Down-integrate from internal code base.

14 files changed, 1498 insertions, 1439 deletions

diff --git a/python/google/protobuf/pyext/descriptor.cc b/python/google/protobuf/pyext/descriptor.cc
index 61a3d237..a875a7be 100644
--- a/python/google/protobuf/pyext/descriptor.cc
+++ b/python/google/protobuf/pyext/descriptor.cc
@@ -203,6 +203,14 @@ static PyObject* GetOrBuildOptions(const DescriptorClass *descriptor) {
   PyObject* message_class(cdescriptor_pool::GetMessageClass(
       pool, message_type));
   if (message_class == NULL) {
+    // The Options message was not found in the current DescriptorPool.
+    // In this case, there cannot be extensions to these options, and we can
+    // try to use the basic pool instead.
+    PyErr_Clear();
+    message_class = cdescriptor_pool::GetMessageClass(
+      GetDefaultDescriptorPool(), message_type);
+  }
+  if (message_class == NULL) {
     PyErr_Format(PyExc_TypeError, "Could not retrieve class for Options: %s",
                  message_type->full_name().c_str());
     return NULL;
@@ -211,6 +219,12 @@ static PyObject* GetOrBuildOptions(const DescriptorClass *descriptor) {
   if (value == NULL) {
     return NULL;
   }
+  if (!PyObject_TypeCheck(value.get(), &CMessage_Type)) {
+      PyErr_Format(PyExc_TypeError, "Invalid class for %s: %s",
+                   message_type->full_name().c_str(),
+                   Py_TYPE(value.get())->tp_name);
+      return NULL;
+  }
   CMessage* cmsg = reinterpret_cast<CMessage*>(value.get());
 
   const Reflection* reflection = options.GetReflection();
@@ -327,7 +341,8 @@ PyObject* NewInternedDescriptor(PyTypeObject* type,
   PyDescriptorPool* pool = GetDescriptorPool_FromPool(
       GetFileDescriptor(descriptor)->pool());
   if (pool == NULL) {
-    Py_DECREF(py_descriptor);
+    // Don't DECREF, the object is not fully initialized.
+    PyObject_Del(py_descriptor);
     return NULL;
   }
   Py_INCREF(pool);
@@ -1213,6 +1228,13 @@ static void Dealloc(PyFileDescriptor* self) {
   descriptor::Dealloc(&self->base);
 }
 
+static PyObject* GetPool(PyFileDescriptor *self, void *closure) {
+  PyObject* pool = reinterpret_cast<PyObject*>(
+      GetDescriptorPool_FromPool(_GetDescriptor(self)->pool()));
+  Py_XINCREF(pool);
+  return pool;
+}
+
 static PyObject* GetName(PyFileDescriptor *self, void *closure) {
   return PyString_FromCppString(_GetDescriptor(self)->name());
 }
@@ -1292,6 +1314,7 @@ static PyObject* CopyToProto(PyFileDescriptor *self, PyObject *target) {
 }
 
 static PyGetSetDef Getters[] = {
+  { "pool", (getter)GetPool, NULL, "pool"},
   { "name", (getter)GetName, NULL, "name"},
   { "package", (getter)GetPackage, NULL, "package"},
   { "serialized_pb", (getter)GetSerializedPb},
@@ -1354,8 +1377,8 @@ PyTypeObject PyFileDescriptor_Type = {
   0,                                    // tp_descr_set
   0,                                    // tp_dictoffset
   0,                                    // tp_init
-  PyType_GenericAlloc,                  // tp_alloc
-  PyType_GenericNew,                    // tp_new
+  0,                                    // tp_alloc
+  0,                                    // tp_new
   PyObject_Del,                         // tp_free
 };
 
diff --git a/python/google/protobuf/pyext/descriptor_database.cc b/python/google/protobuf/pyext/descriptor_database.cc
new file mode 100644
index 00000000..514722b4
--- /dev/null
+++ b/python/google/protobuf/pyext/descriptor_database.cc
@@ -0,0 +1,145 @@
+// 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.
+
+// This file defines a C++ DescriptorDatabase, which wraps a Python Database
+// and delegate all its operations to Python methods.
+
+#include <google/protobuf/pyext/descriptor_database.h>
+
+#include <google/protobuf/stubs/logging.h>
+#include <google/protobuf/stubs/common.h>
+#include <google/protobuf/descriptor.pb.h>
+#include <google/protobuf/pyext/message.h>
+#include <google/protobuf/pyext/scoped_pyobject_ptr.h>
+
+namespace google {
+namespace protobuf {
+namespace python {
+
+PyDescriptorDatabase::PyDescriptorDatabase(PyObject* py_database)
+    : py_database_(py_database) {
+  Py_INCREF(py_database_);
+}
+
+PyDescriptorDatabase::~PyDescriptorDatabase() { Py_DECREF(py_database_); }
+
+// Convert a Python object to a FileDescriptorProto pointer.
+// Handles all kinds of Python errors, which are simply logged.
+static bool GetFileDescriptorProto(PyObject* py_descriptor,
+                                   FileDescriptorProto* output) {
+  if (py_descriptor == NULL) {
+    if (PyErr_ExceptionMatches(PyExc_KeyError)) {
+      // Expected error: item was simply not found.
+      PyErr_Clear();
+    } else {
+      GOOGLE_LOG(ERROR) << "DescriptorDatabase method raised an error";
+      PyErr_Print();
+    }
+    return false;
+  }
+  const Descriptor* filedescriptor_descriptor =
+      FileDescriptorProto::default_instance().GetDescriptor();
+  CMessage* message = reinterpret_cast<CMessage*>(py_descriptor);
+  if (PyObject_TypeCheck(py_descriptor, &CMessage_Type) &&
+      message->message->GetDescriptor() == filedescriptor_descriptor) {
+    // Fast path: Just use the pointer.
+    FileDescriptorProto* file_proto =
+        static_cast<FileDescriptorProto*>(message->message);
+    *output = *file_proto;
+    return true;
+  } else {
+    // Slow path: serialize the message. This allows to use databases which
+    // use a different implementation of FileDescriptorProto.
+    ScopedPyObjectPtr serialized_pb(
+        PyObject_CallMethod(py_descriptor, "SerializeToString", NULL));
+    if (serialized_pb == NULL) {
+      GOOGLE_LOG(ERROR)
+          << "DescriptorDatabase method did not return a FileDescriptorProto";
+      PyErr_Print();
+      return false;
+    }
+    char* str;
+    Py_ssize_t len;
+    if (PyBytes_AsStringAndSize(serialized_pb.get(), &str, &len) < 0) {
+      GOOGLE_LOG(ERROR)
+          << "DescriptorDatabase method did not return a FileDescriptorProto";
+      PyErr_Print();
+      return false;
+    }
+    FileDescriptorProto file_proto;
+    if (!file_proto.ParseFromArray(str, len)) {
+      GOOGLE_LOG(ERROR)
+          << "DescriptorDatabase method did not return a FileDescriptorProto";
+      return false;
+    }
+    *output = file_proto;
+    return true;
+  }
+}
+
+// Find a file by file name.
+bool PyDescriptorDatabase::FindFileByName(const string& filename,
+                                          FileDescriptorProto* output) {
+  ScopedPyObjectPtr py_descriptor(PyObject_CallMethod(
+      py_database_, "FindFileByName", "s#", filename.c_str(), filename.size()));
+  return GetFileDescriptorProto(py_descriptor.get(), output);
+}
+
+// Find the file that declares the given fully-qualified symbol name.
+bool PyDescriptorDatabase::FindFileContainingSymbol(
+    const string& symbol_name, FileDescriptorProto* output) {
+  ScopedPyObjectPtr py_descriptor(
+      PyObject_CallMethod(py_database_, "FindFileContainingSymbol", "s#",
+                          symbol_name.c_str(), symbol_name.size()));
+  return GetFileDescriptorProto(py_descriptor.get(), output);
+}
+
+// Find the file which defines an extension extending the given message type
+// with the given field number.
+// Python DescriptorDatabases are not required to implement this method.
+bool PyDescriptorDatabase::FindFileContainingExtension(
+    const string& containing_type, int field_number,
+    FileDescriptorProto* output) {
+  ScopedPyObjectPtr py_method(
+      PyObject_GetAttrString(py_database_, "FindFileContainingExtension"));
+  if (py_method == NULL) {
+    // This method is not implemented, returns without error.
+    PyErr_Clear();
+    return false;
+  }
+  ScopedPyObjectPtr py_descriptor(
+      PyObject_CallFunction(py_method.get(), "s#i", containing_type.c_str(),
+                            containing_type.size(), field_number));
+  return GetFileDescriptorProto(py_descriptor.get(), output);
+}
+
+}  // namespace python
+}  // namespace protobuf
+}  // namespace google
diff --git a/python/google/protobuf/pyext/descriptor_database.h b/python/google/protobuf/pyext/descriptor_database.h
new file mode 100644
index 00000000..fc71c4bc
--- /dev/null
+++ b/python/google/protobuf/pyext/descriptor_database.h
@@ -0,0 +1,75 @@
+// 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_DESCRIPTOR_DATABASE_H__
+#define GOOGLE_PROTOBUF_PYTHON_CPP_DESCRIPTOR_DATABASE_H__
+
+#include <Python.h>
+
+#include <google/protobuf/descriptor_database.h>
+
+namespace google {
+namespace protobuf {
+namespace python {
+
+class PyDescriptorDatabase : public DescriptorDatabase {
+ public:
+  explicit PyDescriptorDatabase(PyObject* py_database);
+  ~PyDescriptorDatabase();
+
+  // Implement the abstract interface. All these functions fill the output
+  // with a copy of FileDescriptorProto.
+
+  // Find a file by file name.
+  bool FindFileByName(const string& filename,
+                      FileDescriptorProto* output);
+
+  // Find the file that declares the given fully-qualified symbol name.
+  bool FindFileContainingSymbol(const string& symbol_name,
+                                FileDescriptorProto* output);
+
+  // Find the file which defines an extension extending the given message type
+  // with the given field number.
+  // Containing_type must be a fully-qualified type name.
+  // Python objects are not required to implement this method.
+  bool FindFileContainingExtension(const string& containing_type,
+                                   int field_number,
+                                   FileDescriptorProto* output);
+
+ private:
+  // The python object that implements the database. The reference is owned.
+  PyObject* py_database_;
+};
+
+}  // namespace python
+}  // namespace protobuf
+
+}  // namespace google
+#endif  // GOOGLE_PROTOBUF_PYTHON_CPP_DESCRIPTOR_DATABASE_H__
diff --git a/python/google/protobuf/pyext/descriptor_pool.cc b/python/google/protobuf/pyext/descriptor_pool.cc
index 0f7487fa..0bc76bc9 100644
--- a/python/google/protobuf/pyext/descriptor_pool.cc
+++ b/python/google/protobuf/pyext/descriptor_pool.cc
@@ -34,8 +34,9 @@
 
 #include <google/protobuf/descriptor.pb.h>
 #include <google/protobuf/dynamic_message.h>
-#include <google/protobuf/pyext/descriptor_pool.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/scoped_pyobject_ptr.h>
 
@@ -60,38 +61,93 @@ static hash_map<const DescriptorPool*, PyDescriptorPool*> descriptor_pool_map;
 
 namespace cdescriptor_pool {
 
-static PyDescriptorPool* NewDescriptorPool() {
-  PyDescriptorPool* cdescriptor_pool = PyObject_New(
+// Create a Python DescriptorPool object, but does not fill the "pool"
+// attribute.
+static PyDescriptorPool* _CreateDescriptorPool() {
+  PyDescriptorPool* cpool = PyObject_New(
       PyDescriptorPool, &PyDescriptorPool_Type);
-  if (cdescriptor_pool == NULL) {
+  if (cpool == NULL) {
     return NULL;
   }
 
-  // Build a DescriptorPool for messages only declared in Python libraries.
-  // generated_pool() contains all messages linked in C++ libraries, and is used
-  // as underlay.
-  cdescriptor_pool->pool = new DescriptorPool(DescriptorPool::generated_pool());
+  cpool->underlay = NULL;
+  cpool->database = NULL;
 
   DynamicMessageFactory* message_factory = new DynamicMessageFactory();
   // This option might be the default some day.
   message_factory->SetDelegateToGeneratedFactory(true);
-  cdescriptor_pool->message_factory = message_factory;
+  cpool->message_factory = message_factory;
 
   // TODO(amauryfa): Rewrite the SymbolDatabase in C so that it uses the same
   // storage.
-  cdescriptor_pool->classes_by_descriptor =
+  cpool->classes_by_descriptor =
       new PyDescriptorPool::ClassesByMessageMap();
-  cdescriptor_pool->descriptor_options =
+  cpool->descriptor_options =
       new hash_map<const void*, PyObject *>();
 
+  return cpool;
+}
+
+// Create a Python DescriptorPool, using the given pool as an underlay:
+// new messages will be added to a custom pool, not to the underlay.
+//
+// Ownership of the underlay is not transferred, its pointer should
+// stay alive.
+static PyDescriptorPool* PyDescriptorPool_NewWithUnderlay(
+    const DescriptorPool* underlay) {
+  PyDescriptorPool* cpool = _CreateDescriptorPool();
+  if (cpool == NULL) {
+    return NULL;
+  }
+  cpool->pool = new DescriptorPool(underlay);
+  cpool->underlay = underlay;
+
   if (!descriptor_pool_map.insert(
-      std::make_pair(cdescriptor_pool->pool, cdescriptor_pool)).second) {
+      std::make_pair(cpool->pool, cpool)).second) {
     // Should never happen -- would indicate an internal error / bug.
     PyErr_SetString(PyExc_ValueError, "DescriptorPool already registered");
     return NULL;
   }
 
-  return cdescriptor_pool;
+  return cpool;
+}
+
+static PyDescriptorPool* PyDescriptorPool_NewWithDatabase(
+    DescriptorDatabase* database) {
+  PyDescriptorPool* cpool = _CreateDescriptorPool();
+  if (cpool == NULL) {
+    return NULL;
+  }
+  if (database != NULL) {
+    cpool->pool = new DescriptorPool(database);
+    cpool->database = database;
+  } else {
+    cpool->pool = new DescriptorPool();
+  }
+
+  if (!descriptor_pool_map.insert(std::make_pair(cpool->pool, cpool)).second) {
+    // Should never happen -- would indicate an internal error / bug.
+    PyErr_SetString(PyExc_ValueError, "DescriptorPool already registered");
+    return NULL;
+  }
+
+  return cpool;
+}
+
+// The public DescriptorPool constructor.
+static PyObject* New(PyTypeObject* type,
+                     PyObject* args, PyObject* kwargs) {
+  static char* kwlist[] = {"descriptor_db", 0};
+  PyObject* py_database = NULL;
+  if (!PyArg_ParseTupleAndKeywords(args, kwargs, "|O", kwlist, &py_database)) {
+    return NULL;
+  }
+  DescriptorDatabase* database = NULL;
+  if (py_database && py_database != Py_None) {
+    database = new PyDescriptorDatabase(py_database);
+  }
+  return reinterpret_cast<PyObject*>(
+      PyDescriptorPool_NewWithDatabase(database));
 }
 
 static void Dealloc(PyDescriptorPool* self) {
@@ -108,8 +164,9 @@ static void Dealloc(PyDescriptorPool* self) {
     Py_DECREF(it->second);
   }
   delete self->descriptor_options;
-  delete self->pool;
   delete self->message_factory;
+  delete self->database;
+  delete self->pool;
   Py_TYPE(self)->tp_free(reinterpret_cast<PyObject*>(self));
 }
 
@@ -354,6 +411,14 @@ PyObject* AddSerializedFile(PyDescriptorPool* self, PyObject* serialized_pb) {
   char* message_type;
   Py_ssize_t message_len;
 
+  if (self->database != NULL) {
+    PyErr_SetString(
+        PyExc_ValueError,
+        "Cannot call Add on a DescriptorPool that uses a DescriptorDatabase. "
+        "Add your file to the underlying database.");
+    return NULL;
+  }
+
   if (PyBytes_AsStringAndSize(serialized_pb, &message_type, &message_len) < 0) {
     return NULL;
   }
@@ -366,8 +431,10 @@ PyObject* AddSerializedFile(PyDescriptorPool* self, PyObject* serialized_pb) {
 
   // If the file was already part of a C++ library, all its descriptors are in
   // the underlying pool.  No need to do anything else.
-  const FileDescriptor* generated_file =
-      DescriptorPool::generated_pool()->FindFileByName(file_proto.name());
+  const FileDescriptor* generated_file = NULL;
+  if (self->underlay) {
+    generated_file = self->underlay->FindFileByName(file_proto.name());
+  }
   if (generated_file != NULL) {
     return PyFileDescriptor_FromDescriptorWithSerializedPb(
         generated_file, serialized_pb);
@@ -470,7 +537,7 @@ PyTypeObject PyDescriptorPool_Type = {
   0,                                   // tp_dictoffset
   0,                                   // tp_init
   0,                                   // tp_alloc
-  0,                                   // tp_new
+  cdescriptor_pool::New,               // tp_new
   PyObject_Del,                        // tp_free
 };
 
@@ -482,7 +549,11 @@ bool InitDescriptorPool() {
   if (PyType_Ready(&PyDescriptorPool_Type) < 0)
     return false;
 
-  python_generated_pool = cdescriptor_pool::NewDescriptorPool();
+  // The Pool of messages declared in Python libraries.
+  // generated_pool() contains all messages already linked in C++ libraries, and
+  // is used as underlay.
+  python_generated_pool = cdescriptor_pool::PyDescriptorPool_NewWithUnderlay(
+      DescriptorPool::generated_pool());
   if (python_generated_pool == NULL) {
     return false;
   }
@@ -494,6 +565,10 @@ bool InitDescriptorPool() {
   return true;
 }
 
+// The default DescriptorPool used everywhere in this module.
+// Today it's the python_generated_pool.
+// TODO(amauryfa): Remove all usages of this function: the pool should be
+// derived from the context.
 PyDescriptorPool* GetDefaultDescriptorPool() {
   return python_generated_pool;
 }
diff --git a/python/google/protobuf/pyext/descriptor_pool.h b/python/google/protobuf/pyext/descriptor_pool.h
index eda73d38..16bc910c 100644
--- a/python/google/protobuf/pyext/descriptor_pool.h
+++ b/python/google/protobuf/pyext/descriptor_pool.h
@@ -55,8 +55,17 @@ namespace python {
 typedef struct PyDescriptorPool {
   PyObject_HEAD
 
+  // The C++ pool containing Descriptors.
   DescriptorPool* pool;
 
+  // The C++ pool acting as an underlay. Can be NULL.
+  // This pointer is not owned and must stay alive.
+  const DescriptorPool* underlay;
+
+  // The C++ descriptor database used to fetch unknown protos. Can be NULL.
+  // 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.
@@ -138,6 +147,7 @@ PyObject* FindOneofByName(PyDescriptorPool* self, PyObject* arg);
 // Retrieve the global descriptor pool owned by the _message module.
 // This is the one used by pb2.py generated modules.
 // Returns a *borrowed* reference.
+// "Default" pool used to register messages from _pb2.py modules.
 PyDescriptorPool* GetDefaultDescriptorPool();
 
 // Retrieve the python descriptor pool owning a C++ descriptor pool.
diff --git a/python/google/protobuf/pyext/extension_dict.cc b/python/google/protobuf/pyext/extension_dict.cc
index b361b342..555bd293 100644
--- a/python/google/protobuf/pyext/extension_dict.cc
+++ b/python/google/protobuf/pyext/extension_dict.cc
@@ -94,13 +94,13 @@ PyObject* subscript(ExtensionDict* self, PyObject* key) {
   if (descriptor == NULL) {
     return NULL;
   }
-  if (!CheckFieldBelongsToMessage(descriptor, self->parent->message)) {
+  if (!CheckFieldBelongsToMessage(descriptor, self->message)) {
     return NULL;
   }
 
   if (descriptor->label() != FieldDescriptor::LABEL_REPEATED &&
       descriptor->cpp_type() != FieldDescriptor::CPPTYPE_MESSAGE) {
-    return cmessage::InternalGetScalar(self->parent->message, descriptor);
+    return cmessage::InternalGetScalar(self->message, descriptor);
   }
 
   PyObject* value = PyDict_GetItem(self->values, key);
@@ -109,6 +109,14 @@ PyObject* subscript(ExtensionDict* self, PyObject* key) {
     return value;
   }
 
+  if (self->parent == NULL) {
+    // We are in "detached" state. Don't allow further modifications.
+    // TODO(amauryfa): Support adding non-scalars to a detached extension dict.
+    // This probably requires to store the type of the main message.
+    PyErr_SetObject(PyExc_KeyError, key);
+    return NULL;
+  }
+
   if (descriptor->label() != FieldDescriptor::LABEL_REPEATED &&
       descriptor->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
     PyObject* sub_message = cmessage::InternalGetSubMessage(
@@ -154,7 +162,7 @@ int ass_subscript(ExtensionDict* self, PyObject* key, PyObject* value) {
   if (descriptor == NULL) {
     return -1;
   }
-  if (!CheckFieldBelongsToMessage(descriptor, self->parent->message)) {
+  if (!CheckFieldBelongsToMessage(descriptor, self->message)) {
     return -1;
   }
 
@@ -164,9 +172,11 @@ int ass_subscript(ExtensionDict* self, PyObject* key, PyObject* value) {
                     "type");
     return -1;
   }
-  cmessage::AssureWritable(self->parent);
-  if (cmessage::InternalSetScalar(self->parent, descriptor, value) < 0) {
-    return -1;
+  if (self->parent) {
+    cmessage::AssureWritable(self->parent);
+    if (cmessage::InternalSetScalar(self->parent, descriptor, value) < 0) {
+      return -1;
+    }
   }
   // TODO(tibell): We shouldn't write scalars to the cache.
   PyDict_SetItem(self->values, key, value);
@@ -180,15 +190,17 @@ PyObject* ClearExtension(ExtensionDict* self, PyObject* extension) {
     return NULL;
   }
   PyObject* value = PyDict_GetItem(self->values, extension);
-  if (value != NULL) {
-    if (ReleaseExtension(self, value, descriptor) < 0) {
+  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 (ScopedPyObjectPtr(cmessage::ClearFieldByDescriptor(
-          self->parent, descriptor)) == NULL) {
-    return NULL;
-  }
   if (PyDict_DelItem(self->values, extension) < 0) {
     PyErr_Clear();
   }
@@ -201,8 +213,15 @@ PyObject* HasExtension(ExtensionDict* self, PyObject* extension) {
   if (descriptor == NULL) {
     return NULL;
   }
-  PyObject* result = cmessage::HasFieldByDescriptor(self->parent, descriptor);
-  return result;
+  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) {
diff --git a/python/google/protobuf/pyext/extension_dict.h b/python/google/protobuf/pyext/extension_dict.h
index 7a66cb23..d92cf956 100644
--- a/python/google/protobuf/pyext/extension_dict.h
+++ b/python/google/protobuf/pyext/extension_dict.h
@@ -117,11 +117,6 @@ int ass_subscript(ExtensionDict* self, PyObject* key, PyObject* value);
 PyObject* ClearExtension(ExtensionDict* self,
                                        PyObject* extension);
 
-// Checks if the dict has an extension.
-//
-// Returns a new python boolean reference.
-PyObject* HasExtension(ExtensionDict* self, PyObject* extension);
-
 // Gets an extension from the dict given the extension name as opposed to
 // descriptor.
 //
diff --git a/python/google/protobuf/pyext/map_container.cc b/python/google/protobuf/pyext/map_container.cc
new file mode 100644
index 00000000..c39f7b83
--- /dev/null
+++ b/python/google/protobuf/pyext/map_container.cc
@@ -0,0 +1,912 @@
+// 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.
+
+// Author: haberman@google.com (Josh Haberman)
+
+#include <google/protobuf/pyext/map_container.h>
+
+#include <google/protobuf/stubs/logging.h>
+#include <google/protobuf/stubs/common.h>
+#include <google/protobuf/stubs/scoped_ptr.h>
+#include <google/protobuf/map_field.h>
+#include <google/protobuf/map.h>
+#include <google/protobuf/message.h>
+#include <google/protobuf/pyext/message.h>
+#include <google/protobuf/pyext/scoped_pyobject_ptr.h>
+
+#if PY_MAJOR_VERSION >= 3
+  #define PyInt_FromLong PyLong_FromLong
+  #define PyInt_FromSize_t PyLong_FromSize_t
+#endif
+
+namespace google {
+namespace protobuf {
+namespace python {
+
+// Functions that need access to map reflection functionality.
+// They need to be contained in this class because it is friended.
+class MapReflectionFriend {
+ public:
+  // Methods that are in common between the map types.
+  static PyObject* Contains(PyObject* _self, PyObject* key);
+  static Py_ssize_t Length(PyObject* _self);
+  static PyObject* GetIterator(PyObject *_self);
+  static PyObject* IterNext(PyObject* _self);
+
+  // Methods that differ between the map types.
+  static PyObject* ScalarMapGetItem(PyObject* _self, PyObject* key);
+  static PyObject* MessageMapGetItem(PyObject* _self, PyObject* key);
+  static int ScalarMapSetItem(PyObject* _self, PyObject* key, PyObject* v);
+  static int MessageMapSetItem(PyObject* _self, PyObject* key, PyObject* v);
+};
+
+struct MapIterator {
+  PyObject_HEAD;
+
+  scoped_ptr< ::google::protobuf::MapIterator> iter;
+
+  // A pointer back to the container, so we can notice changes to the version.
+  // We own a ref on this.
+  MapContainer* container;
+
+  // We need to keep a ref on the Message* too, because
+  // MapIterator::~MapIterator() accesses it.  Normally this would be ok because
+  // the ref on container (above) would guarantee outlive semantics.  However in
+  // the case of ClearField(), InitializeAndCopyToParentContainer() resets the
+  // message pointer (and the owner) to a different message, a copy of the
+  // original.  But our iterator still points to the original, which could now
+  // get deleted before us.
+  //
+  // To prevent this, we ensure that the Message will always stay alive as long
+  // as this iterator does.  This is solely for the benefit of the MapIterator
+  // destructor -- we should never actually access the iterator in this state
+  // except to delete it.
+  shared_ptr<Message> owner;
+
+  // The version of the map when we took the iterator to it.
+  //
+  // We store this so that if the map is modified during iteration we can throw
+  // an error.
+  uint64 version;
+
+  // True if the container is empty.  We signal this separately to avoid calling
+  // any of the iteration methods, which are non-const.
+  bool empty;
+};
+
+Message* MapContainer::GetMutableMessage() {
+  cmessage::AssureWritable(parent);
+  return const_cast<Message*>(message);
+}
+
+// Consumes a reference on the Python string object.
+static bool PyStringToSTL(PyObject* py_string, string* stl_string) {
+  char *value;
+  Py_ssize_t value_len;
+
+  if (!py_string) {
+    return false;
+  }
+  if (PyBytes_AsStringAndSize(py_string, &value, &value_len) < 0) {
+    Py_DECREF(py_string);
+    return false;
+  } else {
+    stl_string->assign(value, value_len);
+    Py_DECREF(py_string);
+    return true;
+  }
+}
+
+static bool PythonToMapKey(PyObject* obj,
+                           const FieldDescriptor* field_descriptor,
+                           MapKey* key) {
+  switch (field_descriptor->cpp_type()) {
+    case FieldDescriptor::CPPTYPE_INT32: {
+      GOOGLE_CHECK_GET_INT32(obj, value, false);
+      key->SetInt32Value(value);
+      break;
+    }
+    case FieldDescriptor::CPPTYPE_INT64: {
+      GOOGLE_CHECK_GET_INT64(obj, value, false);
+      key->SetInt64Value(value);
+      break;
+    }
+    case FieldDescriptor::CPPTYPE_UINT32: {
+      GOOGLE_CHECK_GET_UINT32(obj, value, false);
+      key->SetUInt32Value(value);
+      break;
+    }
+    case FieldDescriptor::CPPTYPE_UINT64: {
+      GOOGLE_CHECK_GET_UINT64(obj, value, false);
+      key->SetUInt64Value(value);
+      break;
+    }
+    case FieldDescriptor::CPPTYPE_BOOL: {
+      GOOGLE_CHECK_GET_BOOL(obj, value, false);
+      key->SetBoolValue(value);
+      break;
+    }
+    case FieldDescriptor::CPPTYPE_STRING: {
+      string str;
+      if (!PyStringToSTL(CheckString(obj, field_descriptor), &str)) {
+        return false;
+      }
+      key->SetStringValue(str);
+      break;
+    }
+    default:
+      PyErr_Format(
+          PyExc_SystemError, "Type %d cannot be a map key",
+          field_descriptor->cpp_type());
+      return false;
+  }
+  return true;
+}
+
+static PyObject* MapKeyToPython(const FieldDescriptor* field_descriptor,
+                                const MapKey& key) {
+  switch (field_descriptor->cpp_type()) {
+    case FieldDescriptor::CPPTYPE_INT32:
+      return PyInt_FromLong(key.GetInt32Value());
+    case FieldDescriptor::CPPTYPE_INT64:
+      return PyLong_FromLongLong(key.GetInt64Value());
+    case FieldDescriptor::CPPTYPE_UINT32:
+      return PyInt_FromSize_t(key.GetUInt32Value());
+    case FieldDescriptor::CPPTYPE_UINT64:
+      return PyLong_FromUnsignedLongLong(key.GetUInt64Value());
+    case FieldDescriptor::CPPTYPE_BOOL:
+      return PyBool_FromLong(key.GetBoolValue());
+    case FieldDescriptor::CPPTYPE_STRING:
+      return ToStringObject(field_descriptor, key.GetStringValue());
+    default:
+      PyErr_Format(
+          PyExc_SystemError, "Couldn't convert type %d to value",
+          field_descriptor->cpp_type());
+      return NULL;
+  }
+}
+
+// This is only used for ScalarMap, so we don't need to handle the
+// CPPTYPE_MESSAGE case.
+PyObject* MapValueRefToPython(const FieldDescriptor* field_descriptor,
+                              MapValueRef* value) {
+  switch (field_descriptor->cpp_type()) {
+    case FieldDescriptor::CPPTYPE_INT32:
+      return PyInt_FromLong(value->GetInt32Value());
+    case FieldDescriptor::CPPTYPE_INT64:
+      return PyLong_FromLongLong(value->GetInt64Value());
+    case FieldDescriptor::CPPTYPE_UINT32:
+      return PyInt_FromSize_t(value->GetUInt32Value());
+    case FieldDescriptor::CPPTYPE_UINT64:
+      return PyLong_FromUnsignedLongLong(value->GetUInt64Value());
+    case FieldDescriptor::CPPTYPE_FLOAT:
+      return PyFloat_FromDouble(value->GetFloatValue());
+    case FieldDescriptor::CPPTYPE_DOUBLE:
+      return PyFloat_FromDouble(value->GetDoubleValue());
+    case FieldDescriptor::CPPTYPE_BOOL:
+      return PyBool_FromLong(value->GetBoolValue());
+    case FieldDescriptor::CPPTYPE_STRING:
+      return ToStringObject(field_descriptor, value->GetStringValue());
+    case FieldDescriptor::CPPTYPE_ENUM:
+      return PyInt_FromLong(value->GetEnumValue());
+    default:
+      PyErr_Format(
+          PyExc_SystemError, "Couldn't convert type %d to value",
+          field_descriptor->cpp_type());
+      return NULL;
+  }
+}
+
+// This is only used for ScalarMap, so we don't need to handle the
+// CPPTYPE_MESSAGE case.
+static bool PythonToMapValueRef(PyObject* obj,
+                                const FieldDescriptor* field_descriptor,
+                                bool allow_unknown_enum_values,
+                                MapValueRef* value_ref) {
+  switch (field_descriptor->cpp_type()) {
+    case FieldDescriptor::CPPTYPE_INT32: {
+      GOOGLE_CHECK_GET_INT32(obj, value, false);
+      value_ref->SetInt32Value(value);
+      return true;
+    }
+    case FieldDescriptor::CPPTYPE_INT64: {
+      GOOGLE_CHECK_GET_INT64(obj, value, false);
+      value_ref->SetInt64Value(value);
+      return true;
+    }
+    case FieldDescriptor::CPPTYPE_UINT32: {
+      GOOGLE_CHECK_GET_UINT32(obj, value, false);
+      value_ref->SetUInt32Value(value);
+      return true;
+    }
+    case FieldDescriptor::CPPTYPE_UINT64: {
+      GOOGLE_CHECK_GET_UINT64(obj, value, false);
+      value_ref->SetUInt64Value(value);
+      return true;
+    }
+    case FieldDescriptor::CPPTYPE_FLOAT: {
+      GOOGLE_CHECK_GET_FLOAT(obj, value, false);
+      value_ref->SetFloatValue(value);
+      return true;
+    }
+    case FieldDescriptor::CPPTYPE_DOUBLE: {
+      GOOGLE_CHECK_GET_DOUBLE(obj, value, false);
+      value_ref->SetDoubleValue(value);
+      return true;
+    }
+    case FieldDescriptor::CPPTYPE_BOOL: {
+      GOOGLE_CHECK_GET_BOOL(obj, value, false);
+      value_ref->SetBoolValue(value);
+      return true;;
+    }
+    case FieldDescriptor::CPPTYPE_STRING: {
+      string str;
+      if (!PyStringToSTL(CheckString(obj, field_descriptor), &str)) {
+        return false;
+      }
+      value_ref->SetStringValue(str);
+      return true;
+    }
+    case FieldDescriptor::CPPTYPE_ENUM: {
+      GOOGLE_CHECK_GET_INT32(obj, value, false);
+      if (allow_unknown_enum_values) {
+        value_ref->SetEnumValue(value);
+        return true;
+      } else {
+        const EnumDescriptor* enum_descriptor = field_descriptor->enum_type();
+        const EnumValueDescriptor* enum_value =
+            enum_descriptor->FindValueByNumber(value);
+        if (enum_value != NULL) {
+          value_ref->SetEnumValue(value);
+          return true;
+        } else {
+          PyErr_Format(PyExc_ValueError, "Unknown enum value: %d", value);
+          return false;
+        }
+      }
+      break;
+    }
+    default:
+      PyErr_Format(
+          PyExc_SystemError, "Setting value to a field of unknown type %d",
+          field_descriptor->cpp_type());
+      return false;
+  }
+}
+
+// Map methods common to ScalarMap and MessageMap //////////////////////////////
+
+static MapContainer* GetMap(PyObject* obj) {
+  return reinterpret_cast<MapContainer*>(obj);
+}
+
+Py_ssize_t MapReflectionFriend::Length(PyObject* _self) {
+  MapContainer* self = GetMap(_self);
+  const google::protobuf::Message* message = self->message;
+  return message->GetReflection()->MapSize(*message,
+                                           self->parent_field_descriptor);
+}
+
+PyObject* Clear(PyObject* _self) {
+  MapContainer* self = GetMap(_self);
+  Message* message = self->GetMutableMessage();
+  const Reflection* reflection = message->GetReflection();
+
+  reflection->ClearField(message, self->parent_field_descriptor);
+
+  Py_RETURN_NONE;
+}
+
+PyObject* MapReflectionFriend::Contains(PyObject* _self, PyObject* key) {
+  MapContainer* self = GetMap(_self);
+
+  const Message* message = self->message;
+  const Reflection* reflection = message->GetReflection();
+  MapKey map_key;
+
+  if (!PythonToMapKey(key, self->key_field_descriptor, &map_key)) {
+    return NULL;
+  }
+
+  if (reflection->ContainsMapKey(*message, self->parent_field_descriptor,
+                                 map_key)) {
+    Py_RETURN_TRUE;
+  } else {
+    Py_RETURN_FALSE;
+  }
+}
+
+// Initializes the underlying Message object of "to" so it becomes a new parent
+// repeated scalar, and copies all the values from "from" to it. A child scalar
+// container can be released by passing it as both from and to (e.g. making it
+// the recipient of the new parent message and copying the values from itself).
+static int InitializeAndCopyToParentContainer(MapContainer* from,
+                                              MapContainer* to) {
+  // For now we require from == to, re-evaluate if we want to support deep copy
+  // as in repeated_scalar_container.cc.
+  GOOGLE_DCHECK(from == to);
+  Message* new_message = from->message->New();
+
+  if (MapReflectionFriend::Length(reinterpret_cast<PyObject*>(from)) > 0) {
+    // A somewhat roundabout way of copying just one field from old_message to
+    // new_message.  This is the best we can do with what Reflection gives us.
+    Message* mutable_old = from->GetMutableMessage();
+    vector<const FieldDescriptor*> fields;
+    fields.push_back(from->parent_field_descriptor);
+
+    // Move the map field into the new message.
+    mutable_old->GetReflection()->SwapFields(mutable_old, new_message, fields);
+
+    // If/when we support from != to, this will be required also to copy the
+    // map field back into the existing message:
+    // mutable_old->MergeFrom(*new_message);
+  }
+
+  // If from == to this could delete old_message.
+  to->owner.reset(new_message);
+
+  to->parent = NULL;
+  to->parent_field_descriptor = from->parent_field_descriptor;
+  to->message = new_message;
+
+  // Invalidate iterators, since they point to the old copy of the field.
+  to->version++;
+
+  return 0;
+}
+
+int MapContainer::Release() {
+  return InitializeAndCopyToParentContainer(this, this);
+}
+
+
+// ScalarMap ///////////////////////////////////////////////////////////////////
+
+PyObject *NewScalarMapContainer(
+    CMessage* parent, const google::protobuf::FieldDescriptor* parent_field_descriptor) {
+  if (!CheckFieldBelongsToMessage(parent_field_descriptor, parent->message)) {
+    return NULL;
+  }
+
+  ScopedPyObjectPtr obj(PyType_GenericAlloc(&ScalarMapContainer_Type, 0));
+  if (obj.get() == NULL) {
+    return PyErr_Format(PyExc_RuntimeError,
+                        "Could not allocate new container.");
+  }
+
+  MapContainer* self = GetMap(obj.get());
+
+  self->message = parent->message;
+  self->parent = parent;
+  self->parent_field_descriptor = parent_field_descriptor;
+  self->owner = parent->owner;
+  self->version = 0;
+
+  self->key_field_descriptor =
+      parent_field_descriptor->message_type()->FindFieldByName("key");
+  self->value_field_descriptor =
+      parent_field_descriptor->message_type()->FindFieldByName("value");
+
+  if (self->key_field_descriptor == NULL ||
+      self->value_field_descriptor == NULL) {
+    return PyErr_Format(PyExc_KeyError,
+                        "Map entry descriptor did not have key/value fields");
+  }
+
+  return obj.release();
+}
+
+PyObject* MapReflectionFriend::ScalarMapGetItem(PyObject* _self,
+                                                PyObject* key) {
+  MapContainer* self = GetMap(_self);
+
+  Message* message = self->GetMutableMessage();
+  const Reflection* reflection = message->GetReflection();
+  MapKey map_key;
+  MapValueRef value;
+
+  if (!PythonToMapKey(key, self->key_field_descriptor, &map_key)) {
+    return NULL;
+  }
+
+  if (reflection->InsertOrLookupMapValue(message, self->parent_field_descriptor,
+                                         map_key, &value)) {
+    self->version++;
+  }
+
+  return MapValueRefToPython(self->value_field_descriptor, &value);
+}
+
+int MapReflectionFriend::ScalarMapSetItem(PyObject* _self, PyObject* key,
+                                          PyObject* v) {
+  MapContainer* self = GetMap(_self);
+
+  Message* message = self->GetMutableMessage();
+  const Reflection* reflection = message->GetReflection();
+  MapKey map_key;
+  MapValueRef value;
+
+  if (!PythonToMapKey(key, self->key_field_descriptor, &map_key)) {
+    return -1;
+  }
+
+  self->version++;
+
+  if (v) {
+    // Set item to v.
+    reflection->InsertOrLookupMapValue(message, self->parent_field_descriptor,
+                                       map_key, &value);
+
+    return PythonToMapValueRef(v, self->value_field_descriptor,
+                               reflection->SupportsUnknownEnumValues(), &value)
+               ? 0
+               : -1;
+  } else {
+    // Delete key from map.
+    if (reflection->DeleteMapValue(message, self->parent_field_descriptor,
+                                   map_key)) {
+      return 0;
+    } else {
+      PyErr_Format(PyExc_KeyError, "Key not present in map");
+      return -1;
+    }
+  }
+}
+
+static PyObject* ScalarMapGet(PyObject* self, PyObject* args) {
+  PyObject* key;
+  PyObject* default_value = NULL;
+  if (PyArg_ParseTuple(args, "O|O", &key, &default_value) < 0) {
+    return NULL;
+  }
+
+  ScopedPyObjectPtr is_present(MapReflectionFriend::Contains(self, key));
+  if (is_present.get() == NULL) {
+    return NULL;
+  }
+
+  if (PyObject_IsTrue(is_present.get())) {
+    return MapReflectionFriend::ScalarMapGetItem(self, key);
+  } else {
+    if (default_value != NULL) {
+      Py_INCREF(default_value);
+      return default_value;
+    } else {
+      Py_RETURN_NONE;
+    }
+  }
+}
+
+static void ScalarMapDealloc(PyObject* _self) {
+  MapContainer* self = GetMap(_self);
+  self->owner.reset();
+  Py_TYPE(_self)->tp_free(_self);
+}
+
+static PyMappingMethods ScalarMapMappingMethods = {
+  MapReflectionFriend::Length,             // mp_length
+  MapReflectionFriend::ScalarMapGetItem,   // mp_subscript
+  MapReflectionFriend::ScalarMapSetItem,   // mp_ass_subscript
+};
+
+static PyMethodDef ScalarMapMethods[] = {
+  { "__contains__", MapReflectionFriend::Contains, METH_O,
+    "Tests whether a key is a member of the map." },
+  { "clear", (PyCFunction)Clear, METH_NOARGS,
+    "Removes all elements from the map." },
+  { "get", ScalarMapGet, METH_VARARGS,
+    "Gets the value for the given key if present, or otherwise a default" },
+  /*
+  { "__deepcopy__", (PyCFunction)DeepCopy, METH_VARARGS,
+    "Makes a deep copy of the class." },
+  { "__reduce__", (PyCFunction)Reduce, METH_NOARGS,
+    "Outputs picklable representation of the repeated field." },
+  */
+  {NULL, NULL},
+};
+
+PyTypeObject ScalarMapContainer_Type = {
+  PyVarObject_HEAD_INIT(&PyType_Type, 0)
+  FULL_MODULE_NAME ".ScalarMapContainer",  //  tp_name
+  sizeof(MapContainer),                //  tp_basicsize
+  0,                                   //  tp_itemsize
+  ScalarMapDealloc,                    //  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
+  &ScalarMapMappingMethods,            //  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,                  //  tp_flags
+  "A scalar map container",            //  tp_doc
+  0,                                   //  tp_traverse
+  0,                                   //  tp_clear
+  0,                                   //  tp_richcompare
+  0,                                   //  tp_weaklistoffset
+  MapReflectionFriend::GetIterator,    //  tp_iter
+  0,                                   //  tp_iternext
+  ScalarMapMethods,                    //  tp_methods
+  0,                                   //  tp_members
+  0,                                   //  tp_getset
+  0,                                   //  tp_base
+  0,                                   //  tp_dict
+  0,                                   //  tp_descr_get
+  0,                                   //  tp_descr_set
+  0,                                   //  tp_dictoffset
+  0,                                   //  tp_init
+};
+
+
+// MessageMap //////////////////////////////////////////////////////////////////
+
+static MessageMapContainer* GetMessageMap(PyObject* obj) {
+  return reinterpret_cast<MessageMapContainer*>(obj);
+}
+
+static PyObject* GetCMessage(MessageMapContainer* self, Message* message) {
+  // Get or create the CMessage object corresponding to this message.
+  ScopedPyObjectPtr key(PyLong_FromVoidPtr(message));
+  PyObject* ret = PyDict_GetItem(self->message_dict, key.get());
+
+  if (ret == NULL) {
+    CMessage* cmsg = cmessage::NewEmptyMessage(self->subclass_init,
+                                               message->GetDescriptor());
+    ret = reinterpret_cast<PyObject*>(cmsg);
+
+    if (cmsg == NULL) {
+      return NULL;
+    }
+    cmsg->owner = self->owner;
+    cmsg->message = message;
+    cmsg->parent = self->parent;
+
+    if (PyDict_SetItem(self->message_dict, key.get(), ret) < 0) {
+      Py_DECREF(ret);
+      return NULL;
+    }
+  } else {
+    Py_INCREF(ret);
+  }
+
+  return ret;
+}
+
+PyObject* NewMessageMapContainer(
+    CMessage* parent, const google::protobuf::FieldDescriptor* parent_field_descriptor,
+    PyObject* concrete_class) {
+  if (!CheckFieldBelongsToMessage(parent_field_descriptor, parent->message)) {
+    return NULL;
+  }
+
+  PyObject* obj = PyType_GenericAlloc(&MessageMapContainer_Type, 0);
+  if (obj == NULL) {
+    return PyErr_Format(PyExc_RuntimeError,
+                        "Could not allocate new container.");
+  }
+
+  MessageMapContainer* self = GetMessageMap(obj);
+
+  self->message = parent->message;
+  self->parent = parent;
+  self->parent_field_descriptor = parent_field_descriptor;
+  self->owner = parent->owner;
+  self->version = 0;
+
+  self->key_field_descriptor =
+      parent_field_descriptor->message_type()->FindFieldByName("key");
+  self->value_field_descriptor =
+      parent_field_descriptor->message_type()->FindFieldByName("value");
+
+  self->message_dict = PyDict_New();
+  if (self->message_dict == NULL) {
+    return PyErr_Format(PyExc_RuntimeError,
+                        "Could not allocate message dict.");
+  }
+
+  Py_INCREF(concrete_class);
+  self->subclass_init = concrete_class;
+
+  if (self->key_field_descriptor == NULL ||
+      self->value_field_descriptor == NULL) {
+    Py_DECREF(obj);
+    return PyErr_Format(PyExc_KeyError,
+                        "Map entry descriptor did not have key/value fields");
+  }
+
+  return obj;
+}
+
+int MapReflectionFriend::MessageMapSetItem(PyObject* _self, PyObject* key,
+                                           PyObject* v) {
+  if (v) {
+    PyErr_Format(PyExc_ValueError,
+                 "Direct assignment of submessage not allowed");
+    return -1;
+  }
+
+  // Now we know that this is a delete, not a set.
+
+  MessageMapContainer* self = GetMessageMap(_self);
+  Message* message = self->GetMutableMessage();
+  const Reflection* reflection = message->GetReflection();
+  MapKey map_key;
+  MapValueRef value;
+
+  self->version++;
+
+  if (!PythonToMapKey(key, self->key_field_descriptor, &map_key)) {
+    return -1;
+  }
+
+  // Delete key from map.
+  if (reflection->DeleteMapValue(message, self->parent_field_descriptor,
+                                 map_key)) {
+    return 0;
+  } else {
+    PyErr_Format(PyExc_KeyError, "Key not present in map");
+    return -1;
+  }
+}
+
+PyObject* MapReflectionFriend::MessageMapGetItem(PyObject* _self,
+                                                 PyObject* key) {
+  MessageMapContainer* self = GetMessageMap(_self);
+
+  Message* message = self->GetMutableMessage();
+  const Reflection* reflection = message->GetReflection();
+  MapKey map_key;
+  MapValueRef value;
+
+  if (!PythonToMapKey(key, self->key_field_descriptor, &map_key)) {
+    return NULL;
+  }
+
+  if (reflection->InsertOrLookupMapValue(message, self->parent_field_descriptor,
+                                         map_key, &value)) {
+    self->version++;
+  }
+
+  return GetCMessage(self, value.MutableMessageValue());
+}
+
+PyObject* MessageMapGet(PyObject* self, PyObject* args) {
+  PyObject* key;
+  PyObject* default_value = NULL;
+  if (PyArg_ParseTuple(args, "O|O", &key, &default_value) < 0) {
+    return NULL;
+  }
+
+  ScopedPyObjectPtr is_present(MapReflectionFriend::Contains(self, key));
+  if (is_present.get() == NULL) {
+    return NULL;
+  }
+
+  if (PyObject_IsTrue(is_present.get())) {
+    return MapReflectionFriend::MessageMapGetItem(self, key);
+  } else {
+    if (default_value != NULL) {
+      Py_INCREF(default_value);
+      return default_value;
+    } else {
+      Py_RETURN_NONE;
+    }
+  }
+}
+
+static void MessageMapDealloc(PyObject* _self) {
+  MessageMapContainer* self = GetMessageMap(_self);
+  self->owner.reset();
+  Py_DECREF(self->message_dict);
+  Py_TYPE(_self)->tp_free(_self);
+}
+
+static PyMappingMethods MessageMapMappingMethods = {
+  MapReflectionFriend::Length,              // mp_length
+  MapReflectionFriend::MessageMapGetItem,   // mp_subscript
+  MapReflectionFriend::MessageMapSetItem,   // mp_ass_subscript
+};
+
+static PyMethodDef MessageMapMethods[] = {
+  { "__contains__", (PyCFunction)MapReflectionFriend::Contains, METH_O,
+    "Tests whether the map contains this element."},
+  { "clear", (PyCFunction)Clear, METH_NOARGS,
+    "Removes all elements from the map."},
+  { "get", MessageMapGet, METH_VARARGS,
+    "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." },
+  /*
+  { "__deepcopy__", (PyCFunction)DeepCopy, METH_VARARGS,
+    "Makes a deep copy of the class." },
+  { "__reduce__", (PyCFunction)Reduce, METH_NOARGS,
+    "Outputs picklable representation of the repeated field." },
+  */
+  {NULL, NULL},
+};
+
+PyTypeObject MessageMapContainer_Type = {
+  PyVarObject_HEAD_INIT(&PyType_Type, 0)
+  FULL_MODULE_NAME ".MessageMapContainer",  //  tp_name
+  sizeof(MessageMapContainer),         //  tp_basicsize
+  0,                                   //  tp_itemsize
+  MessageMapDealloc,                   //  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
+  &MessageMapMappingMethods,           //  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,                  //  tp_flags
+  "A map container for message",       //  tp_doc
+  0,                                   //  tp_traverse
+  0,                                   //  tp_clear
+  0,                                   //  tp_richcompare
+  0,                                   //  tp_weaklistoffset
+  MapReflectionFriend::GetIterator,    //  tp_iter
+  0,                                   //  tp_iternext
+  MessageMapMethods,                   //  tp_methods
+  0,                                   //  tp_members
+  0,                                   //  tp_getset
+  0,                                   //  tp_base
+  0,                                   //  tp_dict
+  0,                                   //  tp_descr_get
+  0,                                   //  tp_descr_set
+  0,                                   //  tp_dictoffset
+  0,                                   //  tp_init
+};
+
+// MapIterator /////////////////////////////////////////////////////////////////
+
+static MapIterator* GetIter(PyObject* obj) {
+  return reinterpret_cast<MapIterator*>(obj);
+}
+
+PyObject* MapReflectionFriend::GetIterator(PyObject *_self) {
+  MapContainer* self = GetMap(_self);
+
+  ScopedPyObjectPtr obj(PyType_GenericAlloc(&MapIterator_Type, 0));
+  if (obj == NULL) {
+    return PyErr_Format(PyExc_KeyError, "Could not allocate iterator");
+  }
+
+  MapIterator* iter = GetIter(obj.get());
+
+  Py_INCREF(self);
+  iter->container = self;
+  iter->version = self->version;
+  iter->owner = self->owner;
+
+  if (MapReflectionFriend::Length(_self) > 0) {
+    Message* message = self->GetMutableMessage();
+    const Reflection* reflection = message->GetReflection();
+
+    iter->iter.reset(new ::google::protobuf::MapIterator(
+        reflection->MapBegin(message, self->parent_field_descriptor)));
+  }
+
+  return obj.release();
+}
+
+PyObject* MapReflectionFriend::IterNext(PyObject* _self) {
+  MapIterator* self = GetIter(_self);
+
+  // This won't catch mutations to the map performed by MergeFrom(); no easy way
+  // to address that.
+  if (self->version != self->container->version) {
+    return PyErr_Format(PyExc_RuntimeError,
+                        "Map modified during iteration.");
+  }
+
+  if (self->iter.get() == NULL) {
+    return NULL;
+  }
+
+  Message* message = self->container->GetMutableMessage();
+  const Reflection* reflection = message->GetReflection();
+
+  if (*self->iter ==
+      reflection->MapEnd(message, self->container->parent_field_descriptor)) {
+    return NULL;
+  }
+
+  PyObject* ret = MapKeyToPython(self->container->key_field_descriptor,
+                                 self->iter->GetKey());
+
+  ++(*self->iter);
+
+  return ret;
+}
+
+static void DeallocMapIterator(PyObject* _self) {
+  MapIterator* self = GetIter(_self);
+  self->iter.reset();
+  self->owner.reset();
+  Py_XDECREF(self->container);
+  Py_TYPE(_self)->tp_free(_self);
+}
+
+PyTypeObject MapIterator_Type = {
+  PyVarObject_HEAD_INIT(&PyType_Type, 0)
+  FULL_MODULE_NAME ".MapIterator",     //  tp_name
+  sizeof(MapIterator),                 //  tp_basicsize
+  0,                                   //  tp_itemsize
+  DeallocMapIterator,                  //  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,                  //  tp_flags
+  "A scalar map iterator",             //  tp_doc
+  0,                                   //  tp_traverse
+  0,                                   //  tp_clear
+  0,                                   //  tp_richcompare
+  0,                                   //  tp_weaklistoffset
+  PyObject_SelfIter,                   //  tp_iter
+  MapReflectionFriend::IterNext,       //  tp_iternext
+  0,                                   //  tp_methods
+  0,                                   //  tp_members
+  0,                                   //  tp_getset
+  0,                                   //  tp_base
+  0,                                   //  tp_dict
+  0,                                   //  tp_descr_get
+  0,                                   //  tp_descr_set
+  0,                                   //  tp_dictoffset
+  0,                                   //  tp_init
+};
+
+}  // namespace python
+}  // namespace protobuf
+}  // namespace google
diff --git a/python/google/protobuf/pyext/message_map_container.h b/python/google/protobuf/pyext/map_container.h
index 4f6cb26a..2de61187 100644
--- a/python/google/protobuf/pyext/message_map_container.h
+++ b/python/google/protobuf/pyext/map_container.h
@@ -28,8 +28,8 @@
 // (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_MAP_CONTAINER_H__
-#define GOOGLE_PROTOBUF_PYTHON_CPP_MESSAGE_MAP_CONTAINER_H__
+#ifndef GOOGLE_PROTOBUF_PYTHON_CPP_MAP_CONTAINER_H__
+#define GOOGLE_PROTOBUF_PYTHON_CPP_MAP_CONTAINER_H__
 
 #include <Python.h>
 
@@ -39,6 +39,7 @@
 #endif
 
 #include <google/protobuf/descriptor.h>
+#include <google/protobuf/message.h>
 
 namespace google {
 namespace protobuf {
@@ -55,18 +56,23 @@ namespace python {
 
 struct CMessage;
 
-struct MessageMapContainer {
+// This struct is used directly for ScalarMap, and is the base class of
+// MessageMapContainer, which is used for MessageMap.
+struct MapContainer {
   PyObject_HEAD;
 
   // This is the top-level C++ Message object that owns the whole
-  // proto tree.  Every Python MessageMapContainer holds a
+  // proto tree.  Every Python MapContainer holds a
   // reference to it in order to keep it alive as long as there's a
   // Python object that references any part of the tree.
   shared_ptr<Message> owner;
 
   // Pointer to the C++ Message that contains this container.  The
-  // MessageMapContainer does not own this pointer.
-  Message* message;
+  // MapContainer does not own this pointer.
+  const Message* message;
+
+  // Use to get a mutable message when necessary.
+  Message* GetMutableMessage();
 
   // Weak reference to a parent CMessage object (i.e. may be NULL.)
   //
@@ -82,45 +88,46 @@ struct MessageMapContainer {
   const FieldDescriptor* key_field_descriptor;
   const FieldDescriptor* value_field_descriptor;
 
+  // We bump this whenever we perform a mutation, to invalidate existing
+  // iterators.
+  uint64 version;
+
+  // Releases the messages in the container to a new message.
+  //
+  // Returns 0 on success, -1 on failure.
+  int Release();
+
+  // Set the owner field of self and any children of self.
+  void SetOwner(const shared_ptr<Message>& new_owner) {
+    owner = new_owner;
+  }
+};
+
+struct MessageMapContainer : public MapContainer {
   // A callable that is used to create new child messages.
   PyObject* subclass_init;
 
   // A dict mapping Message* -> CMessage.
   PyObject* message_dict;
-
-  // We bump this whenever we perform a mutation, to invalidate existing
-  // iterators.
-  uint64 version;
 };
 
-#if PY_MAJOR_VERSION >= 3
-  extern PyObject *MessageMapContainer_Type;
-  extern PyType_Spec MessageMapContainer_Type_spec;
-#else
-  extern PyTypeObject MessageMapContainer_Type;
-#endif
-extern PyTypeObject MessageMapIterator_Type;
+extern PyTypeObject ScalarMapContainer_Type;
+extern PyTypeObject MessageMapContainer_Type;
+extern PyTypeObject MapIterator_Type;  // Both map types use the same iterator.
 
-namespace message_map_container {
-
-// Builds a MessageMapContainer object, from a parent message and a
+// Builds a MapContainer object, from a parent message and a
 // field descriptor.
-extern PyObject* NewContainer(CMessage* parent,
-                              const FieldDescriptor* parent_field_descriptor,
-                              PyObject* concrete_class);
-
-// Releases the messages in the container to a new message.
-//
-// Returns 0 on success, -1 on failure.
-int Release(MessageMapContainer* self);
+extern PyObject* NewScalarMapContainer(
+    CMessage* parent, const FieldDescriptor* parent_field_descriptor);
 
-// Set the owner field of self and any children of self.
-void SetOwner(MessageMapContainer* self,
-              const shared_ptr<Message>& new_owner);
+// Builds a MessageMap object, from a parent message and a
+// field descriptor.
+extern PyObject* NewMessageMapContainer(
+    CMessage* parent, const FieldDescriptor* parent_field_descriptor,
+    PyObject* concrete_class);
 
-}  // namespace message_map_container
 }  // namespace python
 }  // namespace protobuf
 
 }  // namespace google
-#endif  // GOOGLE_PROTOBUF_PYTHON_CPP_MESSAGE_MAP_CONTAINER_H__
+#endif  // GOOGLE_PROTOBUF_PYTHON_CPP_MAP_CONTAINER_H__
diff --git a/python/google/protobuf/pyext/message.cc b/python/google/protobuf/pyext/message.cc
index 72f51ec1..863cde01 100644
--- a/python/google/protobuf/pyext/message.cc
+++ b/python/google/protobuf/pyext/message.cc
@@ -33,6 +33,7 @@
 
 #include <google/protobuf/pyext/message.h>
 
+#include <map>
 #include <memory>
 #ifndef _SHARED_PTR_H
 #include <google/protobuf/stubs/shared_ptr.h>
@@ -61,8 +62,7 @@
 #include <google/protobuf/pyext/extension_dict.h>
 #include <google/protobuf/pyext/repeated_composite_container.h>
 #include <google/protobuf/pyext/repeated_scalar_container.h>
-#include <google/protobuf/pyext/message_map_container.h>
-#include <google/protobuf/pyext/scalar_map_container.h>
+#include <google/protobuf/pyext/map_container.h>
 #include <google/protobuf/pyext/scoped_pyobject_ptr.h>
 #include <google/protobuf/stubs/strutil.h>
 
@@ -96,6 +96,7 @@ static PyObject* k_extensions_by_number;
 PyObject* EnumTypeWrapper_class;
 static PyObject* PythonMessage_class;
 static PyObject* kEmptyWeakref;
+static PyObject* WKT_classes = NULL;
 
 // Defines the Metaclass of all Message classes.
 // It allows us to cache some C++ pointers in the class object itself, they are
@@ -274,8 +275,32 @@ static PyObject* New(PyTypeObject* type,
 
   // Build the arguments to the base metaclass.
   // We change the __bases__ classes.
-  ScopedPyObjectPtr new_args(Py_BuildValue(
-      "s(OO)O", name, &CMessage_Type, PythonMessage_class, dict));
+  ScopedPyObjectPtr new_args;
+  const Descriptor* message_descriptor =
+      PyMessageDescriptor_AsDescriptor(py_descriptor);
+  if (message_descriptor == NULL) {
+    return NULL;
+  }
+
+  if (WKT_classes == NULL) {
+    ScopedPyObjectPtr well_known_types(PyImport_ImportModule(
+        "google.protobuf.internal.well_known_types"));
+    GOOGLE_DCHECK(well_known_types != NULL);
+
+    WKT_classes = PyObject_GetAttrString(well_known_types.get(), "WKTBASES");
+    GOOGLE_DCHECK(WKT_classes != NULL);
+  }
+
+  PyObject* well_known_class = PyDict_GetItemString(
+      WKT_classes, message_descriptor->full_name().c_str());
+  if (well_known_class == NULL) {
+    new_args.reset(Py_BuildValue("s(OO)O", name, &CMessage_Type,
+                                 PythonMessage_class, dict));
+  } else {
+    new_args.reset(Py_BuildValue("s(OOO)O", name, &CMessage_Type,
+                                 PythonMessage_class, well_known_class, dict));
+  }
+
   if (new_args == NULL) {
     return NULL;
   }
@@ -448,21 +473,9 @@ static int VisitCompositeField(const FieldDescriptor* descriptor,
   if (descriptor->label() == FieldDescriptor::LABEL_REPEATED) {
     if (descriptor->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
       if (descriptor->is_map()) {
-        const Descriptor* entry_type = descriptor->message_type();
-        const FieldDescriptor* value_type =
-            entry_type->FindFieldByName("value");
-        if (value_type->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
-          MessageMapContainer* container =
-            reinterpret_cast<MessageMapContainer*>(child);
-          if (visitor.VisitMessageMapContainer(container) == -1) {
-            return -1;
-          }
-        } else {
-          ScalarMapContainer* container =
-            reinterpret_cast<ScalarMapContainer*>(child);
-          if (visitor.VisitScalarMapContainer(container) == -1) {
-            return -1;
-          }
+        MapContainer* container = reinterpret_cast<MapContainer*>(child);
+        if (visitor.VisitMapContainer(container) == -1) {
+          return -1;
         }
       } else {
         RepeatedCompositeContainer* container =
@@ -579,12 +592,14 @@ bool CheckAndGetInteger(
   if (PyObject_RichCompareBool(min, arg, Py_LE) != 1 ||
       PyObject_RichCompareBool(max, arg, Py_GE) != 1) {
 #endif
-    PyObject *s = PyObject_Str(arg);
-    if (s) {
-      PyErr_Format(PyExc_ValueError,
-                   "Value out of range: %s",
-                   PyString_AsString(s));
-      Py_DECREF(s);
+    if (!PyErr_Occurred()) {
+      PyObject *s = PyObject_Str(arg);
+      if (s) {
+        PyErr_Format(PyExc_ValueError,
+                     "Value out of range: %s",
+                     PyString_AsString(s));
+        Py_DECREF(s);
+      }
     }
     return false;
   }
@@ -642,38 +657,51 @@ bool CheckAndGetBool(PyObject* arg, bool* value) {
   return true;
 }
 
-bool CheckAndSetString(
-    PyObject* arg, Message* message,
-    const FieldDescriptor* descriptor,
-    const Reflection* reflection,
-    bool append,
-    int index) {
+// Checks whether the given object (which must be "bytes" or "unicode") contains
+// valid UTF-8.
+bool IsValidUTF8(PyObject* obj) {
+  if (PyBytes_Check(obj)) {
+    PyObject* unicode = PyUnicode_FromEncodedObject(obj, "utf-8", NULL);
+
+    // Clear the error indicator; we report our own error when desired.
+    PyErr_Clear();
+
+    if (unicode) {
+      Py_DECREF(unicode);
+      return true;
+    } else {
+      return false;
+    }
+  } else {
+    // Unicode object, known to be valid UTF-8.
+    return true;
+  }
+}
+
+bool AllowInvalidUTF8(const FieldDescriptor* field) { return false; }
+
+PyObject* CheckString(PyObject* arg, const FieldDescriptor* descriptor) {
   GOOGLE_DCHECK(descriptor->type() == FieldDescriptor::TYPE_STRING ||
          descriptor->type() == FieldDescriptor::TYPE_BYTES);
   if (descriptor->type() == FieldDescriptor::TYPE_STRING) {
     if (!PyBytes_Check(arg) && !PyUnicode_Check(arg)) {
       FormatTypeError(arg, "bytes, unicode");
-      return false;
+      return NULL;
     }
 
-    if (PyBytes_Check(arg)) {
-      PyObject* unicode = PyUnicode_FromEncodedObject(arg, "utf-8", NULL);
-      if (unicode == NULL) {
-        PyObject* repr = PyObject_Repr(arg);
-        PyErr_Format(PyExc_ValueError,
-                     "%s has type str, but isn't valid UTF-8 "
-                     "encoding. Non-UTF-8 strings must be converted to "
-                     "unicode objects before being added.",
-                     PyString_AsString(repr));
-        Py_DECREF(repr);
-        return false;
-      } else {
-        Py_DECREF(unicode);
-      }
+    if (!IsValidUTF8(arg) && !AllowInvalidUTF8(descriptor)) {
+      PyObject* repr = PyObject_Repr(arg);
+      PyErr_Format(PyExc_ValueError,
+                   "%s has type str, but isn't valid UTF-8 "
+                   "encoding. Non-UTF-8 strings must be converted to "
+                   "unicode objects before being added.",
+                   PyString_AsString(repr));
+      Py_DECREF(repr);
+      return NULL;
     }
   } else if (!PyBytes_Check(arg)) {
     FormatTypeError(arg, "bytes");
-    return false;
+    return NULL;
   }
 
   PyObject* encoded_string = NULL;
@@ -691,14 +719,24 @@ bool CheckAndSetString(
     Py_INCREF(encoded_string);
   }
 
-  if (encoded_string == NULL) {
+  return encoded_string;
+}
+
+bool CheckAndSetString(
+    PyObject* arg, Message* message,
+    const FieldDescriptor* descriptor,
+    const Reflection* reflection,
+    bool append,
+    int index) {
+  ScopedPyObjectPtr encoded_string(CheckString(arg, descriptor));
+
+  if (encoded_string.get() == NULL) {
     return false;
   }
 
   char* value;
   Py_ssize_t value_len;
-  if (PyBytes_AsStringAndSize(encoded_string, &value, &value_len) < 0) {
-    Py_DECREF(encoded_string);
+  if (PyBytes_AsStringAndSize(encoded_string.get(), &value, &value_len) < 0) {
     return false;
   }
 
@@ -710,7 +748,6 @@ bool CheckAndSetString(
   } else {
     reflection->SetRepeatedString(message, descriptor, index, value_string);
   }
-  Py_DECREF(encoded_string);
   return true;
 }
 
@@ -823,12 +860,7 @@ struct FixupMessageReference : public ChildVisitor {
     return 0;
   }
 
-  int VisitScalarMapContainer(ScalarMapContainer* container) {
-    container->message = message_;
-    return 0;
-  }
-
-  int VisitMessageMapContainer(MessageMapContainer* container) {
+  int VisitMapContainer(MapContainer* container) {
     container->message = message_;
     return 0;
   }
@@ -870,9 +902,8 @@ int AssureWritable(CMessage* self) {
   // When a CMessage is made writable its Message pointer is updated
   // to point to a new mutable Message.  When that happens we need to
   // update any references to the old, read-only CMessage.  There are
-  // five places such references occur: RepeatedScalarContainer,
-  // RepeatedCompositeContainer, ScalarMapContainer, MessageMapContainer,
-  // and ExtensionDict.
+  // four places such references occur: RepeatedScalarContainer,
+  // RepeatedCompositeContainer, MapContainer, and ExtensionDict.
   if (self->extensions != NULL)
     self->extensions->message = self->message;
   if (ForEachCompositeField(self, FixupMessageReference(self->message)) == -1)
@@ -1054,7 +1085,8 @@ int InitAttributes(CMessage* self, PyObject* kwargs) {
     }
     const FieldDescriptor* descriptor = GetFieldDescriptor(self, name);
     if (descriptor == NULL) {
-      PyErr_Format(PyExc_ValueError, "Protocol message has no \"%s\" field.",
+      PyErr_Format(PyExc_ValueError, "Protocol message %s has no \"%s\" field.",
+                   self->message->GetDescriptor()->name().c_str(),
                    PyString_AsString(name));
       return -1;
     }
@@ -1203,18 +1235,6 @@ CMessage* NewEmptyMessage(PyObject* type, const Descriptor *descriptor) {
 
   self->composite_fields = NULL;
 
-  // If there are extension_ranges, the message is "extendable". Allocate a
-  // dictionary to store the extension fields.
-  if (descriptor->extension_range_count() > 0) {
-    // TODO(amauryfa): Delay the construction of this dict until extensions are
-    // really used on the object.
-    ExtensionDict* extension_dict = extension_dict::NewExtensionDict(self);
-    if (extension_dict == NULL) {
-      return NULL;
-    }
-    self->extensions = extension_dict;
-  }
-
   return self;
 }
 
@@ -1285,12 +1305,7 @@ struct ClearWeakReferences : public ChildVisitor {
     return 0;
   }
 
-  int VisitScalarMapContainer(ScalarMapContainer* container) {
-    container->parent = NULL;
-    return 0;
-  }
-
-  int VisitMessageMapContainer(MessageMapContainer* container) {
+  int VisitMapContainer(MapContainer* container) {
     container->parent = NULL;
     return 0;
   }
@@ -1305,6 +1320,9 @@ struct ClearWeakReferences : public ChildVisitor {
 static void Dealloc(CMessage* self) {
   // Null out all weak references from children to this message.
   GOOGLE_CHECK_EQ(0, ForEachCompositeField(self, ClearWeakReferences()));
+  if (self->extensions) {
+    self->extensions->parent = NULL;
+  }
 
   Py_CLEAR(self->extensions);
   Py_CLEAR(self->composite_fields);
@@ -1466,20 +1484,27 @@ PyObject* HasField(CMessage* self, PyObject* arg) {
   Py_RETURN_FALSE;
 }
 
-PyObject* ClearExtension(CMessage* self, PyObject* arg) {
+PyObject* ClearExtension(CMessage* self, PyObject* extension) {
   if (self->extensions != NULL) {
-    return extension_dict::ClearExtension(self->extensions, arg);
+    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;
+    }
   }
-  PyErr_SetString(PyExc_TypeError, "Message is not extendable");
-  return NULL;
+  Py_RETURN_NONE;
 }
 
-PyObject* HasExtension(CMessage* self, PyObject* arg) {
-  if (self->extensions != NULL) {
-    return extension_dict::HasExtension(self->extensions, arg);
+PyObject* HasExtension(CMessage* self, PyObject* extension) {
+  const FieldDescriptor* descriptor = GetExtensionDescriptor(extension);
+  if (descriptor == NULL) {
+    return NULL;
   }
-  PyErr_SetString(PyExc_TypeError, "Message is not extendable");
-  return NULL;
+  return HasFieldByDescriptor(self, descriptor);
 }
 
 // ---------------------------------------------------------------------
@@ -1529,13 +1554,8 @@ struct SetOwnerVisitor : public ChildVisitor {
     return 0;
   }
 
-  int VisitScalarMapContainer(ScalarMapContainer* container) {
-    scalar_map_container::SetOwner(container, new_owner_);
-    return 0;
-  }
-
-  int VisitMessageMapContainer(MessageMapContainer* container) {
-    message_map_container::SetOwner(container, new_owner_);
+  int VisitMapContainer(MapContainer* container) {
+    container->SetOwner(new_owner_);
     return 0;
   }
 
@@ -1608,14 +1628,8 @@ struct ReleaseChild : public ChildVisitor {
         reinterpret_cast<RepeatedScalarContainer*>(container));
   }
 
-  int VisitScalarMapContainer(ScalarMapContainer* container) {
-    return scalar_map_container::Release(
-        reinterpret_cast<ScalarMapContainer*>(container));
-  }
-
-  int VisitMessageMapContainer(MessageMapContainer* container) {
-    return message_map_container::Release(
-        reinterpret_cast<MessageMapContainer*>(container));
+  int VisitMapContainer(MapContainer* container) {
+    return reinterpret_cast<MapContainer*>(container)->Release();
   }
 
   int VisitCMessage(CMessage* cmessage,
@@ -1707,17 +1721,7 @@ PyObject* Clear(CMessage* self) {
   AssureWritable(self);
   if (ForEachCompositeField(self, ReleaseChild(self)) == -1)
     return NULL;
-
-  // The old ExtensionDict still aliases this CMessage, but all its
-  // fields have been released.
-  if (self->extensions != NULL) {
-    Py_CLEAR(self->extensions);
-    ExtensionDict* extension_dict = extension_dict::NewExtensionDict(self);
-    if (extension_dict == NULL) {
-      return NULL;
-    }
-    self->extensions = extension_dict;
-  }
+  Py_CLEAR(self->extensions);
   if (self->composite_fields) {
     PyDict_Clear(self->composite_fields);
   }
@@ -1997,7 +2001,6 @@ static PyObject* RegisterExtension(PyObject* cls,
   if (descriptor->is_extension() &&
       descriptor->containing_type()->options().message_set_wire_format() &&
       descriptor->type() == FieldDescriptor::TYPE_MESSAGE &&
-      descriptor->message_type() == descriptor->extension_scope() &&
       descriptor->label() == FieldDescriptor::LABEL_OPTIONAL) {
     ScopedPyObjectPtr message_name(PyString_FromStringAndSize(
         descriptor->message_type()->full_name().c_str(),
@@ -2042,6 +2045,8 @@ static PyObject* WhichOneof(CMessage* self, PyObject* arg) {
   }
 }
 
+static PyObject* GetExtensionDict(CMessage* self, void *closure);
+
 static PyObject* ListFields(CMessage* self) {
   vector<const FieldDescriptor*> fields;
   self->message->GetReflection()->ListFields(*self->message, &fields);
@@ -2079,12 +2084,13 @@ static PyObject* ListFields(CMessage* self) {
         PyErr_Clear();
         continue;
       }
-      PyObject* extensions = reinterpret_cast<PyObject*>(self->extensions);
+      ScopedPyObjectPtr extensions(GetExtensionDict(self, NULL));
       if (extensions == NULL) {
         return NULL;
       }
       // 'extension' reference later stolen by PyTuple_SET_ITEM.
-      PyObject* extension = PyObject_GetItem(extensions, extension_field.get());
+      PyObject* extension = PyObject_GetItem(
+          extensions.get(), extension_field.get());
       if (extension == NULL) {
         return NULL;
       }
@@ -2493,9 +2499,31 @@ PyObject* _CheckCalledFromGeneratedFile(PyObject* unused,
   Py_RETURN_NONE;
 }
 
-static PyMemberDef Members[] = {
-  {"Extensions", T_OBJECT_EX, offsetof(CMessage, extensions), 0,
-   "Extension dict"},
+static PyObject* GetExtensionDict(CMessage* self, void *closure) {
+  if (self->extensions)  {
+    Py_INCREF(self->extensions);
+    return reinterpret_cast<PyObject*>(self->extensions);
+  }
+
+  // If there are extension_ranges, the message is "extendable". Allocate a
+  // dictionary to store the extension fields.
+  const Descriptor* descriptor = GetMessageDescriptor(Py_TYPE(self));
+  if (descriptor->extension_range_count() > 0) {
+    ExtensionDict* extension_dict = extension_dict::NewExtensionDict(self);
+    if (extension_dict == NULL) {
+      return NULL;
+    }
+    self->extensions = extension_dict;
+    Py_INCREF(self->extensions);
+    return reinterpret_cast<PyObject*>(self->extensions);
+  }
+
+  PyErr_SetNone(PyExc_AttributeError);
+  return NULL;
+}
+
+static PyGetSetDef Getters[] = {
+  {"Extensions", (getter)GetExtensionDict, NULL, "Extension dict"},
   {NULL}
 };
 
@@ -2592,10 +2620,10 @@ PyObject* GetAttr(CMessage* self, PyObject* name) {
       if (value_class == NULL) {
         return NULL;
       }
-      py_container = message_map_container::NewContainer(self, field_descriptor,
-                                                         value_class);
+      py_container =
+          NewMessageMapContainer(self, field_descriptor, value_class);
     } else {
-      py_container = scalar_map_container::NewContainer(self, field_descriptor);
+      py_container = NewScalarMapContainer(self, field_descriptor);
     }
     if (py_container == NULL) {
       return NULL;
@@ -2672,7 +2700,10 @@ int SetAttr(CMessage* self, PyObject* name, PyObject* value) {
     }
   }
 
-  PyErr_Format(PyExc_AttributeError, "Assignment not allowed");
+  PyErr_Format(PyExc_AttributeError,
+               "Assignment not allowed "
+               "(no field \"%s\"in protocol message object).",
+               PyString_AsString(name));
   return -1;
 }
 
@@ -2707,8 +2738,8 @@ PyTypeObject CMessage_Type = {
   0,                                   //  tp_iter
   0,                                   //  tp_iternext
   cmessage::Methods,                   //  tp_methods
-  cmessage::Members,                   //  tp_members
-  0,                                   //  tp_getset
+  0,                                   //  tp_members
+  cmessage::Getters,                   //  tp_getset
   0,                                   //  tp_base
   0,                                   //  tp_dict
   0,                                   //  tp_descr_get
@@ -2910,12 +2941,12 @@ bool InitProto2MessageModule(PyObject *m) {
                        reinterpret_cast<PyObject*>(&ScalarMapContainer_Type));
 #endif
 
-    if (PyType_Ready(&ScalarMapIterator_Type) < 0) {
+    if (PyType_Ready(&MapIterator_Type) < 0) {
       return false;
     }
 
-    PyModule_AddObject(m, "ScalarMapIterator",
-                       reinterpret_cast<PyObject*>(&ScalarMapIterator_Type));
+    PyModule_AddObject(m, "MapIterator",
+                       reinterpret_cast<PyObject*>(&MapIterator_Type));
 
 
 #if PY_MAJOR_VERSION >= 3
@@ -2934,13 +2965,6 @@ bool InitProto2MessageModule(PyObject *m) {
     PyModule_AddObject(m, "MessageMapContainer",
                        reinterpret_cast<PyObject*>(&MessageMapContainer_Type));
 #endif
-
-    if (PyType_Ready(&MessageMapIterator_Type) < 0) {
-      return false;
-    }
-
-    PyModule_AddObject(m, "MessageMapIterator",
-                       reinterpret_cast<PyObject*>(&MessageMapIterator_Type));
   }
 
   if (PyType_Ready(&ExtensionDict_Type) < 0) {
@@ -2957,6 +2981,9 @@ bool InitProto2MessageModule(PyObject *m) {
   PyModule_AddObject(m, "default_pool",
                      reinterpret_cast<PyObject*>(GetDefaultDescriptorPool()));
 
+  PyModule_AddObject(m, "DescriptorPool", reinterpret_cast<PyObject*>(
+      &PyDescriptorPool_Type));
+
   // This implementation provides full Descriptor types, we advertise it so that
   // descriptor.py can use them in replacement of the Python classes.
   PyModule_AddIntConstant(m, "_USE_C_DESCRIPTORS", 1);
diff --git a/python/google/protobuf/pyext/message.h b/python/google/protobuf/pyext/message.h
index 94de4551..cc0012e9 100644
--- a/python/google/protobuf/pyext/message.h
+++ b/python/google/protobuf/pyext/message.h
@@ -307,6 +307,7 @@ bool CheckAndGetInteger(
 bool CheckAndGetDouble(PyObject* arg, double* value);
 bool CheckAndGetFloat(PyObject* arg, float* value);
 bool CheckAndGetBool(PyObject* arg, bool* value);
+PyObject* CheckString(PyObject* arg, const FieldDescriptor* descriptor);
 bool CheckAndSetString(
     PyObject* arg, Message* message,
     const FieldDescriptor* descriptor,
diff --git a/python/google/protobuf/pyext/message_map_container.cc b/python/google/protobuf/pyext/message_map_container.cc
deleted file mode 100644
index 8902fa00..00000000
--- a/python/google/protobuf/pyext/message_map_container.cc
+++ /dev/null
@@ -1,569 +0,0 @@
-// 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.
-
-// Author: haberman@google.com (Josh Haberman)
-
-#include <google/protobuf/pyext/message_map_container.h>
-
-#include <google/protobuf/stubs/logging.h>
-#include <google/protobuf/stubs/common.h>
-#include <google/protobuf/message.h>
-#include <google/protobuf/pyext/message.h>
-#include <google/protobuf/pyext/scoped_pyobject_ptr.h>
-
-namespace google {
-namespace protobuf {
-namespace python {
-
-struct MessageMapIterator {
-  PyObject_HEAD;
-
-  // This dict contains the full contents of what we want to iterate over.
-  // There's no way to avoid building this, because the list representation
-  // (which is canonical) can contain duplicate keys.  So at the very least we
-  // need a set that lets us skip duplicate keys.  And at the point that we're
-  // doing that, we might as well just build the actual dict we're iterating
-  // over and use dict's built-in iterator.
-  PyObject* dict;
-
-  // An iterator on dict.
-  PyObject* iter;
-
-  // A pointer back to the container, so we can notice changes to the version.
-  MessageMapContainer* container;
-
-  // The version of the map when we took the iterator to it.
-  //
-  // We store this so that if the map is modified during iteration we can throw
-  // an error.
-  uint64 version;
-};
-
-static MessageMapIterator* GetIter(PyObject* obj) {
-  return reinterpret_cast<MessageMapIterator*>(obj);
-}
-
-namespace message_map_container {
-
-static MessageMapContainer* GetMap(PyObject* obj) {
-  return reinterpret_cast<MessageMapContainer*>(obj);
-}
-
-// The private constructor of MessageMapContainer objects.
-PyObject* NewContainer(CMessage* parent,
-                       const google::protobuf::FieldDescriptor* parent_field_descriptor,
-                       PyObject* concrete_class) {
-  if (!CheckFieldBelongsToMessage(parent_field_descriptor, parent->message)) {
-    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
-  if (obj == NULL) {
-    return PyErr_Format(PyExc_RuntimeError,
-                        "Could not allocate new container.");
-  }
-
-  MessageMapContainer* self = GetMap(obj);
-
-  self->message = parent->message;
-  self->parent = parent;
-  self->parent_field_descriptor = parent_field_descriptor;
-  self->owner = parent->owner;
-  self->version = 0;
-
-  self->key_field_descriptor =
-      parent_field_descriptor->message_type()->FindFieldByName("key");
-  self->value_field_descriptor =
-      parent_field_descriptor->message_type()->FindFieldByName("value");
-
-  self->message_dict = PyDict_New();
-  if (self->message_dict == NULL) {
-    return PyErr_Format(PyExc_RuntimeError,
-                        "Could not allocate message dict.");
-  }
-
-  Py_INCREF(concrete_class);
-  self->subclass_init = concrete_class;
-
-  if (self->key_field_descriptor == NULL ||
-      self->value_field_descriptor == NULL) {
-    Py_DECREF(obj);
-    return PyErr_Format(PyExc_KeyError,
-                        "Map entry descriptor did not have key/value fields");
-  }
-
-  return obj;
-}
-
-// Initializes the underlying Message object of "to" so it becomes a new parent
-// repeated scalar, and copies all the values from "from" to it. A child scalar
-// container can be released by passing it as both from and to (e.g. making it
-// the recipient of the new parent message and copying the values from itself).
-static int InitializeAndCopyToParentContainer(
-    MessageMapContainer* from,
-    MessageMapContainer* to) {
-  // For now we require from == to, re-evaluate if we want to support deep copy
-  // as in repeated_composite_container.cc.
-  GOOGLE_DCHECK(from == to);
-  Message* old_message = from->message;
-  Message* new_message = old_message->New();
-  to->parent = NULL;
-  to->parent_field_descriptor = from->parent_field_descriptor;
-  to->message = new_message;
-  to->owner.reset(new_message);
-
-  vector<const FieldDescriptor*> fields;
-  fields.push_back(from->parent_field_descriptor);
-  old_message->GetReflection()->SwapFields(old_message, new_message, fields);
-  return 0;
-}
-
-static PyObject* GetCMessage(MessageMapContainer* self, Message* entry) {
-  // Get or create the CMessage object corresponding to this message.
-  Message* message = entry->GetReflection()->MutableMessage(
-      entry, self->value_field_descriptor);
-  ScopedPyObjectPtr key(PyLong_FromVoidPtr(message));
-  PyObject* ret = PyDict_GetItem(self->message_dict, key.get());
-
-  if (ret == NULL) {
-    CMessage* cmsg = cmessage::NewEmptyMessage(self->subclass_init,
-                                               message->GetDescriptor());
-    ret = reinterpret_cast<PyObject*>(cmsg);
-
-    if (cmsg == NULL) {
-      return NULL;
-    }
-    cmsg->owner = self->owner;
-    cmsg->message = message;
-    cmsg->parent = self->parent;
-
-    if (PyDict_SetItem(self->message_dict, key.get(), ret) < 0) {
-      Py_DECREF(ret);
-      return NULL;
-    }
-  } else {
-    Py_INCREF(ret);
-  }
-
-  return ret;
-}
-
-int Release(MessageMapContainer* self) {
-  InitializeAndCopyToParentContainer(self, self);
-  return 0;
-}
-
-void SetOwner(MessageMapContainer* self,
-              const shared_ptr<Message>& new_owner) {
-  self->owner = new_owner;
-}
-
-Py_ssize_t Length(PyObject* _self) {
-  MessageMapContainer* self = GetMap(_self);
-  google::protobuf::Message* message = self->message;
-  return message->GetReflection()->FieldSize(*message,
-                                             self->parent_field_descriptor);
-}
-
-int MapKeyMatches(MessageMapContainer* self, const Message* entry,
-                  PyObject* key) {
-  // TODO(haberman): do we need more strict type checking?
-  ScopedPyObjectPtr entry_key(
-      cmessage::InternalGetScalar(entry, self->key_field_descriptor));
-  int ret = PyObject_RichCompareBool(key, entry_key.get(), Py_EQ);
-  return ret;
-}
-
-int SetItem(PyObject *_self, PyObject *key, PyObject *v) {
-  if (v) {
-    PyErr_Format(PyExc_ValueError,
-                 "Direct assignment of submessage not allowed");
-    return -1;
-  }
-
-  // Now we know that this is a delete, not a set.
-
-  MessageMapContainer* self = GetMap(_self);
-  cmessage::AssureWritable(self->parent);
-
-  Message* message = self->message;
-  const Reflection* reflection = message->GetReflection();
-  size_t size =
-      reflection->FieldSize(*message, self->parent_field_descriptor);
-
-  // Right now the Reflection API doesn't support map lookup, so we implement it
-  // via linear search.  We need to search from the end because the underlying
-  // representation can have duplicates if a user calls MergeFrom(); the last
-  // one needs to win.
-  //
-  // TODO(haberman): add lookup API to Reflection API.
-  bool found = false;
-  for (int i = size - 1; i >= 0; i--) {
-    Message* entry = reflection->MutableRepeatedMessage(
-        message, self->parent_field_descriptor, i);
-    int matches = MapKeyMatches(self, entry, key);
-    if (matches < 0) return -1;
-    if (matches) {
-      found = true;
-      if (i != (int)size - 1) {
-        reflection->SwapElements(message, self->parent_field_descriptor, i,
-                                 size - 1);
-      }
-      reflection->RemoveLast(message, self->parent_field_descriptor);
-
-      // Can't exit now, the repeated field representation of maps allows
-      // duplicate keys, and we have to be sure to remove all of them.
-    }
-  }
-
-  if (!found) {
-    PyErr_Format(PyExc_KeyError, "Key not present in map");
-    return -1;
-  }
-
-  self->version++;
-
-  return 0;
-}
-
-PyObject* GetIterator(PyObject *_self) {
-  MessageMapContainer* self = GetMap(_self);
-
-  ScopedPyObjectPtr obj(PyType_GenericAlloc(&MessageMapIterator_Type, 0));
-  if (obj == NULL) {
-    return PyErr_Format(PyExc_KeyError, "Could not allocate iterator");
-  }
-
-  MessageMapIterator* iter = GetIter(obj.get());
-
-  Py_INCREF(self);
-  iter->container = self;
-  iter->version = self->version;
-  iter->dict = PyDict_New();
-  if (iter->dict == NULL) {
-    return PyErr_Format(PyExc_RuntimeError,
-                        "Could not allocate dict for iterator.");
-  }
-
-  // Build the entire map into a dict right now.  Start from the beginning so
-  // that later entries win in the case of duplicates.
-  Message* message = self->message;
-  const Reflection* reflection = message->GetReflection();
-
-  // Right now the Reflection API doesn't support map lookup, so we implement it
-  // via linear search.  We need to search from the end because the underlying
-  // representation can have duplicates if a user calls MergeFrom(); the last
-  // one needs to win.
-  //
-  // TODO(haberman): add lookup API to Reflection API.
-  size_t size =
-      reflection->FieldSize(*message, self->parent_field_descriptor);
-  for (int i = size - 1; i >= 0; i--) {
-    Message* entry = reflection->MutableRepeatedMessage(
-        message, self->parent_field_descriptor, i);
-    ScopedPyObjectPtr key(
-        cmessage::InternalGetScalar(entry, self->key_field_descriptor));
-    if (PyDict_SetItem(iter->dict, key.get(), GetCMessage(self, entry)) < 0) {
-      return PyErr_Format(PyExc_RuntimeError,
-                          "SetItem failed in iterator construction.");
-    }
-  }
-
-  iter->iter = PyObject_GetIter(iter->dict);
-
-  return obj.release();
-}
-
-PyObject* GetItem(PyObject* _self, PyObject* key) {
-  MessageMapContainer* self = GetMap(_self);
-  cmessage::AssureWritable(self->parent);
-  Message* message = self->message;
-  const Reflection* reflection = message->GetReflection();
-
-  // Right now the Reflection API doesn't support map lookup, so we implement it
-  // via linear search.  We need to search from the end because the underlying
-  // representation can have duplicates if a user calls MergeFrom(); the last
-  // one needs to win.
-  //
-  // TODO(haberman): add lookup API to Reflection API.
-  size_t size =
-      reflection->FieldSize(*message, self->parent_field_descriptor);
-  for (int i = size - 1; i >= 0; i--) {
-    Message* entry = reflection->MutableRepeatedMessage(
-        message, self->parent_field_descriptor, i);
-    int matches = MapKeyMatches(self, entry, key);
-    if (matches < 0) return NULL;
-    if (matches) {
-      return GetCMessage(self, entry);
-    }
-  }
-
-  // Key is not already present; insert a new entry.
-  Message* entry =
-      reflection->AddMessage(message, self->parent_field_descriptor);
-
-  self->version++;
-
-  if (cmessage::InternalSetNonOneofScalar(entry, self->key_field_descriptor,
-                                          key) < 0) {
-    reflection->RemoveLast(message, self->parent_field_descriptor);
-    return NULL;
-  }
-
-  return GetCMessage(self, entry);
-}
-
-PyObject* Contains(PyObject* _self, PyObject* key) {
-  MessageMapContainer* self = GetMap(_self);
-  Message* message = self->message;
-  const Reflection* reflection = message->GetReflection();
-
-  // Right now the Reflection API doesn't support map lookup, so we implement it
-  // via linear search.
-  //
-  // TODO(haberman): add lookup API to Reflection API.
-  int size =
-      reflection->FieldSize(*message, self->parent_field_descriptor);
-  for (int i = 0; i < size; i++) {
-    Message* entry = reflection->MutableRepeatedMessage(
-        message, self->parent_field_descriptor, i);
-    int matches = MapKeyMatches(self, entry, key);
-    if (matches < 0) return NULL;
-    if (matches) {
-      Py_RETURN_TRUE;
-    }
-  }
-
-  Py_RETURN_FALSE;
-}
-
-PyObject* Clear(PyObject* _self) {
-  MessageMapContainer* self = GetMap(_self);
-  cmessage::AssureWritable(self->parent);
-  Message* message = self->message;
-  const Reflection* reflection = message->GetReflection();
-
-  self->version++;
-  reflection->ClearField(message, self->parent_field_descriptor);
-
-  Py_RETURN_NONE;
-}
-
-PyObject* Get(PyObject* self, PyObject* args) {
-  PyObject* key;
-  PyObject* default_value = NULL;
-  if (PyArg_ParseTuple(args, "O|O", &key, &default_value) < 0) {
-    return NULL;
-  }
-
-  ScopedPyObjectPtr is_present(Contains(self, key));
-  if (is_present.get() == NULL) {
-    return NULL;
-  }
-
-  if (PyObject_IsTrue(is_present.get())) {
-    return GetItem(self, key);
-  } else {
-    if (default_value != NULL) {
-      Py_INCREF(default_value);
-      return default_value;
-    } else {
-      Py_RETURN_NONE;
-    }
-  }
-}
-
-static void Dealloc(PyObject* _self) {
-  MessageMapContainer* self = GetMap(_self);
-  self->owner.reset();
-  Py_DECREF(self->message_dict);
-  Py_TYPE(_self)->tp_free(_self);
-}
-
-static PyMethodDef Methods[] = {
-  { "__contains__", (PyCFunction)Contains, METH_O,
-    "Tests whether the map contains this element."},
-  { "clear", (PyCFunction)Clear, METH_NOARGS,
-    "Removes all elements from the map."},
-  { "get", Get, METH_VARARGS,
-    "Gets the value for the given key if present, or otherwise a default" },
-  { "get_or_create", GetItem, METH_O,
-    "Alias for getitem, useful to make explicit that the map is mutated." },
-  /*
-  { "__deepcopy__", (PyCFunction)DeepCopy, METH_VARARGS,
-    "Makes a deep copy of the class." },
-  { "__reduce__", (PyCFunction)Reduce, METH_NOARGS,
-    "Outputs picklable representation of the repeated field." },
-  */
-  {NULL, NULL},
-};
-
-}  // namespace message_map_container
-
-namespace message_map_iterator {
-
-static void Dealloc(PyObject* _self) {
-  MessageMapIterator* self = GetIter(_self);
-  Py_DECREF(self->dict);
-  Py_DECREF(self->iter);
-  Py_DECREF(self->container);
-  Py_TYPE(_self)->tp_free(_self);
-}
-
-PyObject* IterNext(PyObject* _self) {
-  MessageMapIterator* self = GetIter(_self);
-
-  // This won't catch mutations to the map performed by MergeFrom(); no easy way
-  // to address that.
-  if (self->version != self->container->version) {
-    return PyErr_Format(PyExc_RuntimeError,
-                        "Map modified during iteration.");
-  }
-
-  return PyIter_Next(self->iter);
-}
-
-}  // namespace message_map_iterator
-
-#if PY_MAJOR_VERSION >= 3
-  static PyType_Slot MessageMapContainer_Type_slots[] = {
-      {Py_tp_dealloc, (void *)message_map_container::Dealloc},
-      {Py_mp_length, (void *)message_map_container::Length},
-      {Py_mp_subscript, (void *)message_map_container::GetItem},
-      {Py_mp_ass_subscript, (void *)message_map_container::SetItem},
-      {Py_tp_methods, (void *)message_map_container::Methods},
-      {Py_tp_iter, (void *)message_map_container::GetIterator},
-      {0, 0}
-  };
-
-  PyType_Spec MessageMapContainer_Type_spec = {
-      FULL_MODULE_NAME ".MessageMapContainer",
-      sizeof(MessageMapContainer),
-      0,
-      Py_TPFLAGS_DEFAULT,
-      MessageMapContainer_Type_slots
-  };
-
-  PyObject *MessageMapContainer_Type;
-
-#else
-  static PyMappingMethods MpMethods = {
-    message_map_container::Length,    // mp_length
-    message_map_container::GetItem,   // mp_subscript
-    message_map_container::SetItem,   // mp_ass_subscript
-  };
-
-  PyTypeObject MessageMapContainer_Type = {
-    PyVarObject_HEAD_INIT(&PyType_Type, 0)
-    FULL_MODULE_NAME ".MessageMapContainer",  //  tp_name
-    sizeof(MessageMapContainer),         //  tp_basicsize
-    0,                                   //  tp_itemsize
-    message_map_container::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
-    &MpMethods,                          //  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,                  //  tp_flags
-    "A map container for message",       //  tp_doc
-    0,                                   //  tp_traverse
-    0,                                   //  tp_clear
-    0,                                   //  tp_richcompare
-    0,                                   //  tp_weaklistoffset
-    message_map_container::GetIterator,  //  tp_iter
-    0,                                   //  tp_iternext
-    message_map_container::Methods,      //  tp_methods
-    0,                                   //  tp_members
-    0,                                   //  tp_getset
-    0,                                   //  tp_base
-    0,                                   //  tp_dict
-    0,                                   //  tp_descr_get
-    0,                                   //  tp_descr_set
-    0,                                   //  tp_dictoffset
-    0,                                   //  tp_init
-  };
-#endif
-
-PyTypeObject MessageMapIterator_Type = {
-  PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  FULL_MODULE_NAME ".MessageMapIterator",  //  tp_name
-  sizeof(MessageMapIterator),          //  tp_basicsize
-  0,                                   //  tp_itemsize
-  message_map_iterator::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,                  //  tp_flags
-  "A scalar map iterator",             //  tp_doc
-  0,                                   //  tp_traverse
-  0,                                   //  tp_clear
-  0,                                   //  tp_richcompare
-  0,                                   //  tp_weaklistoffset
-  PyObject_SelfIter,                   //  tp_iter
-  message_map_iterator::IterNext,       //  tp_iternext
-  0,                                   //  tp_methods
-  0,                                   //  tp_members
-  0,                                   //  tp_getset
-  0,                                   //  tp_base
-  0,                                   //  tp_dict
-  0,                                   //  tp_descr_get
-  0,                                   //  tp_descr_set
-  0,                                   //  tp_dictoffset
-  0,                                   //  tp_init
-};
-
-}  // namespace python
-}  // namespace protobuf
-}  // namespace google
diff --git a/python/google/protobuf/pyext/scalar_map_container.cc b/python/google/protobuf/pyext/scalar_map_container.cc
deleted file mode 100644
index 0b0d5a3d..00000000
--- a/python/google/protobuf/pyext/scalar_map_container.cc
+++ /dev/null
@@ -1,542 +0,0 @@
-// 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.
-
-// Author: haberman@google.com (Josh Haberman)
-
-#include <google/protobuf/pyext/scalar_map_container.h>
-
-#include <google/protobuf/stubs/logging.h>
-#include <google/protobuf/stubs/common.h>
-#include <google/protobuf/message.h>
-#include <google/protobuf/pyext/message.h>
-#include <google/protobuf/pyext/scoped_pyobject_ptr.h>
-
-namespace google {
-namespace protobuf {
-namespace python {
-
-struct ScalarMapIterator {
-  PyObject_HEAD;
-
-  // This dict contains the full contents of what we want to iterate over.
-  // There's no way to avoid building this, because the list representation
-  // (which is canonical) can contain duplicate keys.  So at the very least we
-  // need a set that lets us skip duplicate keys.  And at the point that we're
-  // doing that, we might as well just build the actual dict we're iterating
-  // over and use dict's built-in iterator.
-  PyObject* dict;
-
-  // An iterator on dict.
-  PyObject* iter;
-
-  // A pointer back to the container, so we can notice changes to the version.
-  ScalarMapContainer* container;
-
-  // The version of the map when we took the iterator to it.
-  //
-  // We store this so that if the map is modified during iteration we can throw
-  // an error.
-  uint64 version;
-};
-
-static ScalarMapIterator* GetIter(PyObject* obj) {
-  return reinterpret_cast<ScalarMapIterator*>(obj);
-}
-
-namespace scalar_map_container {
-
-static ScalarMapContainer* GetMap(PyObject* obj) {
-  return reinterpret_cast<ScalarMapContainer*>(obj);
-}
-
-// The private constructor of ScalarMapContainer objects.
-PyObject *NewContainer(
-    CMessage* parent, const google::protobuf::FieldDescriptor* parent_field_descriptor) {
-  if (!CheckFieldBelongsToMessage(parent_field_descriptor, parent->message)) {
-    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
-  if (obj.get() == NULL) {
-    return PyErr_Format(PyExc_RuntimeError,
-                        "Could not allocate new container.");
-  }
-
-  ScalarMapContainer* self = GetMap(obj.get());
-
-  self->message = parent->message;
-  self->parent = parent;
-  self->parent_field_descriptor = parent_field_descriptor;
-  self->owner = parent->owner;
-  self->version = 0;
-
-  self->key_field_descriptor =
-      parent_field_descriptor->message_type()->FindFieldByName("key");
-  self->value_field_descriptor =
-      parent_field_descriptor->message_type()->FindFieldByName("value");
-
-  if (self->key_field_descriptor == NULL ||
-      self->value_field_descriptor == NULL) {
-    return PyErr_Format(PyExc_KeyError,
-                        "Map entry descriptor did not have key/value fields");
-  }
-
-  return obj.release();
-}
-
-// Initializes the underlying Message object of "to" so it becomes a new parent
-// repeated scalar, and copies all the values from "from" to it. A child scalar
-// container can be released by passing it as both from and to (e.g. making it
-// the recipient of the new parent message and copying the values from itself).
-static int InitializeAndCopyToParentContainer(
-    ScalarMapContainer* from,
-    ScalarMapContainer* to) {
-  // For now we require from == to, re-evaluate if we want to support deep copy
-  // as in repeated_scalar_container.cc.
-  GOOGLE_DCHECK(from == to);
-  Message* old_message = from->message;
-  Message* new_message = old_message->New();
-  to->parent = NULL;
-  to->parent_field_descriptor = from->parent_field_descriptor;
-  to->message = new_message;
-  to->owner.reset(new_message);
-
-  vector<const FieldDescriptor*> fields;
-  fields.push_back(from->parent_field_descriptor);
-  old_message->GetReflection()->SwapFields(old_message, new_message, fields);
-  return 0;
-}
-
-int Release(ScalarMapContainer* self) {
-  return InitializeAndCopyToParentContainer(self, self);
-}
-
-void SetOwner(ScalarMapContainer* self,
-              const shared_ptr<Message>& new_owner) {
-  self->owner = new_owner;
-}
-
-Py_ssize_t Length(PyObject* _self) {
-  ScalarMapContainer* self = GetMap(_self);
-  google::protobuf::Message* message = self->message;
-  return message->GetReflection()->FieldSize(*message,
-                                             self->parent_field_descriptor);
-}
-
-int MapKeyMatches(ScalarMapContainer* self, const Message* entry,
-                  PyObject* key) {
-  // TODO(haberman): do we need more strict type checking?
-  ScopedPyObjectPtr entry_key(
-      cmessage::InternalGetScalar(entry, self->key_field_descriptor));
-  int ret = PyObject_RichCompareBool(key, entry_key.get(), Py_EQ);
-  return ret;
-}
-
-PyObject* GetItem(PyObject* _self, PyObject* key) {
-  ScalarMapContainer* self = GetMap(_self);
-
-  Message* message = self->message;
-  const Reflection* reflection = message->GetReflection();
-
-  // Right now the Reflection API doesn't support map lookup, so we implement it
-  // via linear search.
-  //
-  // TODO(haberman): add lookup API to Reflection API.
-  size_t size = reflection->FieldSize(*message, self->parent_field_descriptor);
-  for (int i = size - 1; i >= 0; i--) {
-    const Message& entry = reflection->GetRepeatedMessage(
-        *message, self->parent_field_descriptor, i);
-    int matches = MapKeyMatches(self, &entry, key);
-    if (matches < 0) return NULL;
-    if (matches) {
-      return cmessage::InternalGetScalar(&entry, self->value_field_descriptor);
-    }
-  }
-
-  // Need to add a new entry.
-  Message* entry =
-      reflection->AddMessage(message, self->parent_field_descriptor);
-  PyObject* ret = NULL;
-
-  if (cmessage::InternalSetNonOneofScalar(entry, self->key_field_descriptor,
-                                          key) >= 0) {
-    ret = cmessage::InternalGetScalar(entry, self->value_field_descriptor);
-  }
-
-  self->version++;
-
-  // If there was a type error above, it set the Python exception.
-  return ret;
-}
-
-int SetItem(PyObject *_self, PyObject *key, PyObject *v) {
-  ScalarMapContainer* self = GetMap(_self);
-  cmessage::AssureWritable(self->parent);
-
-  Message* message = self->message;
-  const Reflection* reflection = message->GetReflection();
-  size_t size =
-      reflection->FieldSize(*message, self->parent_field_descriptor);
-  self->version++;
-
-  if (v) {
-    // Set item.
-    //
-    // Right now the Reflection API doesn't support map lookup, so we implement
-    // it via linear search.
-    //
-    // TODO(haberman): add lookup API to Reflection API.
-    for (int i = size - 1; i >= 0; i--) {
-      Message* entry = reflection->MutableRepeatedMessage(
-          message, self->parent_field_descriptor, i);
-      int matches = MapKeyMatches(self, entry, key);
-      if (matches < 0) return -1;
-      if (matches) {
-        return cmessage::InternalSetNonOneofScalar(
-            entry, self->value_field_descriptor, v);
-      }
-    }
-
-    // Key is not already present; insert a new entry.
-    Message* entry =
-        reflection->AddMessage(message, self->parent_field_descriptor);
-
-    if (cmessage::InternalSetNonOneofScalar(entry, self->key_field_descriptor,
-                                            key) < 0 ||
-        cmessage::InternalSetNonOneofScalar(entry, self->value_field_descriptor,
-                                            v) < 0) {
-      reflection->RemoveLast(message, self->parent_field_descriptor);
-      return -1;
-    }
-
-    return 0;
-  } else {
-    bool found = false;
-    for (int i = size - 1; i >= 0; i--) {
-      Message* entry = reflection->MutableRepeatedMessage(
-          message, self->parent_field_descriptor, i);
-      int matches = MapKeyMatches(self, entry, key);
-      if (matches < 0) return -1;
-      if (matches) {
-        found = true;
-        if (i != (int)size - 1) {
-          reflection->SwapElements(message, self->parent_field_descriptor, i,
-                                   size - 1);
-        }
-        reflection->RemoveLast(message, self->parent_field_descriptor);
-
-        // Can't exit now, the repeated field representation of maps allows
-        // duplicate keys, and we have to be sure to remove all of them.
-      }
-    }
-
-    if (found) {
-      return 0;
-    } else {
-      PyErr_Format(PyExc_KeyError, "Key not present in map");
-      return -1;
-    }
-  }
-}
-
-PyObject* GetIterator(PyObject *_self) {
-  ScalarMapContainer* self = GetMap(_self);
-
-  ScopedPyObjectPtr obj(PyType_GenericAlloc(&ScalarMapIterator_Type, 0));
-  if (obj == NULL) {
-    return PyErr_Format(PyExc_KeyError, "Could not allocate iterator");
-  }
-
-  ScalarMapIterator* iter = GetIter(obj.get());
-
-  Py_INCREF(self);
-  iter->container = self;
-  iter->version = self->version;
-  iter->dict = PyDict_New();
-  if (iter->dict == NULL) {
-    return PyErr_Format(PyExc_RuntimeError,
-                        "Could not allocate dict for iterator.");
-  }
-
-  // Build the entire map into a dict right now.  Start from the beginning so
-  // that later entries win in the case of duplicates.
-  Message* message = self->message;
-  const Reflection* reflection = message->GetReflection();
-
-  // Right now the Reflection API doesn't support map lookup, so we implement it
-  // via linear search.  We need to search from the end because the underlying
-  // representation can have duplicates if a user calls MergeFrom(); the last
-  // one needs to win.
-  //
-  // TODO(haberman): add lookup API to Reflection API.
-  size_t size =
-      reflection->FieldSize(*message, self->parent_field_descriptor);
-  for (size_t i = 0; i < size; i++) {
-    Message* entry = reflection->MutableRepeatedMessage(
-        message, self->parent_field_descriptor, i);
-    ScopedPyObjectPtr key(
-        cmessage::InternalGetScalar(entry, self->key_field_descriptor));
-    ScopedPyObjectPtr val(
-        cmessage::InternalGetScalar(entry, self->value_field_descriptor));
-    if (PyDict_SetItem(iter->dict, key.get(), val.get()) < 0) {
-      return PyErr_Format(PyExc_RuntimeError,
-                          "SetItem failed in iterator construction.");
-    }
-  }
-
-
-  iter->iter = PyObject_GetIter(iter->dict);
-
-
-  return obj.release();
-}
-
-PyObject* Clear(PyObject* _self) {
-  ScalarMapContainer* self = GetMap(_self);
-  cmessage::AssureWritable(self->parent);
-  Message* message = self->message;
-  const Reflection* reflection = message->GetReflection();
-
-  reflection->ClearField(message, self->parent_field_descriptor);
-
-  Py_RETURN_NONE;
-}
-
-PyObject* Contains(PyObject* _self, PyObject* key) {
-  ScalarMapContainer* self = GetMap(_self);
-
-  Message* message = self->message;
-  const Reflection* reflection = message->GetReflection();
-
-  // Right now the Reflection API doesn't support map lookup, so we implement it
-  // via linear search.
-  //
-  // TODO(haberman): add lookup API to Reflection API.
-  size_t size = reflection->FieldSize(*message, self->parent_field_descriptor);
-  for (int i = size - 1; i >= 0; i--) {
-    const Message& entry = reflection->GetRepeatedMessage(
-        *message, self->parent_field_descriptor, i);
-    int matches = MapKeyMatches(self, &entry, key);
-    if (matches < 0) return NULL;
-    if (matches) {
-      Py_RETURN_TRUE;
-    }
-  }
-
-  Py_RETURN_FALSE;
-}
-
-PyObject* Get(PyObject* self, PyObject* args) {
-  PyObject* key;
-  PyObject* default_value = NULL;
-  if (PyArg_ParseTuple(args, "O|O", &key, &default_value) < 0) {
-    return NULL;
-  }
-
-  ScopedPyObjectPtr is_present(Contains(self, key));
-  if (is_present.get() == NULL) {
-    return NULL;
-  }
-
-  if (PyObject_IsTrue(is_present.get())) {
-    return GetItem(self, key);
-  } else {
-    if (default_value != NULL) {
-      Py_INCREF(default_value);
-      return default_value;
-    } else {
-      Py_RETURN_NONE;
-    }
-  }
-}
-
-static void Dealloc(PyObject* _self) {
-  ScalarMapContainer* self = GetMap(_self);
-  self->owner.reset();
-  Py_TYPE(_self)->tp_free(_self);
-}
-
-static PyMethodDef Methods[] = {
-  { "__contains__", Contains, METH_O,
-    "Tests whether a key is a member of the map." },
-  { "clear", (PyCFunction)Clear, METH_NOARGS,
-    "Removes all elements from the map." },
-  { "get", Get, METH_VARARGS,
-    "Gets the value for the given key if present, or otherwise a default" },
-  /*
-  { "__deepcopy__", (PyCFunction)DeepCopy, METH_VARARGS,
-    "Makes a deep copy of the class." },
-  { "__reduce__", (PyCFunction)Reduce, METH_NOARGS,
-    "Outputs picklable representation of the repeated field." },
-  */
-  {NULL, NULL},
-};
-
-}  // namespace scalar_map_container
-
-namespace scalar_map_iterator {
-
-static void Dealloc(PyObject* _self) {
-  ScalarMapIterator* self = GetIter(_self);
-  Py_DECREF(self->dict);
-  Py_DECREF(self->iter);
-  Py_DECREF(self->container);
-  Py_TYPE(_self)->tp_free(_self);
-}
-
-PyObject* IterNext(PyObject* _self) {
-  ScalarMapIterator* self = GetIter(_self);
-
-  // This won't catch mutations to the map performed by MergeFrom(); no easy way
-  // to address that.
-  if (self->version != self->container->version) {
-    return PyErr_Format(PyExc_RuntimeError,
-                        "Map modified during iteration.");
-  }
-
-  return PyIter_Next(self->iter);
-}
-
-}  // namespace scalar_map_iterator
-
- 
-#if PY_MAJOR_VERSION >= 3
-  static PyType_Slot ScalarMapContainer_Type_slots[] = {
-      {Py_tp_dealloc, (void *)scalar_map_container::Dealloc},
-      {Py_mp_length, (void *)scalar_map_container::Length},
-      {Py_mp_subscript, (void *)scalar_map_container::GetItem},
-      {Py_mp_ass_subscript, (void *)scalar_map_container::SetItem},
-      {Py_tp_methods, (void *)scalar_map_container::Methods},
-      {Py_tp_iter, (void *)scalar_map_container::GetIterator},
-      {0, 0},
-  };
-
-  PyType_Spec ScalarMapContainer_Type_spec = {
-      FULL_MODULE_NAME ".ScalarMapContainer",
-      sizeof(ScalarMapContainer),
-      0,
-      Py_TPFLAGS_DEFAULT,
-      ScalarMapContainer_Type_slots
-  };
-  PyObject *ScalarMapContainer_Type;
-#else
-  static PyMappingMethods MpMethods = {
-    scalar_map_container::Length,    // mp_length
-    scalar_map_container::GetItem,   // mp_subscript
-    scalar_map_container::SetItem,   // mp_ass_subscript
-  };
-
-  PyTypeObject ScalarMapContainer_Type = {
-    PyVarObject_HEAD_INIT(&PyType_Type, 0)
-    FULL_MODULE_NAME ".ScalarMapContainer",  //  tp_name
-    sizeof(ScalarMapContainer),          //  tp_basicsize
-    0,                                   //  tp_itemsize
-    scalar_map_container::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
-    &MpMethods,                          //  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,                  //  tp_flags
-    "A scalar map container",            //  tp_doc
-    0,                                   //  tp_traverse
-    0,                                   //  tp_clear
-    0,                                   //  tp_richcompare
-    0,                                   //  tp_weaklistoffset
-    scalar_map_container::GetIterator,   //  tp_iter
-    0,                                   //  tp_iternext
-    scalar_map_container::Methods,       //  tp_methods
-    0,                                   //  tp_members
-    0,                                   //  tp_getset
-    0,                                   //  tp_base
-    0,                                   //  tp_dict
-    0,                                   //  tp_descr_get
-    0,                                   //  tp_descr_set
-    0,                                   //  tp_dictoffset
-    0,                                   //  tp_init
-  };
-#endif
-
-PyTypeObject ScalarMapIterator_Type = {
-  PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  FULL_MODULE_NAME ".ScalarMapIterator",  //  tp_name
-  sizeof(ScalarMapIterator),           //  tp_basicsize
-  0,                                   //  tp_itemsize
-  scalar_map_iterator::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,                  //  tp_flags
-  "A scalar map iterator",             //  tp_doc
-  0,                                   //  tp_traverse
-  0,                                   //  tp_clear
-  0,                                   //  tp_richcompare
-  0,                                   //  tp_weaklistoffset
-  PyObject_SelfIter,                   //  tp_iter
-  scalar_map_iterator::IterNext,       //  tp_iternext
-  0,                                   //  tp_methods
-  0,                                   //  tp_members
-  0,                                   //  tp_getset
-  0,                                   //  tp_base
-  0,                                   //  tp_dict
-  0,                                   //  tp_descr_get
-  0,                                   //  tp_descr_set
-  0,                                   //  tp_dictoffset
-  0,                                   //  tp_init
-};
-
-}  // namespace python
-}  // namespace protobuf
-}  // namespace google
diff --git a/python/google/protobuf/pyext/scalar_map_container.h b/python/google/protobuf/pyext/scalar_map_container.h
deleted file mode 100644
index 4d663b88..00000000
--- a/python/google/protobuf/pyext/scalar_map_container.h
+++ /dev/null
@@ -1,119 +0,0 @@
-// 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_SCALAR_MAP_CONTAINER_H__
-#define GOOGLE_PROTOBUF_PYTHON_CPP_SCALAR_MAP_CONTAINER_H__
-
-#include <Python.h>
-
-#include <memory>
-#ifndef _SHARED_PTR_H
-#include <google/protobuf/stubs/shared_ptr.h>
-#endif
-
-#include <google/protobuf/descriptor.h>
-
-namespace google {
-namespace protobuf {
-
-class Message;
-
-#ifdef _SHARED_PTR_H
-using std::shared_ptr;
-#else
-using internal::shared_ptr;
-#endif
-
-namespace python {
-
-struct CMessage;
-
-struct ScalarMapContainer {
-  PyObject_HEAD;
-
-  // This is the top-level C++ Message object that owns the whole
-  // proto tree.  Every Python ScalarMapContainer holds a
-  // reference to it in order to keep it alive as long as there's a
-  // Python object that references any part of the tree.
-  shared_ptr<Message> owner;
-
-  // Pointer to the C++ Message that contains this container.  The
-  // ScalarMapContainer does not own this pointer.
-  Message* message;
-
-  // Weak reference to a parent CMessage object (i.e. may be NULL.)
-  //
-  // Used to make sure all ancestors are also mutable when first
-  // modifying the container.
-  CMessage* parent;
-
-  // Pointer to the parent's descriptor that describes this
-  // field.  Used together with the parent's message when making a
-  // default message instance mutable.
-  // The pointer is owned by the global DescriptorPool.
-  const FieldDescriptor* parent_field_descriptor;
-  const FieldDescriptor* key_field_descriptor;
-  const FieldDescriptor* value_field_descriptor;
-
-  // We bump this whenever we perform a mutation, to invalidate existing
-  // iterators.
-  uint64 version;
-};
-
-#if PY_MAJOR_VERSION >= 3
-  extern PyObject *ScalarMapContainer_Type;
-  extern PyType_Spec ScalarMapContainer_Type_spec;
-#else
-  extern PyTypeObject ScalarMapContainer_Type;
-#endif
-extern PyTypeObject ScalarMapIterator_Type;
-
-namespace scalar_map_container {
-
-// Builds a ScalarMapContainer object, from a parent message and a
-// field descriptor.
-extern PyObject *NewContainer(
-    CMessage* parent, const FieldDescriptor* parent_field_descriptor);
-
-// Releases the messages in the container to a new message.
-//
-// Returns 0 on success, -1 on failure.
-int Release(ScalarMapContainer* self);
-
-// Set the owner field of self and any children of self.
-void SetOwner(ScalarMapContainer* self,
-              const shared_ptr<Message>& new_owner);
-
-}  // namespace scalar_map_container
-}  // namespace python
-}  // namespace protobuf
-
-}  // namespace google
-#endif  // GOOGLE_PROTOBUF_PYTHON_CPP_SCALAR_MAP_CONTAINER_H__