-  // Add the message to the DescriptorPool.
-  if (cdescriptor_pool::RegisterMessageClass(newtype->py_descriptor_pool,
-                                             descriptor, newtype) < 0) {
+  // Register the message in the MessageFactory.
+  // TODO(amauryfa): Move this call to MessageFactory.GetPrototype() when the
+  // MessageFactory is fully implemented in C++.
+  if (message_factory::RegisterMessageClass(newtype->py_message_factory,
+                                            descriptor, newtype) < 0) {
     return NULL;
   }
 
@@ -321,8 +331,8 @@ static PyObject* New(PyTypeObject* type,
 }
 
 static void Dealloc(CMessageClass *self) {
-  Py_DECREF(self->py_message_descriptor);
-  Py_DECREF(self->py_descriptor_pool);
+  Py_XDECREF(self->py_message_descriptor);
+  Py_XDECREF(self->py_message_factory);
   Py_TYPE(self)->tp_free(reinterpret_cast<PyObject*>(self));
 }
 
@@ -752,15 +762,9 @@ bool CheckFieldBelongsToMessage(const FieldDescriptor* field_descriptor,
 
 namespace cmessage {
 
-PyDescriptorPool* GetDescriptorPoolForMessage(CMessage* message) {
-  // No need to check the type: the type of instances of CMessage is always
-  // an instance of CMessageClass. Let's prove it with a debug-only check.
+PyMessageFactory* GetFactoryForMessage(CMessage* message) {
   GOOGLE_DCHECK(PyObject_TypeCheck(message, &CMessage_Type));
-  return reinterpret_cast<CMessageClass*>(Py_TYPE(message))->py_descriptor_pool;
-}
-
-MessageFactory* GetFactoryForMessage(CMessage* message) {
-  return GetDescriptorPoolForMessage(message)->message_factory;
+  return reinterpret_cast<CMessageClass*>(Py_TYPE(message))->py_message_factory;
 }
 
 static int MaybeReleaseOverlappingOneofField(
@@ -813,7 +817,8 @@ static Message* GetMutableMessage(
     return NULL;
   }
   return reflection->MutableMessage(
-      parent_message, parent_field, GetFactoryForMessage(parent));
+      parent_message, parent_field,
+      GetFactoryForMessage(parent)->message_factory);
 }
 
 struct FixupMessageReference : public ChildVisitor {
@@ -1172,6 +1177,8 @@ int InitAttributes(CMessage* self, PyObject* args, PyObject* kwargs) {
       }
       CMessage* cmessage = reinterpret_cast<CMessage*>(message.get());
       if (PyDict_Check(value)) {
+        // Make the message exist even if the dict is empty.
+        AssureWritable(cmessage);
         if (InitAttributes(cmessage, NULL, value) < 0) {
           return -1;
         }
@@ -1231,7 +1238,7 @@ static PyObject* New(PyTypeObject* cls,
   if (message_descriptor == NULL) {
     return NULL;
   }
-  const Message* default_message = type->py_descriptor_pool->message_factory
+  const Message* default_message = type->py_message_factory->message_factory
                                    ->GetPrototype(message_descriptor);
   if (default_message == NULL) {
     PyErr_SetString(PyExc_TypeError, message_descriptor->full_name().c_str());
@@ -1292,6 +1299,9 @@ struct ClearWeakReferences : public ChildVisitor {
 };
 
 static void Dealloc(CMessage* self) {
+  if (self->weakreflist) {
+    PyObject_ClearWeakRefs(reinterpret_cast<PyObject*>(self));
+  }
   // Null out all weak references from children to this message.
   GOOGLE_CHECK_EQ(0, ForEachCompositeField(self, ClearWeakReferences()));
   if (self->extensions) {
@@ -1459,18 +1469,20 @@ PyObject* HasField(CMessage* self, PyObject* arg) {
 }
 
 PyObject* ClearExtension(CMessage* self, PyObject* extension) {
+  const FieldDescriptor* descriptor = GetExtensionDescriptor(extension);
+  if (descriptor == NULL) {
+    return NULL;
+  }
   if (self->extensions != NULL) {
-    return extension_dict::ClearExtension(self->extensions, extension);
-  } else {
-    const FieldDescriptor* descriptor = GetExtensionDescriptor(extension);
-    if (descriptor == NULL) {
-      return NULL;
-    }
-    if (ScopedPyObjectPtr(ClearFieldByDescriptor(self, descriptor)) == NULL) {
-      return NULL;
+    PyObject* value = PyDict_GetItem(self->extensions->values, extension);
+    if (value != NULL) {
+      if (InternalReleaseFieldByDescriptor(self, descriptor, value) < 0) {
+        return NULL;
+      }
+      PyDict_DelItem(self->extensions->values, extension);
     }
   }
-  Py_RETURN_NONE;
+  return ClearFieldByDescriptor(self, descriptor);
 }
 
 PyObject* HasExtension(CMessage* self, PyObject* extension) {
@@ -1556,7 +1568,7 @@ int SetOwner(CMessage* self, const shared_ptr<Message>& new_owner) {
 Message* ReleaseMessage(CMessage* self,
                         const Descriptor* descriptor,
                         const FieldDescriptor* field_descriptor) {
-  MessageFactory* message_factory = GetFactoryForMessage(self);
+  MessageFactory* message_factory = GetFactoryForMessage(self)->message_factory;
   Message* released_message = self->message->GetReflection()->ReleaseMessage(
       self->message, field_descriptor, message_factory);
   // ReleaseMessage will return NULL which differs from
@@ -1624,12 +1636,19 @@ int InternalReleaseFieldByDescriptor(
 
 PyObject* ClearFieldByDescriptor(
     CMessage* self,
-    const FieldDescriptor* descriptor) {
-  if (!CheckFieldBelongsToMessage(descriptor, self->message)) {
+    const FieldDescriptor* field_descriptor) {
+  if (!CheckFieldBelongsToMessage(field_descriptor, self->message)) {
     return NULL;
   }
   AssureWritable(self);
-  self->message->GetReflection()->ClearField(self->message, descriptor);
+  Message* message = self->message;
+  message->GetReflection()->ClearField(message, field_descriptor);
+  if (field_descriptor->cpp_type() == FieldDescriptor::CPPTYPE_ENUM &&
+      !message->GetReflection()->SupportsUnknownEnumValues()) {
+    UnknownFieldSet* unknown_field_set =
+        message->GetReflection()->MutableUnknownFields(message);
+    unknown_field_set->DeleteByNumber(field_descriptor->number());
+  }
   Py_RETURN_NONE;
 }
 
@@ -1665,27 +1684,17 @@ PyObject* ClearField(CMessage* self, PyObject* arg) {
     arg = arg_in_oneof.get();
   }
 
-  PyObject* composite_field = self->composite_fields ?
-      PyDict_GetItem(self->composite_fields, arg) : NULL;
-
-  // Only release the field if there's a possibility that there are
-  // references to it.
-  if (composite_field != NULL) {
-    if (InternalReleaseFieldByDescriptor(self, field_descriptor,
-                                         composite_field) < 0) {
-      return NULL;
+  // Release the field if it exists in the dict of composite fields.
+  if (self->composite_fields) {
+    PyObject* value = PyDict_GetItem(self->composite_fields, arg);
+    if (value != NULL) {
+      if (InternalReleaseFieldByDescriptor(self, field_descriptor, value) < 0) {
+        return NULL;
+      }
+      PyDict_DelItem(self->composite_fields, arg);
     }
-    PyDict_DelItem(self->composite_fields, arg);
-  }
-  message->GetReflection()->ClearField(message, field_descriptor);
-  if (field_descriptor->cpp_type() == FieldDescriptor::CPPTYPE_ENUM &&
-      !message->GetReflection()->SupportsUnknownEnumValues()) {
-    UnknownFieldSet* unknown_field_set =
-        message->GetReflection()->MutableUnknownFields(message);
-    unknown_field_set->DeleteByNumber(field_descriptor->number());
   }
-
-  Py_RETURN_NONE;
+  return ClearFieldByDescriptor(self, field_descriptor);
 }
 
 PyObject* Clear(CMessage* self) {
@@ -1927,8 +1936,8 @@ static PyObject* MergeFromString(CMessage* self, PyObject* arg) {
   if (allow_oversize_protos) {
     input.SetTotalBytesLimit(INT_MAX, INT_MAX);
   }
-  PyDescriptorPool* pool = GetDescriptorPoolForMessage(self);
-  input.SetExtensionRegistry(pool->pool, pool->message_factory);
+  PyMessageFactory* factory = GetFactoryForMessage(self);
+  input.SetExtensionRegistry(factory->pool->pool, factory->message_factory);
   bool success = self->message->MergePartialFromCodedStream(&input);
   if (success) {
     return PyInt_FromLong(input.CurrentPosition());
@@ -2108,8 +2117,8 @@ static PyObject* ListFields(CMessage* self) {
       // is no message class and we cannot retrieve the value.
       // TODO(amauryfa): consider building the class on the fly!
       if (fields[i]->message_type() != NULL &&
-          cdescriptor_pool::GetMessageClass(
-              GetDescriptorPoolForMessage(self),
+          message_factory::GetMessageClass(
+              GetFactoryForMessage(self),
               fields[i]->message_type()) == NULL) {
         PyErr_Clear();
         continue;
@@ -2306,12 +2315,12 @@ PyObject* InternalGetScalar(const Message* message,
 PyObject* InternalGetSubMessage(
     CMessage* self, const FieldDescriptor* field_descriptor) {
   const Reflection* reflection = self->message->GetReflection();
-  PyDescriptorPool* pool = GetDescriptorPoolForMessage(self);
+  PyMessageFactory* factory = GetFactoryForMessage(self);
   const Message& sub_message = reflection->GetMessage(
-      *self->message, field_descriptor, pool->message_factory);
+      *self->message, field_descriptor, factory->message_factory);
 
-  CMessageClass* message_class = cdescriptor_pool::GetMessageClass(
-      pool, field_descriptor->message_type());
+  CMessageClass* message_class = message_factory::GetMessageClass(
+      factory, field_descriptor->message_type());
   if (message_class == NULL) {
     return NULL;
   }
@@ -2656,8 +2665,8 @@ PyObject* GetAttr(CMessage* self, PyObject* name) {
     const Descriptor* entry_type = field_descriptor->message_type();
     const FieldDescriptor* value_type = entry_type->FindFieldByName("value");
     if (value_type->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
-      CMessageClass* value_class = cdescriptor_pool::GetMessageClass(
-          GetDescriptorPoolForMessage(self), value_type->message_type());
+      CMessageClass* value_class = message_factory::GetMessageClass(
+          GetFactoryForMessage(self), value_type->message_type());
       if (value_class == NULL) {
         return NULL;
       }
@@ -2679,8 +2688,8 @@ PyObject* GetAttr(CMessage* self, PyObject* name) {
   if (field_descriptor->label() == FieldDescriptor::LABEL_REPEATED) {
     PyObject* py_container = NULL;
     if (field_descriptor->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
-      CMessageClass* message_class = cdescriptor_pool::GetMessageClass(
-          GetDescriptorPoolForMessage(self), field_descriptor->message_type());
+      CMessageClass* message_class = message_factory::GetMessageClass(
+          GetFactoryForMessage(self), field_descriptor->message_type());
       if (message_class == NULL) {
         return NULL;
       }
@@ -2775,7 +2784,7 @@ PyTypeObject CMessage_Type = {
   0,                                   //  tp_traverse
   0,                                   //  tp_clear
   (richcmpfunc)cmessage::RichCompare,  //  tp_richcompare
-  0,                                   //  tp_weaklistoffset
+  offsetof(CMessage, weakreflist),     //  tp_weaklistoffset
   0,                                   //  tp_iter
   0,                                   //  tp_iternext
   cmessage::Methods,                   //  tp_methods
@@ -2863,6 +2872,11 @@ bool InitProto2MessageModule(PyObject *m) {
     return false;
   }
 
+  // Initialize types and globals in message_factory.cc
+  if (!InitMessageFactory()) {
+    return false;
+  }
+
   // Initialize constants defined in this file.
   InitGlobals();
 
@@ -2944,69 +2958,15 @@ bool InitProto2MessageModule(PyObject *m) {
   }
 
   // Initialize Map container types.
-  {
-    // ScalarMapContainer_Type derives from our MutableMapping type.
-    ScopedPyObjectPtr containers(PyImport_ImportModule(
-        "google.protobuf.internal.containers"));
-    if (containers == NULL) {
-      return false;
-    }
-
-    ScopedPyObjectPtr mutable_mapping(
-        PyObject_GetAttrString(containers.get(), "MutableMapping"));
-    if (mutable_mapping == NULL) {
-      return false;
-    }
-
-    if (!PyObject_TypeCheck(mutable_mapping.get(), &PyType_Type)) {
-      return false;
-    }
-
-    Py_INCREF(mutable_mapping.get());
-#if PY_MAJOR_VERSION >= 3
-    PyObject* bases = PyTuple_New(1);
-    PyTuple_SET_ITEM(bases, 0, mutable_mapping.get());
-
-    ScalarMapContainer_Type = 
-        PyType_FromSpecWithBases(&ScalarMapContainer_Type_spec, bases);
-    PyModule_AddObject(m, "ScalarMapContainer", ScalarMapContainer_Type);
-#else
-    ScalarMapContainer_Type.tp_base =
-        reinterpret_cast<PyTypeObject*>(mutable_mapping.get());
-
-    if (PyType_Ready(&ScalarMapContainer_Type) < 0) {
-      return false;
-    }
-
-    PyModule_AddObject(m, "ScalarMapContainer",
-                       reinterpret_cast<PyObject*>(&ScalarMapContainer_Type));
-#endif
-
-    if (PyType_Ready(&MapIterator_Type) < 0) {
-      return false;
-    }
-
-    PyModule_AddObject(m, "MapIterator",
-                       reinterpret_cast<PyObject*>(&MapIterator_Type));
-
-
-#if PY_MAJOR_VERSION >= 3
-    MessageMapContainer_Type = 
-        PyType_FromSpecWithBases(&MessageMapContainer_Type_spec, bases);
-    PyModule_AddObject(m, "MessageMapContainer", MessageMapContainer_Type);
-#else
-    Py_INCREF(mutable_mapping.get());
-    MessageMapContainer_Type.tp_base =
-        reinterpret_cast<PyTypeObject*>(mutable_mapping.get());
-
-    if (PyType_Ready(&MessageMapContainer_Type) < 0) {
-      return false;
-    }
-
-    PyModule_AddObject(m, "MessageMapContainer",
-                       reinterpret_cast<PyObject*>(&MessageMapContainer_Type));
-#endif
+  if (!InitMapContainers()) {
+    return false;
   }
+  PyModule_AddObject(m, "ScalarMapContainer",
+                     reinterpret_cast<PyObject*>(ScalarMapContainer_Type));
+  PyModule_AddObject(m, "MessageMapContainer",
+                     reinterpret_cast<PyObject*>(MessageMapContainer_Type));
+  PyModule_AddObject(m, "MapIterator",
+                     reinterpret_cast<PyObject*>(&MapIterator_Type));
 
   if (PyType_Ready(&ExtensionDict_Type) < 0) {
     return false;
diff --git a/python/google/protobuf/pyext/message.h b/python/google/protobuf/pyext/message.h
index c44a2ae2..1550724c 100644
--- a/python/google/protobuf/pyext/message.h
+++ b/python/google/protobuf/pyext/message.h
@@ -62,7 +62,7 @@ using internal::shared_ptr;
 namespace python {
 
 struct ExtensionDict;
-struct PyDescriptorPool;
+struct PyMessageFactory;
 
 typedef struct CMessage {
   PyObject_HEAD;
@@ -112,6 +112,9 @@ typedef struct CMessage {
   // Similar to composite_fields, acting as a cache, but also contains the
   // required extension dict logic.
   ExtensionDict* extensions;
+
+  // Implements the "weakref" protocol for this object.
+  PyObject* weakreflist;
 } CMessage;
 
 extern PyTypeObject CMessage_Type;
@@ -132,14 +135,11 @@ struct CMessageClass {
   // Owned reference, used to keep the pointer above alive.
   PyObject* py_message_descriptor;
 
-  // The Python DescriptorPool used to create the class. It is needed to resolve
+  // The Python MessageFactory used to create the class. It is needed to resolve
   // fields descriptors, including extensions fields; its C++ MessageFactory is
   // used to instantiate submessages.
-  // This can be different from DESCRIPTOR.file.pool, in the case of a custom
-  // DescriptorPool which defines new extensions.
-  // We own the reference, because it's important to keep the descriptors and
-  // factory alive.
-  PyDescriptorPool* py_descriptor_pool;
+  // We own the reference, because it's important to keep the factory alive.
+  PyMessageFactory* py_message_factory;
 
   PyObject* AsPyObject() {
     return reinterpret_cast<PyObject*>(this);
@@ -154,14 +154,6 @@ namespace cmessage {
 // The caller must fill self->message, self->owner and eventually self->parent.
 CMessage* NewEmptyMessage(CMessageClass* type);
 
-// Release a submessage from its proto tree, making it a new top-level messgae.
-// A new message will be created if this is a read-only default instance.
-//
-// Corresponds to reflection api method ReleaseMessage.
-int ReleaseSubMessage(CMessage* self,
-                      const FieldDescriptor* field_descriptor,
-                      CMessage* child_cmessage);
-
 // Retrieves the C++ descriptor of a Python Extension descriptor.
 // On error, return NULL with an exception set.
 const FieldDescriptor* GetExtensionDescriptor(PyObject* extension);
@@ -262,14 +254,13 @@ int SetOwner(CMessage* self, const shared_ptr<Message>& new_owner);
 
 int AssureWritable(CMessage* self);
 
-// Returns the "best" DescriptorPool for the given message.
-// This is often equivalent to message.DESCRIPTOR.pool, but not always, when
-// the message class was created from a MessageFactory using a custom pool which
-// uses the generated pool as an underlay.
+// Returns the message factory for the given message.
+// This is equivalent to message.MESSAGE_FACTORY
 //
-// The returned pool is suitable for finding fields and building submessages,
+// The returned factory is suitable for finding fields and building submessages,
 // even in the case of extensions.
-PyDescriptorPool* GetDescriptorPoolForMessage(CMessage* message);
+// Returns a *borrowed* reference, and never fails because we pass a CMessage.
+PyMessageFactory* GetFactoryForMessage(CMessage* message);
 
 PyObject* SetAllowOversizeProtos(PyObject* m, PyObject* arg);
 
diff --git a/python/google/protobuf/pyext/message_factory.cc b/python/google/protobuf/pyext/message_factory.cc
new file mode 100644
index 00000000..2ad89022
--- /dev/null
+++ b/python/google/protobuf/pyext/message_factory.cc
@@ -0,0 +1,214 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include <Python.h>
+
+#include <google/protobuf/dynamic_message.h>
+#include <google/protobuf/pyext/descriptor.h>
+#include <google/protobuf/pyext/message.h>
+#include <google/protobuf/pyext/message_factory.h>
+#include <google/protobuf/pyext/scoped_pyobject_ptr.h>
+
+#if PY_MAJOR_VERSION >= 3
+  #if PY_VERSION_HEX < 0x03030000
+    #error "Python 3.0 - 3.2 are not supported."
+  #endif
+  #define PyString_AsStringAndSize(ob, charpp, sizep) \
+    (PyUnicode_Check(ob)? \
+       ((*(charpp) = PyUnicode_AsUTF8AndSize(ob, (sizep))) == NULL? -1: 0): \
+       PyBytes_AsStringAndSize(ob, (charpp), (sizep)))
+#endif
+
+namespace google {
+namespace protobuf {
+namespace python {
+
+namespace message_factory {
+
+PyMessageFactory* NewMessageFactory(PyTypeObject* type, PyDescriptorPool* pool) {
+  PyMessageFactory* factory = reinterpret_cast<PyMessageFactory*>(
+      PyType_GenericAlloc(type, 0));
+  if (factory == NULL) {
+    return NULL;
+  }
+
+  DynamicMessageFactory* message_factory = new DynamicMessageFactory();
+  // This option might be the default some day.
+  message_factory->SetDelegateToGeneratedFactory(true);
+  factory->message_factory = message_factory;
+
+  factory->pool = pool;
+  // TODO(amauryfa): When the MessageFactory is not created from the
+  // DescriptorPool this reference should be owned, not borrowed.
+  // Py_INCREF(pool);
+
+  factory->classes_by_descriptor = new PyMessageFactory::ClassesByMessageMap();
+
+  return factory;
+}
+
+PyObject* New(PyTypeObject* type, PyObject* args, PyObject* kwargs) {
+  static char* kwlist[] = {"pool", 0};
+  PyObject* pool = NULL;
+  if (!PyArg_ParseTupleAndKeywords(args, kwargs, "|O", kwlist, &pool)) {
+    return NULL;
+  }
+  ScopedPyObjectPtr owned_pool;
+  if (pool == NULL || pool == Py_None) {
+    owned_pool.reset(PyObject_CallFunction(
+        reinterpret_cast<PyObject*>(&PyDescriptorPool_Type), NULL));
+    if (owned_pool == NULL) {
+      return NULL;
+    }
+    pool = owned_pool.get();
+  } else {
+    if (!PyObject_TypeCheck(pool, &PyDescriptorPool_Type)) {
+      PyErr_Format(PyExc_TypeError, "Expected a DescriptorPool, got %s",
+                   pool->ob_type->tp_name);
+      return NULL;
+    }
+  }
+
+  return reinterpret_cast<PyObject*>(
+      NewMessageFactory(type, reinterpret_cast<PyDescriptorPool*>(pool)));
+}
+
+static void Dealloc(PyMessageFactory* self) {
+  // TODO(amauryfa): When the MessageFactory is not created from the
+  // DescriptorPool this reference should be owned, not borrowed.
+  // Py_CLEAR(self->pool);
+  typedef PyMessageFactory::ClassesByMessageMap::iterator iterator;
+  for (iterator it = self->classes_by_descriptor->begin();
+       it != self->classes_by_descriptor->end(); ++it) {
+    Py_DECREF(it->second);
+  }
+  delete self->classes_by_descriptor;
+  delete self->message_factory;
+  Py_TYPE(self)->tp_free(reinterpret_cast<PyObject*>(self));
+}
+
+// Add a message class to our database.
+int RegisterMessageClass(PyMessageFactory* self,
+                         const Descriptor* message_descriptor,
+                         CMessageClass* message_class) {
+  Py_INCREF(message_class);
+  typedef PyMessageFactory::ClassesByMessageMap::iterator iterator;
+  std::pair<iterator, bool> ret = self->classes_by_descriptor->insert(
+      std::make_pair(message_descriptor, message_class));
+  if (!ret.second) {
+    // Update case: DECREF the previous value.
+    Py_DECREF(ret.first->second);
+    ret.first->second = message_class;
+  }
+  return 0;
+}
+
+// Retrieve the message class added to our database.
+CMessageClass* GetMessageClass(PyMessageFactory* self,
+                               const Descriptor* message_descriptor) {
+  typedef PyMessageFactory::ClassesByMessageMap::iterator iterator;
+  iterator ret = self->classes_by_descriptor->find(message_descriptor);
+  if (ret == self->classes_by_descriptor->end()) {
+    PyErr_Format(PyExc_TypeError, "No message class registered for '%s'",
+                 message_descriptor->full_name().c_str());
+    return NULL;
+  } else {
+    return ret->second;
+  }
+}
+
+static PyMethodDef Methods[] = {
+    {NULL}};
+
+static PyObject* GetPool(PyMessageFactory* self, void* closure) {
+  Py_INCREF(self->pool);
+  return reinterpret_cast<PyObject*>(self->pool);
+}
+
+static PyGetSetDef Getters[] = {
+    {"pool", (getter)GetPool, NULL, "DescriptorPool"},
+    {NULL}
+};
+
+}  // namespace message_factory
+
+PyTypeObject PyMessageFactory_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0) FULL_MODULE_NAME
+    ".MessageFactory",                        // tp_name
+    sizeof(PyMessageFactory),                 // tp_basicsize
+    0,                                        // tp_itemsize
+    (destructor)message_factory::Dealloc,     // tp_dealloc
+    0,                                        // tp_print
+    0,                                        // tp_getattr
+    0,                                        // tp_setattr
+    0,                                        // tp_compare
+    0,                                        // tp_repr
+    0,                                        // tp_as_number
+    0,                                        // tp_as_sequence
+    0,                                        // tp_as_mapping
+    0,                                        // tp_hash
+    0,                                        // tp_call
+    0,                                        // tp_str
+    0,                                        // tp_getattro
+    0,                                        // tp_setattro
+    0,                                        // tp_as_buffer
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,  // tp_flags
+    "A static Message Factory",               // tp_doc
+    0,                                        // tp_traverse
+    0,                                        // tp_clear
+    0,                                        // tp_richcompare
+    0,                                        // tp_weaklistoffset
+    0,                                        // tp_iter
+    0,                                        // tp_iternext
+    message_factory::Methods,                 // tp_methods
+    0,                                        // tp_members
+    message_factory::Getters,                 // tp_getset
+    0,                                        // tp_base
+    0,                                        // tp_dict
+    0,                                        // tp_descr_get
+    0,                                        // tp_descr_set
+    0,                                        // tp_dictoffset
+    0,                                        // tp_init
+    0,                                        // tp_alloc
+    message_factory::New,                     // tp_new
+    PyObject_Del,                             // tp_free
+};
+
+bool InitMessageFactory() {
+  if (PyType_Ready(&PyMessageFactory_Type) < 0) {
+    return false;
+  }
+
+  return true;
+}
+
+}  // namespace python
+}  // namespace protobuf
+}  // namespace google
diff --git a/python/google/protobuf/pyext/message_factory.h b/python/google/protobuf/pyext/message_factory.h
new file mode 100644
index 00000000..07cccbfb
--- /dev/null
+++ b/python/google/protobuf/pyext/message_factory.h
@@ -0,0 +1,103 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef GOOGLE_PROTOBUF_PYTHON_CPP_MESSAGE_FACTORY_H__
+#define GOOGLE_PROTOBUF_PYTHON_CPP_MESSAGE_FACTORY_H__
+
+#include <Python.h>
+
+#include <google/protobuf/stubs/hash.h>
+#include <google/protobuf/descriptor.h>
+#include <google/protobuf/pyext/descriptor_pool.h>
+
+namespace google {
+namespace protobuf {
+class MessageFactory;
+
+namespace python {
+
+// The (meta) type of all Messages classes.
+struct CMessageClass;
+
+struct PyMessageFactory {
+  PyObject_HEAD
+
+  // DynamicMessageFactory used to create C++ instances of messages.
+  // This object cache the descriptors that were used, so the DescriptorPool
+  // needs to get rid of it before it can delete itself.
+  //
+  // Note: A C++ MessageFactory is different from the PyMessageFactory.
+  // The C++ one creates messages, when the Python one creates classes.
+  MessageFactory* message_factory;
+
+  // borrowed reference to a Python DescriptorPool.
+  // TODO(amauryfa): invert the dependency: the MessageFactory owns the
+  // DescriptorPool, not the opposite.
+  PyDescriptorPool* pool;
+
+  // Make our own mapping to retrieve Python classes from C++ descriptors.
+  //
+  // Descriptor pointers stored here are owned by the DescriptorPool above.
+  // Python references to classes are owned by this PyDescriptorPool.
+  typedef hash_map<const Descriptor*, CMessageClass*> ClassesByMessageMap;
+  ClassesByMessageMap* classes_by_descriptor;
+};
+
+extern PyTypeObject PyMessageFactory_Type;
+
+namespace message_factory {
+
+// Creates a new MessageFactory instance.
+PyMessageFactory* NewMessageFactory(PyTypeObject* type, PyDescriptorPool* pool);
+
+// Registers a new Python class for the given message descriptor.
+// On error, returns -1 with a Python exception set.
+int RegisterMessageClass(PyMessageFactory* self,
+                         const Descriptor* message_descriptor,
+                         CMessageClass* message_class);
+
+// Retrieves the Python class registered with the given message descriptor.
+//
+// Returns a *borrowed* reference if found, otherwise returns NULL with an
+// exception set.
+CMessageClass* GetMessageClass(PyMessageFactory* self,
+                               const Descriptor* message_descriptor);
+
+}  // namespace message_factory
+
+// Initialize objects used by this module.
+// On error, returns false with a Python exception set.
+bool InitMessageFactory();
+
+}  // namespace python
+}  // namespace protobuf
+
+}  // namespace google
+#endif  // GOOGLE_PROTOBUF_PYTHON_CPP_MESSAGE_FACTORY_H__
diff --git a/python/google/protobuf/pyext/repeated_composite_container.cc b/python/google/protobuf/pyext/repeated_composite_container.cc
index bb2f6db2..43a2bc12 100644
--- a/python/google/protobuf/pyext/repeated_composite_container.cc
+++ b/python/google/protobuf/pyext/repeated_composite_container.cc
@@ -364,7 +364,7 @@ static int SortPythonMessages(RepeatedCompositeContainer* self,
   ScopedPyObjectPtr m(PyObject_GetAttrString(self->child_messages, "sort"));
   if (m == NULL)
     return -1;
-  if (PyObject_Call(m.get(), args, kwds) == NULL)
+  if (ScopedPyObjectPtr(PyObject_Call(m.get(), args, kwds)) == NULL)
     return -1;
   if (self->message != NULL) {
     ReorderAttached(self);