down-integrate internal changes

16 files changed, 1948 insertions, 401 deletions

diff --git a/python/google/protobuf/pyext/descriptor.cc b/python/google/protobuf/pyext/descriptor.cc
index e77d0bb9..2160757b 100644
--- a/python/google/protobuf/pyext/descriptor.cc
+++ b/python/google/protobuf/pyext/descriptor.cc
@@ -43,8 +43,6 @@
 #include <google/protobuf/pyext/message.h>
 #include <google/protobuf/pyext/scoped_pyobject_ptr.h>
 
-#define C(str) const_cast<char*>(str)
-
 #if PY_MAJOR_VERSION >= 3
   #define PyString_FromStringAndSize PyUnicode_FromStringAndSize
   #define PyString_Check PyUnicode_Check
@@ -257,8 +255,14 @@ namespace descriptor {
 // Creates or retrieve a Python descriptor of the specified type.
 // Objects are interned: the same descriptor will return the same object if it
 // was kept alive.
+// 'was_created' is an optional pointer to a bool, and is set to true if a new
+// object was allocated.
 // Always return a new reference.
-PyObject* NewInternedDescriptor(PyTypeObject* type, const void* descriptor) {
+PyObject* NewInternedDescriptor(PyTypeObject* type, const void* descriptor,
+                                bool* was_created) {
+  if (was_created) {
+    *was_created = false;
+  }
   if (descriptor == NULL) {
     PyErr_BadInternalCall();
     return NULL;
@@ -283,6 +287,9 @@ PyObject* NewInternedDescriptor(PyTypeObject* type, const void* descriptor) {
   GetDescriptorPool()->interned_descriptors->insert(
       std::make_pair(descriptor, reinterpret_cast<PyObject*>(py_descriptor)));
 
+  if (was_created) {
+    *was_created = true;
+  }
   return reinterpret_cast<PyObject*>(py_descriptor);
 }
 
@@ -298,9 +305,7 @@ static PyGetSetDef Getters[] = {
 
 PyTypeObject PyBaseDescriptor_Type = {
   PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  // Keep the fully qualified _message symbol in a line for opensource.
-  "google.protobuf.internal._message."
-  "DescriptorBase",                     // tp_name
+  FULL_MODULE_NAME ".DescriptorBase",   // tp_name
   sizeof(PyBaseDescriptor),             // tp_basicsize
   0,                                    // tp_itemsize
   (destructor)Dealloc,                  // tp_dealloc
@@ -357,7 +362,7 @@ static PyObject* GetFullName(PyBaseDescriptor* self, void *closure) {
 }
 
 static PyObject* GetFile(PyBaseDescriptor *self, void *closure) {
-  return PyFileDescriptor_New(_GetDescriptor(self)->file());
+  return PyFileDescriptor_FromDescriptor(_GetDescriptor(self)->file());
 }
 
 static PyObject* GetConcreteClass(PyBaseDescriptor* self, void *closure) {
@@ -367,17 +372,6 @@ static PyObject* GetConcreteClass(PyBaseDescriptor* self, void *closure) {
   return concrete_class;
 }
 
-static int SetConcreteClass(PyBaseDescriptor *self, PyObject *value,
-                            void *closure) {
-  // This attribute is also set from reflection.py. Check that it's actually a
-  // no-op.
-  if (value != cdescriptor_pool::GetMessageClass(
-          GetDescriptorPool(), _GetDescriptor(self))) {
-    PyErr_SetString(PyExc_AttributeError, "Cannot change _concrete_class");
-  }
-  return 0;
-}
-
 static PyObject* GetFieldsByName(PyBaseDescriptor* self, void *closure) {
   return NewMessageFieldsByName(_GetDescriptor(self));
 }
@@ -452,7 +446,7 @@ static PyObject* GetContainingType(PyBaseDescriptor *self, void *closure) {
   const Descriptor* containing_type =
       _GetDescriptor(self)->containing_type();
   if (containing_type) {
-    return PyMessageDescriptor_New(containing_type);
+    return PyMessageDescriptor_FromDescriptor(containing_type);
   } else {
     Py_RETURN_NONE;
   }
@@ -515,29 +509,34 @@ static PyObject* GetSyntax(PyBaseDescriptor *self, void *closure) {
 }
 
 static PyGetSetDef Getters[] = {
-  { C("name"), (getter)GetName, NULL, "Last name", NULL},
-  { C("full_name"), (getter)GetFullName, NULL, "Full name", NULL},
-  { C("_concrete_class"), (getter)GetConcreteClass, (setter)SetConcreteClass, "concrete class", NULL},
-  { C("file"), (getter)GetFile, NULL, "File descriptor", NULL},
-
-  { C("fields"), (getter)GetFieldsSeq, NULL, "Fields sequence", NULL},
-  { C("fields_by_name"), (getter)GetFieldsByName, NULL, "Fields by name", NULL},
-  { C("fields_by_number"), (getter)GetFieldsByNumber, NULL, "Fields by number", NULL},
-  { C("nested_types"), (getter)GetNestedTypesSeq, NULL, "Nested types sequence", NULL},
-  { C("nested_types_by_name"), (getter)GetNestedTypesByName, NULL, "Nested types by name", NULL},
-  { C("extensions"), (getter)GetExtensions, NULL, "Extensions Sequence", NULL},
-  { C("extensions_by_name"), (getter)GetExtensionsByName, NULL, "Extensions by name", NULL},
-  { C("extension_ranges"), (getter)GetExtensionRanges, NULL, "Extension ranges", NULL},
-  { C("enum_types"), (getter)GetEnumsSeq, NULL, "Enum sequence", NULL},
-  { C("enum_types_by_name"), (getter)GetEnumTypesByName, NULL, "Enum types by name", NULL},
-  { C("enum_values_by_name"), (getter)GetEnumValuesByName, NULL, "Enum values by name", NULL},
-  { C("oneofs_by_name"), (getter)GetOneofsByName, NULL, "Oneofs by name", NULL},
-  { C("oneofs"), (getter)GetOneofsSeq, NULL, "Oneofs by name", NULL},
-  { C("containing_type"), (getter)GetContainingType, (setter)SetContainingType, "Containing type", NULL},
-  { C("is_extendable"), (getter)IsExtendable, (setter)NULL, NULL, NULL},
-  { C("has_options"), (getter)GetHasOptions, (setter)SetHasOptions, "Has Options", NULL},
-  { C("_options"), (getter)NULL, (setter)SetOptions, "Options", NULL},
-  { C("syntax"), (getter)GetSyntax, (setter)NULL, "Syntax", NULL},
+  { "name", (getter)GetName, NULL, "Last name"},
+  { "full_name", (getter)GetFullName, NULL, "Full name"},
+  { "_concrete_class", (getter)GetConcreteClass, NULL, "concrete class"},
+  { "file", (getter)GetFile, NULL, "File descriptor"},
+
+  { "fields", (getter)GetFieldsSeq, NULL, "Fields sequence"},
+  { "fields_by_name", (getter)GetFieldsByName, NULL, "Fields by name"},
+  { "fields_by_number", (getter)GetFieldsByNumber, NULL, "Fields by number"},
+  { "nested_types", (getter)GetNestedTypesSeq, NULL, "Nested types sequence"},
+  { "nested_types_by_name", (getter)GetNestedTypesByName, NULL,
+    "Nested types by name"},
+  { "extensions", (getter)GetExtensions, NULL, "Extensions Sequence"},
+  { "extensions_by_name", (getter)GetExtensionsByName, NULL,
+    "Extensions by name"},
+  { "extension_ranges", (getter)GetExtensionRanges, NULL, "Extension ranges"},
+  { "enum_types", (getter)GetEnumsSeq, NULL, "Enum sequence"},
+  { "enum_types_by_name", (getter)GetEnumTypesByName, NULL,
+    "Enum types by name"},
+  { "enum_values_by_name", (getter)GetEnumValuesByName, NULL,
+    "Enum values by name"},
+  { "oneofs_by_name", (getter)GetOneofsByName, NULL, "Oneofs by name"},
+  { "oneofs", (getter)GetOneofsSeq, NULL, "Oneofs by name"},
+  { "containing_type", (getter)GetContainingType, (setter)SetContainingType,
+    "Containing type"},
+  { "is_extendable", (getter)IsExtendable, (setter)NULL},
+  { "has_options", (getter)GetHasOptions, (setter)SetHasOptions, "Has Options"},
+  { "_options", (getter)NULL, (setter)SetOptions, "Options"},
+  { "syntax", (getter)GetSyntax, (setter)NULL, "Syntax"},
   {NULL}
 };
 
@@ -552,9 +551,7 @@ static PyMethodDef Methods[] = {
 
 PyTypeObject PyMessageDescriptor_Type = {
   PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  // Keep the fully qualified _message symbol in a line for opensource.
-  C("google.protobuf.internal._message."
-    "MessageDescriptor"),      // tp_name
+  FULL_MODULE_NAME ".MessageDescriptor",  // tp_name
   sizeof(PyBaseDescriptor),             // tp_basicsize
   0,                                    // tp_itemsize
   0,                                    // tp_dealloc
@@ -573,7 +570,7 @@ PyTypeObject PyMessageDescriptor_Type = {
   0,                                    // tp_setattro
   0,                                    // tp_as_buffer
   Py_TPFLAGS_DEFAULT,                   // tp_flags
-  C("A Message Descriptor"),            // tp_doc
+  "A Message Descriptor",               // tp_doc
   0,                                    // tp_traverse
   0,                                    // tp_clear
   0,                                    // tp_richcompare
@@ -586,10 +583,10 @@ PyTypeObject PyMessageDescriptor_Type = {
   &descriptor::PyBaseDescriptor_Type,   // tp_base
 };
 
-PyObject* PyMessageDescriptor_New(
+PyObject* PyMessageDescriptor_FromDescriptor(
     const Descriptor* message_descriptor) {
   return descriptor::NewInternedDescriptor(
-      &PyMessageDescriptor_Type, message_descriptor);
+      &PyMessageDescriptor_Type, message_descriptor, NULL);
 }
 
 const Descriptor* PyMessageDescriptor_AsDescriptor(PyObject* obj) {
@@ -715,7 +712,7 @@ static PyObject* GetCDescriptor(PyObject *self, void *closure) {
 static PyObject *GetEnumType(PyBaseDescriptor *self, void *closure) {
   const EnumDescriptor* enum_type = _GetDescriptor(self)->enum_type();
   if (enum_type) {
-    return PyEnumDescriptor_New(enum_type);
+    return PyEnumDescriptor_FromDescriptor(enum_type);
   } else {
     Py_RETURN_NONE;
   }
@@ -728,7 +725,7 @@ static int SetEnumType(PyBaseDescriptor *self, PyObject *value, void *closure) {
 static PyObject *GetMessageType(PyBaseDescriptor *self, void *closure) {
   const Descriptor* message_type = _GetDescriptor(self)->message_type();
   if (message_type) {
-    return PyMessageDescriptor_New(message_type);
+    return PyMessageDescriptor_FromDescriptor(message_type);
   } else {
     Py_RETURN_NONE;
   }
@@ -743,7 +740,7 @@ static PyObject* GetContainingType(PyBaseDescriptor *self, void *closure) {
   const Descriptor* containing_type =
       _GetDescriptor(self)->containing_type();
   if (containing_type) {
-    return PyMessageDescriptor_New(containing_type);
+    return PyMessageDescriptor_FromDescriptor(containing_type);
   } else {
     Py_RETURN_NONE;
   }
@@ -758,7 +755,7 @@ static PyObject* GetExtensionScope(PyBaseDescriptor *self, void *closure) {
   const Descriptor* extension_scope =
       _GetDescriptor(self)->extension_scope();
   if (extension_scope) {
-    return PyMessageDescriptor_New(extension_scope);
+    return PyMessageDescriptor_FromDescriptor(extension_scope);
   } else {
     Py_RETURN_NONE;
   }
@@ -768,7 +765,7 @@ static PyObject* GetContainingOneof(PyBaseDescriptor *self, void *closure) {
   const OneofDescriptor* containing_oneof =
       _GetDescriptor(self)->containing_oneof();
   if (containing_oneof) {
-    return PyOneofDescriptor_New(containing_oneof);
+    return PyOneofDescriptor_FromDescriptor(containing_oneof);
   } else {
     Py_RETURN_NONE;
   }
@@ -803,26 +800,30 @@ static int SetOptions(PyBaseDescriptor *self, PyObject *value,
 
 
 static PyGetSetDef Getters[] = {
-  { C("full_name"), (getter)GetFullName, NULL, "Full name", NULL},
-  { C("name"), (getter)GetName, NULL, "Unqualified name", NULL},
-  { C("type"), (getter)GetType, NULL, "C++ Type", NULL},
-  { C("cpp_type"), (getter)GetCppType, NULL, "C++ Type", NULL},
-  { C("label"), (getter)GetLabel, NULL, "Label", NULL},
-  { C("number"), (getter)GetNumber, NULL, "Number", NULL},
-  { C("index"), (getter)GetIndex, NULL, "Index", NULL},
-  { C("default_value"), (getter)GetDefaultValue, NULL, "Default Value", NULL},
-  { C("has_default_value"), (getter)HasDefaultValue, NULL, NULL, NULL},
-  { C("is_extension"), (getter)IsExtension, NULL, "ID", NULL},
-  { C("id"), (getter)GetID, NULL, "ID", NULL},
-  { C("_cdescriptor"), (getter)GetCDescriptor, NULL, "HAACK REMOVE ME", NULL},
-
-  { C("message_type"), (getter)GetMessageType, (setter)SetMessageType, "Message type", NULL},
-  { C("enum_type"), (getter)GetEnumType, (setter)SetEnumType, "Enum type", NULL},
-  { C("containing_type"), (getter)GetContainingType, (setter)SetContainingType, "Containing type", NULL},
-  { C("extension_scope"), (getter)GetExtensionScope, (setter)NULL, "Extension scope", NULL},
-  { C("containing_oneof"), (getter)GetContainingOneof, (setter)SetContainingOneof, "Containing oneof", NULL},
-  { C("has_options"), (getter)GetHasOptions, (setter)SetHasOptions, "Has Options", NULL},
-  { C("_options"), (getter)NULL, (setter)SetOptions, "Options", NULL},
+  { "full_name", (getter)GetFullName, NULL, "Full name"},
+  { "name", (getter)GetName, NULL, "Unqualified name"},
+  { "type", (getter)GetType, NULL, "C++ Type"},
+  { "cpp_type", (getter)GetCppType, NULL, "C++ Type"},
+  { "label", (getter)GetLabel, NULL, "Label"},
+  { "number", (getter)GetNumber, NULL, "Number"},
+  { "index", (getter)GetIndex, NULL, "Index"},
+  { "default_value", (getter)GetDefaultValue, NULL, "Default Value"},
+  { "has_default_value", (getter)HasDefaultValue},
+  { "is_extension", (getter)IsExtension, NULL, "ID"},
+  { "id", (getter)GetID, NULL, "ID"},
+  { "_cdescriptor", (getter)GetCDescriptor, NULL, "HAACK REMOVE ME"},
+
+  { "message_type", (getter)GetMessageType, (setter)SetMessageType,
+    "Message type"},
+  { "enum_type", (getter)GetEnumType, (setter)SetEnumType, "Enum type"},
+  { "containing_type", (getter)GetContainingType, (setter)SetContainingType,
+    "Containing type"},
+  { "extension_scope", (getter)GetExtensionScope, (setter)NULL,
+    "Extension scope"},
+  { "containing_oneof", (getter)GetContainingOneof, (setter)SetContainingOneof,
+    "Containing oneof"},
+  { "has_options", (getter)GetHasOptions, (setter)SetHasOptions, "Has Options"},
+  { "_options", (getter)NULL, (setter)SetOptions, "Options"},
   {NULL}
 };
 
@@ -835,8 +836,7 @@ static PyMethodDef Methods[] = {
 
 PyTypeObject PyFieldDescriptor_Type = {
   PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  C("google.protobuf.internal."
-    "_message.FieldDescriptor"),        // tp_name
+  FULL_MODULE_NAME ".FieldDescriptor",  // tp_name
   sizeof(PyBaseDescriptor),             // tp_basicsize
   0,                                    // tp_itemsize
   0,                                    // tp_dealloc
@@ -855,7 +855,7 @@ PyTypeObject PyFieldDescriptor_Type = {
   0,                                    // tp_setattro
   0,                                    // tp_as_buffer
   Py_TPFLAGS_DEFAULT,                   // tp_flags
-  C("A Field Descriptor"),              // tp_doc
+  "A Field Descriptor",                 // tp_doc
   0,                                    // tp_traverse
   0,                                    // tp_clear
   0,                                    // tp_richcompare
@@ -868,10 +868,10 @@ PyTypeObject PyFieldDescriptor_Type = {
   &descriptor::PyBaseDescriptor_Type,   // tp_base
 };
 
-PyObject* PyFieldDescriptor_New(
+PyObject* PyFieldDescriptor_FromDescriptor(
     const FieldDescriptor* field_descriptor) {
   return descriptor::NewInternedDescriptor(
-      &PyFieldDescriptor_Type, field_descriptor);
+      &PyFieldDescriptor_Type, field_descriptor, NULL);
 }
 
 const FieldDescriptor* PyFieldDescriptor_AsDescriptor(PyObject* obj) {
@@ -900,7 +900,7 @@ static PyObject* GetName(PyBaseDescriptor *self, void *closure) {
 }
 
 static PyObject* GetFile(PyBaseDescriptor *self, void *closure) {
-  return PyFileDescriptor_New(_GetDescriptor(self)->file());
+  return PyFileDescriptor_FromDescriptor(_GetDescriptor(self)->file());
 }
 
 static PyObject* GetEnumvaluesByName(PyBaseDescriptor* self, void *closure) {
@@ -919,7 +919,7 @@ static PyObject* GetContainingType(PyBaseDescriptor *self, void *closure) {
   const Descriptor* containing_type =
       _GetDescriptor(self)->containing_type();
   if (containing_type) {
-    return PyMessageDescriptor_New(containing_type);
+    return PyMessageDescriptor_FromDescriptor(containing_type);
   } else {
     Py_RETURN_NONE;
   }
@@ -964,16 +964,19 @@ static PyMethodDef Methods[] = {
 };
 
 static PyGetSetDef Getters[] = {
-  { C("full_name"), (getter)GetFullName, NULL, "Full name", NULL},
-  { C("name"), (getter)GetName, NULL, "last name", NULL},
-  { C("file"), (getter)GetFile, NULL, "File descriptor", NULL},
-  { C("values"), (getter)GetEnumvaluesSeq, NULL, "values", NULL},
-  { C("values_by_name"), (getter)GetEnumvaluesByName, NULL, "Enumvalues by name", NULL},
-  { C("values_by_number"), (getter)GetEnumvaluesByNumber, NULL, "Enumvalues by number", NULL},
-
-  { C("containing_type"), (getter)GetContainingType, (setter)SetContainingType, "Containing type", NULL},
-  { C("has_options"), (getter)GetHasOptions, (setter)SetHasOptions, "Has Options", NULL},
-  { C("_options"), (getter)NULL, (setter)SetOptions, "Options", NULL},
+  { "full_name", (getter)GetFullName, NULL, "Full name"},
+  { "name", (getter)GetName, NULL, "last name"},
+  { "file", (getter)GetFile, NULL, "File descriptor"},
+  { "values", (getter)GetEnumvaluesSeq, NULL, "values"},
+  { "values_by_name", (getter)GetEnumvaluesByName, NULL,
+    "Enum values by name"},
+  { "values_by_number", (getter)GetEnumvaluesByNumber, NULL,
+    "Enum values by number"},
+
+  { "containing_type", (getter)GetContainingType, (setter)SetContainingType,
+    "Containing type"},
+  { "has_options", (getter)GetHasOptions, (setter)SetHasOptions, "Has Options"},
+  { "_options", (getter)NULL, (setter)SetOptions, "Options"},
   {NULL}
 };
 
@@ -981,9 +984,7 @@ static PyGetSetDef Getters[] = {
 
 PyTypeObject PyEnumDescriptor_Type = {
   PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  // Keep the fully qualified _message symbol in a line for opensource.
-  C("google.protobuf.internal._message."
-    "EnumDescriptor"),                  // tp_name
+  FULL_MODULE_NAME ".EnumDescriptor",   // tp_name
   sizeof(PyBaseDescriptor),             // tp_basicsize
   0,                                    // tp_itemsize
   0,                                    // tp_dealloc
@@ -1002,7 +1003,7 @@ PyTypeObject PyEnumDescriptor_Type = {
   0,                                    // tp_setattro
   0,                                    // tp_as_buffer
   Py_TPFLAGS_DEFAULT,                   // tp_flags
-  C("A Enum Descriptor"),               // tp_doc
+  "A Enum Descriptor",                  // tp_doc
   0,                                    // tp_traverse
   0,                                    // tp_clear
   0,                                    // tp_richcompare
@@ -1015,10 +1016,10 @@ PyTypeObject PyEnumDescriptor_Type = {
   &descriptor::PyBaseDescriptor_Type,   // tp_base
 };
 
-PyObject* PyEnumDescriptor_New(
+PyObject* PyEnumDescriptor_FromDescriptor(
     const EnumDescriptor* enum_descriptor) {
   return descriptor::NewInternedDescriptor(
-      &PyEnumDescriptor_Type, enum_descriptor);
+      &PyEnumDescriptor_Type, enum_descriptor, NULL);
 }
 
 namespace enumvalue_descriptor {
@@ -1042,7 +1043,7 @@ static PyObject* GetIndex(PyBaseDescriptor *self, void *closure) {
 }
 
 static PyObject* GetType(PyBaseDescriptor *self, void *closure) {
-  return PyEnumDescriptor_New(_GetDescriptor(self)->type());
+  return PyEnumDescriptor_FromDescriptor(_GetDescriptor(self)->type());
 }
 
 static PyObject* GetHasOptions(PyBaseDescriptor *self, void *closure) {
@@ -1069,13 +1070,13 @@ static int SetOptions(PyBaseDescriptor *self, PyObject *value,
 
 
 static PyGetSetDef Getters[] = {
-  { C("name"), (getter)GetName, NULL, "name", NULL},
-  { C("number"), (getter)GetNumber, NULL, "number", NULL},
-  { C("index"), (getter)GetIndex, NULL, "index", NULL},
-  { C("type"), (getter)GetType, NULL, "index", NULL},
+  { "name", (getter)GetName, NULL, "name"},
+  { "number", (getter)GetNumber, NULL, "number"},
+  { "index", (getter)GetIndex, NULL, "index"},
+  { "type", (getter)GetType, NULL, "index"},
 
-  { C("has_options"), (getter)GetHasOptions, (setter)SetHasOptions, "Has Options", NULL},
-  { C("_options"), (getter)NULL, (setter)SetOptions, "Options", NULL},
+  { "has_options", (getter)GetHasOptions, (setter)SetHasOptions, "Has Options"},
+  { "_options", (getter)NULL, (setter)SetOptions, "Options"},
   {NULL}
 };
 
@@ -1088,8 +1089,7 @@ static PyMethodDef Methods[] = {
 
 PyTypeObject PyEnumValueDescriptor_Type = {
   PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  C("google.protobuf.internal."
-    "_message.EnumValueDescriptor"),    // tp_name
+  FULL_MODULE_NAME ".EnumValueDescriptor",  // tp_name
   sizeof(PyBaseDescriptor),             // tp_basicsize
   0,                                    // tp_itemsize
   0,                                    // tp_dealloc
@@ -1108,7 +1108,7 @@ PyTypeObject PyEnumValueDescriptor_Type = {
   0,                                    // tp_setattro
   0,                                    // tp_as_buffer
   Py_TPFLAGS_DEFAULT,                   // tp_flags
-  C("A EnumValue Descriptor"),          // tp_doc
+  "A EnumValue Descriptor",             // tp_doc
   0,                                    // tp_traverse
   0,                                    // tp_clear
   0,                                    // tp_richcompare
@@ -1121,10 +1121,10 @@ PyTypeObject PyEnumValueDescriptor_Type = {
   &descriptor::PyBaseDescriptor_Type,   // tp_base
 };
 
-PyObject* PyEnumValueDescriptor_New(
+PyObject* PyEnumValueDescriptor_FromDescriptor(
     const EnumValueDescriptor* enumvalue_descriptor) {
   return descriptor::NewInternedDescriptor(
-      &PyEnumValueDescriptor_Type, enumvalue_descriptor);
+      &PyEnumValueDescriptor_Type, enumvalue_descriptor, NULL);
 }
 
 namespace file_descriptor {
@@ -1218,18 +1218,20 @@ static PyObject* CopyToProto(PyFileDescriptor *self, PyObject *target) {
 }
 
 static PyGetSetDef Getters[] = {
-  { C("name"), (getter)GetName, NULL, "name", NULL},
-  { C("package"), (getter)GetPackage, NULL, "package", NULL},
-  { C("serialized_pb"), (getter)GetSerializedPb, NULL, NULL, NULL},
-  { C("message_types_by_name"), (getter)GetMessageTypesByName, NULL, "Messages by name", NULL},
-  { C("enum_types_by_name"), (getter)GetEnumTypesByName, NULL, "Enums by name", NULL},
-  { C("extensions_by_name"), (getter)GetExtensionsByName, NULL, "Extensions by name", NULL},
-  { C("dependencies"), (getter)GetDependencies, NULL, "Dependencies", NULL},
-  { C("public_dependencies"), (getter)GetPublicDependencies, NULL, "Dependencies", NULL},
-
-  { C("has_options"), (getter)GetHasOptions, (setter)SetHasOptions, "Has Options", NULL},
-  { C("_options"), (getter)NULL, (setter)SetOptions, "Options", NULL},
-  { C("syntax"), (getter)GetSyntax, (setter)NULL, "Syntax", NULL},
+  { "name", (getter)GetName, NULL, "name"},
+  { "package", (getter)GetPackage, NULL, "package"},
+  { "serialized_pb", (getter)GetSerializedPb},
+  { "message_types_by_name", (getter)GetMessageTypesByName, NULL,
+    "Messages by name"},
+  { "enum_types_by_name", (getter)GetEnumTypesByName, NULL, "Enums by name"},
+  { "extensions_by_name", (getter)GetExtensionsByName, NULL,
+    "Extensions by name"},
+  { "dependencies", (getter)GetDependencies, NULL, "Dependencies"},
+  { "public_dependencies", (getter)GetPublicDependencies, NULL, "Dependencies"},
+
+  { "has_options", (getter)GetHasOptions, (setter)SetHasOptions, "Has Options"},
+  { "_options", (getter)NULL, (setter)SetOptions, "Options"},
+  { "syntax", (getter)GetSyntax, (setter)NULL, "Syntax"},
   {NULL}
 };
 
@@ -1243,11 +1245,10 @@ static PyMethodDef Methods[] = {
 
 PyTypeObject PyFileDescriptor_Type = {
   PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  C("google.protobuf.internal."
-    "_message.FileDescriptor"),         // tp_name
+  FULL_MODULE_NAME ".FileDescriptor",   // tp_name
   sizeof(PyFileDescriptor),             // tp_basicsize
   0,                                    // tp_itemsize
-  (destructor)file_descriptor::Dealloc, // tp_dealloc
+  (destructor)file_descriptor::Dealloc,  // tp_dealloc
   0,                                    // tp_print
   0,                                    // tp_getattr
   0,                                    // tp_setattr
@@ -1263,7 +1264,7 @@ PyTypeObject PyFileDescriptor_Type = {
   0,                                    // tp_setattro
   0,                                    // tp_as_buffer
   Py_TPFLAGS_DEFAULT,                   // tp_flags
-  C("A File Descriptor"),               // tp_doc
+  "A File Descriptor",                  // tp_doc
   0,                                    // tp_traverse
   0,                                    // tp_clear
   0,                                    // tp_richcompare
@@ -1284,23 +1285,28 @@ PyTypeObject PyFileDescriptor_Type = {
   PyObject_Del,                         // tp_free
 };
 
-PyObject* PyFileDescriptor_New(const FileDescriptor* file_descriptor) {
-  return descriptor::NewInternedDescriptor(
-      &PyFileDescriptor_Type, file_descriptor);
+PyObject* PyFileDescriptor_FromDescriptor(
+    const FileDescriptor* file_descriptor) {
+  return PyFileDescriptor_FromDescriptorWithSerializedPb(file_descriptor,
+                                                         NULL);
 }
 
-PyObject* PyFileDescriptor_NewWithPb(
+PyObject* PyFileDescriptor_FromDescriptorWithSerializedPb(
     const FileDescriptor* file_descriptor, PyObject *serialized_pb) {
-  PyObject* py_descriptor = PyFileDescriptor_New(file_descriptor);
+  bool was_created;
+  PyObject* py_descriptor = descriptor::NewInternedDescriptor(
+      &PyFileDescriptor_Type, file_descriptor, &was_created);
   if (py_descriptor == NULL) {
     return NULL;
   }
-  if (serialized_pb != NULL) {
+  if (was_created) {
     PyFileDescriptor* cfile_descriptor =
         reinterpret_cast<PyFileDescriptor*>(py_descriptor);
     Py_XINCREF(serialized_pb);
     cfile_descriptor->serialized_pb = serialized_pb;
   }
+  // TODO(amauryfa): In the case of a cached object, check that serialized_pb
+  // is the same as before.
 
   return py_descriptor;
 }
@@ -1333,19 +1339,19 @@ static PyObject* GetContainingType(PyBaseDescriptor *self, void *closure) {
   const Descriptor* containing_type =
       _GetDescriptor(self)->containing_type();
   if (containing_type) {
-    return PyMessageDescriptor_New(containing_type);
+    return PyMessageDescriptor_FromDescriptor(containing_type);
   } else {
     Py_RETURN_NONE;
   }
 }
 
 static PyGetSetDef Getters[] = {
-  { C("name"), (getter)GetName, NULL, "Name", NULL},
-  { C("full_name"), (getter)GetFullName, NULL, "Full name", NULL},
-  { C("index"), (getter)GetIndex, NULL, "Index", NULL},
+  { "name", (getter)GetName, NULL, "Name"},
+  { "full_name", (getter)GetFullName, NULL, "Full name"},
+  { "index", (getter)GetIndex, NULL, "Index"},
 
-  { C("containing_type"), (getter)GetContainingType, NULL, "Containing type", NULL},
-  { C("fields"), (getter)GetFields, NULL, "Fields", NULL},
+  { "containing_type", (getter)GetContainingType, NULL, "Containing type"},
+  { "fields", (getter)GetFields, NULL, "Fields"},
   {NULL}
 };
 
@@ -1353,8 +1359,7 @@ static PyGetSetDef Getters[] = {
 
 PyTypeObject PyOneofDescriptor_Type = {
   PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  C("google.protobuf.internal."
-    "_message.OneofDescriptor"),        // tp_name
+  FULL_MODULE_NAME ".OneofDescriptor",  // tp_name
   sizeof(PyBaseDescriptor),             // tp_basicsize
   0,                                    // tp_itemsize
   0,                                    // tp_dealloc
@@ -1373,7 +1378,7 @@ PyTypeObject PyOneofDescriptor_Type = {
   0,                                    // tp_setattro
   0,                                    // tp_as_buffer
   Py_TPFLAGS_DEFAULT,                   // tp_flags
-  C("A Oneof Descriptor"),              // tp_doc
+  "A Oneof Descriptor",                 // tp_doc
   0,                                    // tp_traverse
   0,                                    // tp_clear
   0,                                    // tp_richcompare
@@ -1386,10 +1391,10 @@ PyTypeObject PyOneofDescriptor_Type = {
   &descriptor::PyBaseDescriptor_Type,   // tp_base
 };
 
-PyObject* PyOneofDescriptor_New(
+PyObject* PyOneofDescriptor_FromDescriptor(
     const OneofDescriptor* oneof_descriptor) {
   return descriptor::NewInternedDescriptor(
-      &PyOneofDescriptor_Type, oneof_descriptor);
+      &PyOneofDescriptor_Type, oneof_descriptor, NULL);
 }
 
 // Add a enum values to a type dictionary.
diff --git a/python/google/protobuf/pyext/descriptor.h b/python/google/protobuf/pyext/descriptor.h
index ba6e7298..b2550406 100644
--- a/python/google/protobuf/pyext/descriptor.h
+++ b/python/google/protobuf/pyext/descriptor.h
@@ -48,21 +48,24 @@ extern PyTypeObject PyEnumValueDescriptor_Type;
 extern PyTypeObject PyFileDescriptor_Type;
 extern PyTypeObject PyOneofDescriptor_Type;
 
-// Return a new reference to a Descriptor object.
+// Wraps a Descriptor in a Python object.
 // The C++ pointer is usually borrowed from the global DescriptorPool.
 // In any case, it must stay alive as long as the Python object.
-PyObject* PyMessageDescriptor_New(const Descriptor* descriptor);
-PyObject* PyFieldDescriptor_New(const FieldDescriptor* descriptor);
-PyObject* PyEnumDescriptor_New(const EnumDescriptor* descriptor);
-PyObject* PyEnumValueDescriptor_New(const EnumValueDescriptor* descriptor);
-PyObject* PyOneofDescriptor_New(const OneofDescriptor* descriptor);
-PyObject* PyFileDescriptor_New(const FileDescriptor* file_descriptor);
+// Returns a new reference.
+PyObject* PyMessageDescriptor_FromDescriptor(const Descriptor* descriptor);
+PyObject* PyFieldDescriptor_FromDescriptor(const FieldDescriptor* descriptor);
+PyObject* PyEnumDescriptor_FromDescriptor(const EnumDescriptor* descriptor);
+PyObject* PyEnumValueDescriptor_FromDescriptor(
+    const EnumValueDescriptor* descriptor);
+PyObject* PyOneofDescriptor_FromDescriptor(const OneofDescriptor* descriptor);
+PyObject* PyFileDescriptor_FromDescriptor(
+    const FileDescriptor* file_descriptor);
 
 // Alternate constructor of PyFileDescriptor, used when we already have a
 // serialized FileDescriptorProto that can be cached.
 // Returns a new reference.
-PyObject* PyFileDescriptor_NewWithPb(const FileDescriptor* file_descriptor,
-                                     PyObject* serialized_pb);
+PyObject* PyFileDescriptor_FromDescriptorWithSerializedPb(
+    const FileDescriptor* file_descriptor, PyObject* serialized_pb);
 
 // Return the C++ descriptor pointer.
 // This function checks the parameter type; on error, return NULL with a Python
diff --git a/python/google/protobuf/pyext/descriptor_containers.cc b/python/google/protobuf/pyext/descriptor_containers.cc
index 06edebf8..92e11e31 100644
--- a/python/google/protobuf/pyext/descriptor_containers.cc
+++ b/python/google/protobuf/pyext/descriptor_containers.cc
@@ -898,7 +898,7 @@ static ItemDescriptor GetByIndex(PyContainer* self, int index) {
 }
 
 static PyObject* NewObjectFromItem(ItemDescriptor item) {
-  return PyFieldDescriptor_New(item);
+  return PyFieldDescriptor_FromDescriptor(item);
 }
 
 static const string& GetItemName(ItemDescriptor item) {
@@ -956,7 +956,7 @@ static ItemDescriptor GetByIndex(PyContainer* self, int index) {
 }
 
 static PyObject* NewObjectFromItem(ItemDescriptor item) {
-  return PyMessageDescriptor_New(item);
+  return PyMessageDescriptor_FromDescriptor(item);
 }
 
 static const string& GetItemName(ItemDescriptor item) {
@@ -1006,7 +1006,7 @@ static ItemDescriptor GetByIndex(PyContainer* self, int index) {
 }
 
 static PyObject* NewObjectFromItem(ItemDescriptor item) {
-  return PyEnumDescriptor_New(item);
+  return PyEnumDescriptor_FromDescriptor(item);
 }
 
 static const string& GetItemName(ItemDescriptor item) {
@@ -1082,7 +1082,7 @@ static ItemDescriptor GetByIndex(PyContainer* self, int index) {
 }
 
 static PyObject* NewObjectFromItem(ItemDescriptor item) {
-  return PyEnumValueDescriptor_New(item);
+  return PyEnumValueDescriptor_FromDescriptor(item);
 }
 
 static const string& GetItemName(ItemDescriptor item) {
@@ -1124,7 +1124,7 @@ static ItemDescriptor GetByIndex(PyContainer* self, int index) {
 }
 
 static PyObject* NewObjectFromItem(ItemDescriptor item) {
-  return PyFieldDescriptor_New(item);
+  return PyFieldDescriptor_FromDescriptor(item);
 }
 
 static const string& GetItemName(ItemDescriptor item) {
@@ -1174,7 +1174,7 @@ static ItemDescriptor GetByIndex(PyContainer* self, int index) {
 }
 
 static PyObject* NewObjectFromItem(ItemDescriptor item) {
-  return PyOneofDescriptor_New(item);
+  return PyOneofDescriptor_FromDescriptor(item);
 }
 
 static const string& GetItemName(ItemDescriptor item) {
@@ -1238,7 +1238,7 @@ static ItemDescriptor GetByNumber(PyContainer* self, int number) {
 }
 
 static PyObject* NewObjectFromItem(ItemDescriptor item) {
-  return PyEnumValueDescriptor_New(item);
+  return PyEnumValueDescriptor_FromDescriptor(item);
 }
 
 static const string& GetItemName(ItemDescriptor item) {
@@ -1302,7 +1302,7 @@ static ItemDescriptor GetByIndex(PyContainer* self, int index) {
 }
 
 static PyObject* NewObjectFromItem(ItemDescriptor item) {
-  return PyFieldDescriptor_New(item);
+  return PyFieldDescriptor_FromDescriptor(item);
 }
 
 static int GetItemIndex(ItemDescriptor item) {
@@ -1354,7 +1354,7 @@ static ItemDescriptor GetByIndex(PyContainer* self, int index) {
 }
 
 static PyObject* NewObjectFromItem(ItemDescriptor item) {
-  return PyMessageDescriptor_New(item);
+  return PyMessageDescriptor_FromDescriptor(item);
 }
 
 static const string& GetItemName(ItemDescriptor item) {
@@ -1400,7 +1400,7 @@ static ItemDescriptor GetByIndex(PyContainer* self, int index) {
 }
 
 static PyObject* NewObjectFromItem(ItemDescriptor item) {
-  return PyEnumDescriptor_New(item);
+  return PyEnumDescriptor_FromDescriptor(item);
 }
 
 static const string& GetItemName(ItemDescriptor item) {
@@ -1446,7 +1446,7 @@ static ItemDescriptor GetByIndex(PyContainer* self, int index) {
 }
 
 static PyObject* NewObjectFromItem(ItemDescriptor item) {
-  return PyFieldDescriptor_New(item);
+  return PyFieldDescriptor_FromDescriptor(item);
 }
 
 static const string& GetItemName(ItemDescriptor item) {
@@ -1488,7 +1488,7 @@ static ItemDescriptor GetByIndex(PyContainer* self, int index) {
 }
 
 static PyObject* NewObjectFromItem(ItemDescriptor item) {
-  return PyFileDescriptor_New(item);
+  return PyFileDescriptor_FromDescriptor(item);
 }
 
 static DescriptorContainerDef ContainerDef = {
@@ -1522,7 +1522,7 @@ static ItemDescriptor GetByIndex(PyContainer* self, int index) {
 }
 
 static PyObject* NewObjectFromItem(ItemDescriptor item) {
-  return PyFileDescriptor_New(item);
+  return PyFileDescriptor_FromDescriptor(item);
 }
 
 static DescriptorContainerDef ContainerDef = {
diff --git a/python/google/protobuf/pyext/descriptor_containers.h b/python/google/protobuf/pyext/descriptor_containers.h
index d81537de..8fbdaff9 100644
--- a/python/google/protobuf/pyext/descriptor_containers.h
+++ b/python/google/protobuf/pyext/descriptor_containers.h
@@ -28,6 +28,9 @@
 // (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_CONTAINERS_H__
+#define GOOGLE_PROTOBUF_PYTHON_CPP_DESCRIPTOR_CONTAINERS_H__
+
 // Mappings and Sequences of descriptors.
 // They implement containers like fields_by_name, EnumDescriptor.values...
 // See descriptor_containers.cc for more description.
@@ -92,4 +95,6 @@ PyObject* NewFilePublicDependencies(const FileDescriptor* descriptor);
 
 }  // namespace python
 }  // namespace protobuf
+
 }  // namespace google
+#endif  // GOOGLE_PROTOBUF_PYTHON_CPP_DESCRIPTOR_CONTAINERS_H__
diff --git a/python/google/protobuf/pyext/descriptor_pool.cc b/python/google/protobuf/pyext/descriptor_pool.cc
index bc3077bc..ecd90847 100644
--- a/python/google/protobuf/pyext/descriptor_pool.cc
+++ b/python/google/protobuf/pyext/descriptor_pool.cc
@@ -35,10 +35,9 @@
 #include <google/protobuf/descriptor.pb.h>
 #include <google/protobuf/pyext/descriptor_pool.h>
 #include <google/protobuf/pyext/descriptor.h>
+#include <google/protobuf/pyext/message.h>
 #include <google/protobuf/pyext/scoped_pyobject_ptr.h>
 
-#define C(str) const_cast<char*>(str)
-
 #if PY_MAJOR_VERSION >= 3
   #define PyString_FromStringAndSize PyUnicode_FromStringAndSize
   #if PY_VERSION_HEX < 0x03030000
@@ -108,11 +107,11 @@ PyObject* FindMessageByName(PyDescriptorPool* self, PyObject* arg) {
       self->pool->FindMessageTypeByName(string(name, name_size));
 
   if (message_descriptor == NULL) {
-    PyErr_Format(PyExc_TypeError, "Couldn't find message %.200s", name);
+    PyErr_Format(PyExc_KeyError, "Couldn't find message %.200s", name);
     return NULL;
   }
 
-  return PyMessageDescriptor_New(message_descriptor);
+  return PyMessageDescriptor_FromDescriptor(message_descriptor);
 }
 
 // Add a message class to our database.
@@ -158,6 +157,24 @@ PyObject *GetMessageClass(PyDescriptorPool* self,
   }
 }
 
+PyObject* FindFileByName(PyDescriptorPool* self, PyObject* arg) {
+  Py_ssize_t name_size;
+  char* name;
+  if (PyString_AsStringAndSize(arg, &name, &name_size) < 0) {
+    return NULL;
+  }
+
+  const FileDescriptor* file_descriptor =
+      self->pool->FindFileByName(string(name, name_size));
+  if (file_descriptor == NULL) {
+    PyErr_Format(PyExc_KeyError, "Couldn't find file %.200s",
+                 name);
+    return NULL;
+  }
+
+  return PyFileDescriptor_FromDescriptor(file_descriptor);
+}
+
 PyObject* FindFieldByName(PyDescriptorPool* self, PyObject* arg) {
   Py_ssize_t name_size;
   char* name;
@@ -168,12 +185,12 @@ PyObject* FindFieldByName(PyDescriptorPool* self, PyObject* arg) {
   const FieldDescriptor* field_descriptor =
       self->pool->FindFieldByName(string(name, name_size));
   if (field_descriptor == NULL) {
-    PyErr_Format(PyExc_TypeError, "Couldn't find field %.200s",
+    PyErr_Format(PyExc_KeyError, "Couldn't find field %.200s",
                  name);
     return NULL;
   }
 
-  return PyFieldDescriptor_New(field_descriptor);
+  return PyFieldDescriptor_FromDescriptor(field_descriptor);
 }
 
 PyObject* FindExtensionByName(PyDescriptorPool* self, PyObject* arg) {
@@ -186,11 +203,11 @@ PyObject* FindExtensionByName(PyDescriptorPool* self, PyObject* arg) {
   const FieldDescriptor* field_descriptor =
       self->pool->FindExtensionByName(string(name, name_size));
   if (field_descriptor == NULL) {
-    PyErr_Format(PyExc_TypeError, "Couldn't find field %.200s", name);
+    PyErr_Format(PyExc_KeyError, "Couldn't find extension field %.200s", name);
     return NULL;
   }
 
-  return PyFieldDescriptor_New(field_descriptor);
+  return PyFieldDescriptor_FromDescriptor(field_descriptor);
 }
 
 PyObject* FindEnumTypeByName(PyDescriptorPool* self, PyObject* arg) {
@@ -203,11 +220,11 @@ PyObject* FindEnumTypeByName(PyDescriptorPool* self, PyObject* arg) {
   const EnumDescriptor* enum_descriptor =
       self->pool->FindEnumTypeByName(string(name, name_size));
   if (enum_descriptor == NULL) {
-    PyErr_Format(PyExc_TypeError, "Couldn't find enum %.200s", name);
+    PyErr_Format(PyExc_KeyError, "Couldn't find enum %.200s", name);
     return NULL;
   }
 
-  return PyEnumDescriptor_New(enum_descriptor);
+  return PyEnumDescriptor_FromDescriptor(enum_descriptor);
 }
 
 PyObject* FindOneofByName(PyDescriptorPool* self, PyObject* arg) {
@@ -220,70 +237,13 @@ PyObject* FindOneofByName(PyDescriptorPool* self, PyObject* arg) {
   const OneofDescriptor* oneof_descriptor =
       self->pool->FindOneofByName(string(name, name_size));
   if (oneof_descriptor == NULL) {
-    PyErr_Format(PyExc_TypeError, "Couldn't find oneof %.200s", name);
+    PyErr_Format(PyExc_KeyError, "Couldn't find oneof %.200s", name);
     return NULL;
   }
 
-  return PyOneofDescriptor_New(oneof_descriptor);
+  return PyOneofDescriptor_FromDescriptor(oneof_descriptor);
 }
 
-static PyMethodDef Methods[] = {
-  { C("FindFieldByName"),
-    (PyCFunction)FindFieldByName,
-    METH_O,
-    C("Searches for a field descriptor by full name.") },
-  { C("FindExtensionByName"),
-    (PyCFunction)FindExtensionByName,
-    METH_O,
-    C("Searches for extension descriptor by full name.") },
-  {NULL}
-};
-
-}  // namespace cdescriptor_pool
-
-PyTypeObject PyDescriptorPool_Type = {
-  PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  C("google.protobuf.internal."
-    "_message.DescriptorPool"),        // tp_name
-  sizeof(PyDescriptorPool),            // tp_basicsize
-  0,                                   // tp_itemsize
-  (destructor)cdescriptor_pool::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
-  C("A Descriptor Pool"),              // tp_doc
-  0,                                   // tp_traverse
-  0,                                   // tp_clear
-  0,                                   // tp_richcompare
-  0,                                   // tp_weaklistoffset
-  0,                                   // tp_iter
-  0,                                   // tp_iternext
-  cdescriptor_pool::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
-  0,                                   // tp_alloc
-  0,                                   // tp_new
-  PyObject_Del,                        // tp_free
-};
-
 // The code below loads new Descriptors from a serialized FileDescriptorProto.
 
 
@@ -301,6 +261,7 @@ class BuildFileErrorCollector : public DescriptorPool::ErrorCollector {
     if (!had_errors) {
       error_message +=
           ("Invalid proto descriptor for file \"" + filename + "\":\n");
+      had_errors = true;
     }
     // As this only happens on failure and will result in the program not
     // running at all, no effort is made to optimize this string manipulation.
@@ -311,7 +272,7 @@ class BuildFileErrorCollector : public DescriptorPool::ErrorCollector {
   bool had_errors;
 };
 
-PyObject* Python_BuildFile(PyObject* ignored, PyObject* serialized_pb) {
+PyObject* AddSerializedFile(PyDescriptorPool* self, PyObject* serialized_pb) {
   char* message_type;
   Py_ssize_t message_len;
 
@@ -330,13 +291,14 @@ PyObject* Python_BuildFile(PyObject* ignored, PyObject* serialized_pb) {
   const FileDescriptor* generated_file =
       DescriptorPool::generated_pool()->FindFileByName(file_proto.name());
   if (generated_file != NULL) {
-    return PyFileDescriptor_NewWithPb(generated_file, serialized_pb);
+    return PyFileDescriptor_FromDescriptorWithSerializedPb(
+        generated_file, serialized_pb);
   }
 
   BuildFileErrorCollector error_collector;
   const FileDescriptor* descriptor =
-      GetDescriptorPool()->pool->BuildFileCollectingErrors(file_proto,
-                                                           &error_collector);
+      self->pool->BuildFileCollectingErrors(file_proto,
+                                            &error_collector);
   if (descriptor == NULL) {
     PyErr_Format(PyExc_TypeError,
                  "Couldn't build proto file into descriptor pool!\n%s",
@@ -344,9 +306,84 @@ PyObject* Python_BuildFile(PyObject* ignored, PyObject* serialized_pb) {
     return NULL;
   }
 
-  return PyFileDescriptor_NewWithPb(descriptor, serialized_pb);
+  return PyFileDescriptor_FromDescriptorWithSerializedPb(
+      descriptor, serialized_pb);
+}
+
+PyObject* Add(PyDescriptorPool* self, PyObject* file_descriptor_proto) {
+  ScopedPyObjectPtr serialized_pb(
+      PyObject_CallMethod(file_descriptor_proto, "SerializeToString", NULL));
+  if (serialized_pb == NULL) {
+    return NULL;
+  }
+  return AddSerializedFile(self, serialized_pb);
 }
 
+static PyMethodDef Methods[] = {
+  { "Add", (PyCFunction)Add, METH_O,
+    "Adds the FileDescriptorProto and its types to this pool." },
+  { "AddSerializedFile", (PyCFunction)AddSerializedFile, METH_O,
+    "Adds a serialized FileDescriptorProto to this pool." },
+
+  { "FindFileByName", (PyCFunction)FindFileByName, METH_O,
+    "Searches for a file descriptor by its .proto name." },
+  { "FindMessageTypeByName", (PyCFunction)FindMessageByName, METH_O,
+    "Searches for a message descriptor by full name." },
+  { "FindFieldByName", (PyCFunction)FindFieldByName, METH_O,
+    "Searches for a field descriptor by full name." },
+  { "FindExtensionByName", (PyCFunction)FindExtensionByName, METH_O,
+    "Searches for extension descriptor by full name." },
+  { "FindEnumTypeByName", (PyCFunction)FindEnumTypeByName, METH_O,
+    "Searches for enum type descriptor by full name." },
+  { "FindOneofByName", (PyCFunction)FindOneofByName, METH_O,
+    "Searches for oneof descriptor by full name." },
+  {NULL}
+};
+
+}  // namespace cdescriptor_pool
+
+PyTypeObject PyDescriptorPool_Type = {
+  PyVarObject_HEAD_INIT(&PyType_Type, 0)
+  FULL_MODULE_NAME ".DescriptorPool",  // tp_name
+  sizeof(PyDescriptorPool),            // tp_basicsize
+  0,                                   // tp_itemsize
+  (destructor)cdescriptor_pool::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 Descriptor Pool",                 // tp_doc
+  0,                                   // tp_traverse
+  0,                                   // tp_clear
+  0,                                   // tp_richcompare
+  0,                                   // tp_weaklistoffset
+  0,                                   // tp_iter
+  0,                                   // tp_iternext
+  cdescriptor_pool::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
+  0,                                   // tp_alloc
+  0,                                   // tp_new
+  PyObject_Del,                        // tp_free
+};
+
 static PyDescriptorPool* global_cdescriptor_pool = NULL;
 
 bool InitDescriptorPool() {
diff --git a/python/google/protobuf/pyext/descriptor_pool.h b/python/google/protobuf/pyext/descriptor_pool.h
index 4e494b89..efb1abeb 100644
--- a/python/google/protobuf/pyext/descriptor_pool.h
+++ b/python/google/protobuf/pyext/descriptor_pool.h
@@ -95,6 +95,8 @@ const Descriptor* FindMessageTypeByName(PyDescriptorPool* self,
 const Descriptor* RegisterMessageClass(
     PyDescriptorPool* self, PyObject* message_class, PyObject* descriptor);
 
+// The function below are also exposed as methods of the DescriptorPool type.
+
 // Retrieves the Python class registered with the given message descriptor.
 //
 // Returns a *borrowed* reference if found, otherwise returns NULL with an
@@ -134,12 +136,8 @@ PyObject* FindOneofByName(PyDescriptorPool* self, PyObject* arg);
 
 }  // namespace cdescriptor_pool
 
-// Implement the Python "_BuildFile" method, it takes a serialized
-// FileDescriptorProto, and adds it to the C++ DescriptorPool.
-// It returns a new FileDescriptor object, or NULL when an exception is raised.
-PyObject* Python_BuildFile(PyObject* ignored, PyObject* args);
-
 // Retrieve the global descriptor pool owned by the _message module.
+// Returns a *borrowed* reference.
 PyDescriptorPool* GetDescriptorPool();
 
 // Initialize objects used by this module.
diff --git a/python/google/protobuf/pyext/extension_dict.cc b/python/google/protobuf/pyext/extension_dict.cc
index 8e38fc42..b8d18f8d 100644
--- a/python/google/protobuf/pyext/extension_dict.cc
+++ b/python/google/protobuf/pyext/extension_dict.cc
@@ -51,16 +51,6 @@ namespace python {
 
 namespace extension_dict {
 
-// TODO(tibell): Always use self->message for clarity, just like in
-// RepeatedCompositeContainer.
-static Message* GetMessage(ExtensionDict* self) {
-  if (self->parent != NULL) {
-    return self->parent->message;
-  } else {
-    return self->message;
-  }
-}
-
 PyObject* len(ExtensionDict* self) {
 #if PY_MAJOR_VERSION >= 3
   return PyLong_FromLong(PyDict_Size(self->values));
@@ -89,7 +79,7 @@ int ReleaseExtension(ExtensionDict* self,
     }
   } else if (descriptor->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
     if (cmessage::ReleaseSubMessage(
-            GetMessage(self), descriptor,
+            self->parent, descriptor,
             reinterpret_cast<CMessage*>(extension)) < 0) {
       return -1;
     }
@@ -109,7 +99,7 @@ PyObject* subscript(ExtensionDict* self, PyObject* key) {
 
   if (descriptor->label() != FieldDescriptor::LABEL_REPEATED &&
       descriptor->cpp_type() != FieldDescriptor::CPPTYPE_MESSAGE) {
-    return cmessage::InternalGetScalar(self->parent, descriptor);
+    return cmessage::InternalGetScalar(self->parent->message, descriptor);
   }
 
   PyObject* value = PyDict_GetItem(self->values, key);
@@ -266,8 +256,7 @@ static PyMethodDef Methods[] = {
 
 PyTypeObject ExtensionDict_Type = {
   PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  "google.protobuf.internal."
-  "cpp._message.ExtensionDict",        // tp_name
+  FULL_MODULE_NAME ".ExtensionDict",   // tp_name
   sizeof(ExtensionDict),               // tp_basicsize
   0,                                   //  tp_itemsize
   (destructor)extension_dict::dealloc,  //  tp_dealloc
diff --git a/python/google/protobuf/pyext/message.cc b/python/google/protobuf/pyext/message.cc
index a2b357b2..a4843e8d 100644
--- a/python/google/protobuf/pyext/message.cc
+++ b/python/google/protobuf/pyext/message.cc
@@ -59,6 +59,8 @@
 #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/scoped_pyobject_ptr.h>
 #include <google/protobuf/stubs/strutil.h>
 
@@ -93,9 +95,9 @@ static const FieldDescriptor* GetFieldDescriptor(
 static const Descriptor* GetMessageDescriptor(PyTypeObject* cls);
 static string GetMessageName(CMessage* self);
 int InternalReleaseFieldByDescriptor(
+    CMessage* self,
     const FieldDescriptor* field_descriptor,
-    PyObject* composite_field,
-    Message* parent_message);
+    PyObject* composite_field);
 }  // namespace cmessage
 
 // ---------------------------------------------------------------------
@@ -127,10 +129,29 @@ static int VisitCompositeField(const FieldDescriptor* descriptor,
                                Visitor visitor) {
   if (descriptor->label() == FieldDescriptor::LABEL_REPEATED) {
     if (descriptor->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
-      RepeatedCompositeContainer* container =
-        reinterpret_cast<RepeatedCompositeContainer*>(child);
-      if (visitor.VisitRepeatedCompositeContainer(container) == -1)
-        return -1;
+      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;
+          }
+        }
+      } else {
+        RepeatedCompositeContainer* container =
+          reinterpret_cast<RepeatedCompositeContainer*>(child);
+        if (visitor.VisitRepeatedCompositeContainer(container) == -1)
+          return -1;
+      }
     } else {
       RepeatedScalarContainer* container =
         reinterpret_cast<RepeatedScalarContainer*>(child);
@@ -444,7 +465,7 @@ static int MaybeReleaseOverlappingOneofField(
   }
 
   if (InternalReleaseFieldByDescriptor(
-          existing_field, child_message, message) < 0) {
+          cmessage, existing_field, child_message) < 0) {
     return -1;
   }
   return PyDict_DelItemString(cmessage->composite_fields, field_name);
@@ -483,6 +504,16 @@ struct FixupMessageReference : public ChildVisitor {
     return 0;
   }
 
+  int VisitScalarMapContainer(ScalarMapContainer* container) {
+    container->message = message_;
+    return 0;
+  }
+
+  int VisitMessageMapContainer(MessageMapContainer* container) {
+    container->message = message_;
+    return 0;
+  }
+
  private:
   Message* message_;
 };
@@ -500,6 +531,9 @@ int AssureWritable(CMessage* self) {
         self->message->GetDescriptor());
     self->message = prototype->New();
     self->owner.reset(self->message);
+    // Cascade the new owner to eventual children: even if this message is
+    // empty, some submessages or repeated containers might exist already.
+    SetOwner(self, self->owner);
   } else {
     // Otherwise, we need a mutable child message.
     if (AssureWritable(self->parent) == -1)
@@ -520,8 +554,9 @@ 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
-  // three places such references occur: RepeatedScalarContainer,
-  // RepeatedCompositeContainer, and ExtensionDict.
+  // five places such references occur: RepeatedScalarContainer,
+  // RepeatedCompositeContainer, ScalarMapContainer, MessageMapContainer,
+  // and ExtensionDict.
   if (self->extensions != NULL)
     self->extensions->message = self->message;
   if (ForEachCompositeField(self, FixupMessageReference(self->message)) == -1)
@@ -583,15 +618,43 @@ const FieldDescriptor* GetExtensionDescriptor(PyObject* extension) {
   return PyFieldDescriptor_AsDescriptor(extension);
 }
 
+// If value is a string, convert it into an enum value based on the labels in
+// descriptor, otherwise simply return value.  Always returns a new reference.
+static PyObject* GetIntegerEnumValue(const FieldDescriptor& descriptor,
+                                     PyObject* value) {
+  if (PyString_Check(value) || PyUnicode_Check(value)) {
+    const EnumDescriptor* enum_descriptor = descriptor.enum_type();
+    if (enum_descriptor == NULL) {
+      PyErr_SetString(PyExc_TypeError, "not an enum field");
+      return NULL;
+    }
+    char* enum_label;
+    Py_ssize_t size;
+    if (PyString_AsStringAndSize(value, &enum_label, &size) < 0) {
+      return NULL;
+    }
+    const EnumValueDescriptor* enum_value_descriptor =
+        enum_descriptor->FindValueByName(string(enum_label, size));
+    if (enum_value_descriptor == NULL) {
+      PyErr_SetString(PyExc_ValueError, "unknown enum label");
+      return NULL;
+    }
+    return PyInt_FromLong(enum_value_descriptor->number());
+  }
+  Py_INCREF(value);
+  return value;
+}
+
 // If cmessage_list is not NULL, this function releases values into the
 // container CMessages instead of just removing. Repeated composite container
 // needs to do this to make sure CMessages stay alive if they're still
 // referenced after deletion. Repeated scalar container doesn't need to worry.
 int InternalDeleteRepeatedField(
-    Message* message,
+    CMessage* self,
     const FieldDescriptor* field_descriptor,
     PyObject* slice,
     PyObject* cmessage_list) {
+  Message* message = self->message;
   Py_ssize_t length, from, to, step, slice_length;
   const Reflection* reflection = message->GetReflection();
   int min, max;
@@ -665,7 +728,7 @@ int InternalDeleteRepeatedField(
       CMessage* last_cmessage = reinterpret_cast<CMessage*>(
           PyList_GET_ITEM(cmessage_list, PyList_GET_SIZE(cmessage_list) - 1));
       repeated_composite_container::ReleaseLastTo(
-          field_descriptor, message, last_cmessage);
+          self, field_descriptor, last_cmessage);
       if (PySequence_DelItem(cmessage_list, -1) < 0) {
         return -1;
       }
@@ -696,16 +759,90 @@ int InitAttributes(CMessage* self, PyObject* kwargs) {
                    PyString_AsString(name));
       return -1;
     }
-    if (descriptor->label() == FieldDescriptor::LABEL_REPEATED) {
+    if (descriptor->is_map()) {
+      ScopedPyObjectPtr map(GetAttr(self, name));
+      const FieldDescriptor* value_descriptor =
+          descriptor->message_type()->FindFieldByName("value");
+      if (value_descriptor->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
+        Py_ssize_t map_pos = 0;
+        PyObject* map_key;
+        PyObject* map_value;
+        while (PyDict_Next(value, &map_pos, &map_key, &map_value)) {
+          ScopedPyObjectPtr function_return;
+          function_return.reset(PyObject_GetItem(map.get(), map_key));
+          if (function_return.get() == NULL) {
+            return -1;
+          }
+          ScopedPyObjectPtr ok(PyObject_CallMethod(
+              function_return.get(), "MergeFrom", "O", map_value));
+          if (ok.get() == NULL) {
+            return -1;
+          }
+        }
+      } else {
+        ScopedPyObjectPtr function_return;
+        function_return.reset(
+            PyObject_CallMethod(map.get(), "update", "O", value));
+        if (function_return.get() == NULL) {
+          return -1;
+        }
+      }
+    } else if (descriptor->label() == FieldDescriptor::LABEL_REPEATED) {
       ScopedPyObjectPtr container(GetAttr(self, name));
       if (container == NULL) {
         return -1;
       }
       if (descriptor->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
-        if (repeated_composite_container::Extend(
-                reinterpret_cast<RepeatedCompositeContainer*>(container.get()),
-                value)
-            == NULL) {
+        RepeatedCompositeContainer* rc_container =
+            reinterpret_cast<RepeatedCompositeContainer*>(container.get());
+        ScopedPyObjectPtr iter(PyObject_GetIter(value));
+        if (iter == NULL) {
+          PyErr_SetString(PyExc_TypeError, "Value must be iterable");
+          return -1;
+        }
+        ScopedPyObjectPtr next;
+        while ((next.reset(PyIter_Next(iter))) != NULL) {
+          PyObject* kwargs = (PyDict_Check(next) ? next.get() : NULL);
+          ScopedPyObjectPtr new_msg(
+              repeated_composite_container::Add(rc_container, NULL, kwargs));
+          if (new_msg == NULL) {
+            return -1;
+          }
+          if (kwargs == NULL) {
+            // next was not a dict, it's a message we need to merge
+            ScopedPyObjectPtr merged(
+                MergeFrom(reinterpret_cast<CMessage*>(new_msg.get()), next));
+            if (merged == NULL) {
+              return -1;
+            }
+          }
+        }
+        if (PyErr_Occurred()) {
+          // Check to see how PyIter_Next() exited.
+          return -1;
+        }
+      } else if (descriptor->cpp_type() == FieldDescriptor::CPPTYPE_ENUM) {
+        RepeatedScalarContainer* rs_container =
+            reinterpret_cast<RepeatedScalarContainer*>(container.get());
+        ScopedPyObjectPtr iter(PyObject_GetIter(value));
+        if (iter == NULL) {
+          PyErr_SetString(PyExc_TypeError, "Value must be iterable");
+          return -1;
+        }
+        ScopedPyObjectPtr next;
+        while ((next.reset(PyIter_Next(iter))) != NULL) {
+          ScopedPyObjectPtr enum_value(GetIntegerEnumValue(*descriptor, next));
+          if (enum_value == NULL) {
+            return -1;
+          }
+          ScopedPyObjectPtr new_msg(
+              repeated_scalar_container::Append(rs_container, enum_value));
+          if (new_msg == NULL) {
+            return -1;
+          }
+        }
+        if (PyErr_Occurred()) {
+          // Check to see how PyIter_Next() exited.
           return -1;
         }
       } else {
@@ -721,12 +858,26 @@ int InitAttributes(CMessage* self, PyObject* kwargs) {
       if (message == NULL) {
         return -1;
       }
-      if (MergeFrom(reinterpret_cast<CMessage*>(message.get()),
-                             value) == NULL) {
-        return -1;
+      CMessage* cmessage = reinterpret_cast<CMessage*>(message.get());
+      if (PyDict_Check(value)) {
+        if (InitAttributes(cmessage, value) < 0) {
+          return -1;
+        }
+      } else {
+        ScopedPyObjectPtr merged(MergeFrom(cmessage, value));
+        if (merged == NULL) {
+          return -1;
+        }
       }
     } else {
-      if (SetAttr(self, name, value) < 0) {
+      ScopedPyObjectPtr new_val;
+      if (descriptor->cpp_type() == FieldDescriptor::CPPTYPE_ENUM) {
+        new_val.reset(GetIntegerEnumValue(*descriptor, value));
+        if (new_val == NULL) {
+          return -1;
+        }
+      }
+      if (SetAttr(self, name, (new_val == NULL) ? value : new_val) < 0) {
         return -1;
       }
     }
@@ -789,7 +940,6 @@ static PyObject* New(PyTypeObject* type,
   }
   self->message = default_message->New();
   self->owner.reset(self->message);
-
   return reinterpret_cast<PyObject*>(self);
 }
 
@@ -830,6 +980,16 @@ struct ClearWeakReferences : public ChildVisitor {
     return 0;
   }
 
+  int VisitScalarMapContainer(ScalarMapContainer* container) {
+    container->parent = NULL;
+    return 0;
+  }
+
+  int VisitMessageMapContainer(MessageMapContainer* container) {
+    container->parent = NULL;
+    return 0;
+  }
+
   int VisitCMessage(CMessage* cmessage,
                     const FieldDescriptor* field_descriptor) {
     cmessage->parent = NULL;
@@ -1064,6 +1224,16 @@ 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_);
+    return 0;
+  }
+
   int VisitCMessage(CMessage* cmessage,
                     const FieldDescriptor* field_descriptor) {
     return SetOwner(cmessage, new_owner_);
@@ -1084,11 +1254,11 @@ int SetOwner(CMessage* self, const shared_ptr<Message>& new_owner) {
 // Releases the message specified by 'field' and returns the
 // pointer. If the field does not exist a new message is created using
 // 'descriptor'. The caller takes ownership of the returned pointer.
-Message* ReleaseMessage(Message* message,
+Message* ReleaseMessage(CMessage* self,
                         const Descriptor* descriptor,
                         const FieldDescriptor* field_descriptor) {
-  Message* released_message = message->GetReflection()->ReleaseMessage(
-      message, field_descriptor, message_factory);
+  Message* released_message = self->message->GetReflection()->ReleaseMessage(
+      self->message, field_descriptor, message_factory);
   // ReleaseMessage will return NULL which differs from
   // child_cmessage->message, if the field does not exist.  In this case,
   // the latter points to the default instance via a const_cast<>, so we
@@ -1102,12 +1272,12 @@ Message* ReleaseMessage(Message* message,
   return released_message;
 }
 
-int ReleaseSubMessage(Message* message,
+int ReleaseSubMessage(CMessage* self,
                       const FieldDescriptor* field_descriptor,
                       CMessage* child_cmessage) {
   // Release the Message
   shared_ptr<Message> released_message(ReleaseMessage(
-      message, child_cmessage->message->GetDescriptor(), field_descriptor));
+      self, child_cmessage->message->GetDescriptor(), field_descriptor));
   child_cmessage->message = released_message.get();
   child_cmessage->owner.swap(released_message);
   child_cmessage->parent = NULL;
@@ -1119,8 +1289,8 @@ int ReleaseSubMessage(Message* message,
 
 struct ReleaseChild : public ChildVisitor {
   // message must outlive this object.
-  explicit ReleaseChild(Message* parent_message) :
-      parent_message_(parent_message) {}
+  explicit ReleaseChild(CMessage* parent) :
+      parent_(parent) {}
 
   int VisitRepeatedCompositeContainer(RepeatedCompositeContainer* container) {
     return repeated_composite_container::Release(
@@ -1132,23 +1302,33 @@ 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 VisitCMessage(CMessage* cmessage,
                     const FieldDescriptor* field_descriptor) {
-    return ReleaseSubMessage(parent_message_, field_descriptor,
+    return ReleaseSubMessage(parent_, field_descriptor,
         reinterpret_cast<CMessage*>(cmessage));
   }
 
-  Message* parent_message_;
+  CMessage* parent_;
 };
 
 int InternalReleaseFieldByDescriptor(
+    CMessage* self,
     const FieldDescriptor* field_descriptor,
-    PyObject* composite_field,
-    Message* parent_message) {
+    PyObject* composite_field) {
   return VisitCompositeField(
       field_descriptor,
       composite_field,
-      ReleaseChild(parent_message));
+      ReleaseChild(self));
 }
 
 PyObject* ClearFieldByDescriptor(
@@ -1200,8 +1380,8 @@ PyObject* ClearField(CMessage* self, PyObject* arg) {
   // Only release the field if there's a possibility that there are
   // references to it.
   if (composite_field != NULL) {
-    if (InternalReleaseFieldByDescriptor(field_descriptor,
-                                         composite_field, message) < 0) {
+    if (InternalReleaseFieldByDescriptor(self, field_descriptor,
+                                         composite_field) < 0) {
       return NULL;
     }
     PyDict_DelItem(self->composite_fields, arg);
@@ -1219,7 +1399,7 @@ PyObject* ClearField(CMessage* self, PyObject* arg) {
 
 PyObject* Clear(CMessage* self) {
   AssureWritable(self);
-  if (ForEachCompositeField(self, ReleaseChild(self->message)) == -1)
+  if (ForEachCompositeField(self, ReleaseChild(self)) == -1)
     return NULL;
 
   // The old ExtensionDict still aliases this CMessage, but all its
@@ -1582,7 +1762,8 @@ static PyObject* ListFields(CMessage* self) {
     }
 
     if (fields[i]->is_extension()) {
-      ScopedPyObjectPtr extension_field(PyFieldDescriptor_New(fields[i]));
+      ScopedPyObjectPtr extension_field(
+          PyFieldDescriptor_FromDescriptor(fields[i]));
       if (extension_field == NULL) {
         return NULL;
       }
@@ -1616,7 +1797,8 @@ static PyObject* ListFields(CMessage* self) {
         PyErr_SetString(PyExc_ValueError, "bad string");
         return NULL;
       }
-      ScopedPyObjectPtr field_descriptor(PyFieldDescriptor_New(fields[i]));
+      ScopedPyObjectPtr field_descriptor(
+          PyFieldDescriptor_FromDescriptor(fields[i]));
       if (field_descriptor == NULL) {
         return NULL;
       }
@@ -1683,10 +1865,8 @@ static PyObject* RichCompare(CMessage* self, PyObject* other, int opid) {
   }
 }
 
-PyObject* InternalGetScalar(
-    CMessage* self,
-    const FieldDescriptor* field_descriptor) {
-  Message* message = self->message;
+PyObject* InternalGetScalar(const Message* message,
+                            const FieldDescriptor* field_descriptor) {
   const Reflection* reflection = message->GetReflection();
 
   if (!CheckFieldBelongsToMessage(field_descriptor, message)) {
@@ -1739,12 +1919,12 @@ PyObject* InternalGetScalar(
       if (!message->GetReflection()->SupportsUnknownEnumValues() &&
           !message->GetReflection()->HasField(*message, field_descriptor)) {
         // Look for the value in the unknown fields.
-        UnknownFieldSet* unknown_field_set =
-            message->GetReflection()->MutableUnknownFields(message);
-        for (int i = 0; i < unknown_field_set->field_count(); ++i) {
-          if (unknown_field_set->field(i).number() ==
+        const UnknownFieldSet& unknown_field_set =
+            message->GetReflection()->GetUnknownFields(*message);
+        for (int i = 0; i < unknown_field_set.field_count(); ++i) {
+          if (unknown_field_set.field(i).number() ==
               field_descriptor->number()) {
-            result = PyInt_FromLong(unknown_field_set->field(i).varint());
+            result = PyInt_FromLong(unknown_field_set.field(i).varint());
             break;
           }
         }
@@ -1793,21 +1973,16 @@ PyObject* InternalGetSubMessage(
   return reinterpret_cast<PyObject*>(cmsg);
 }
 
-int InternalSetScalar(
-    CMessage* self,
+int InternalSetNonOneofScalar(
+    Message* message,
     const FieldDescriptor* field_descriptor,
     PyObject* arg) {
-  Message* message = self->message;
   const Reflection* reflection = message->GetReflection();
 
   if (!CheckFieldBelongsToMessage(field_descriptor, message)) {
     return -1;
   }
 
-  if (MaybeReleaseOverlappingOneofField(self, field_descriptor) < 0) {
-    return -1;
-  }
-
   switch (field_descriptor->cpp_type()) {
     case FieldDescriptor::CPPTYPE_INT32: {
       GOOGLE_CHECK_GET_INT32(arg, value, -1);
@@ -1878,6 +2053,21 @@ int InternalSetScalar(
   return 0;
 }
 
+int InternalSetScalar(
+    CMessage* self,
+    const FieldDescriptor* field_descriptor,
+    PyObject* arg) {
+  if (!CheckFieldBelongsToMessage(field_descriptor, self->message)) {
+    return -1;
+  }
+
+  if (MaybeReleaseOverlappingOneofField(self, field_descriptor) < 0) {
+    return -1;
+  }
+
+  return InternalSetNonOneofScalar(self->message, field_descriptor, arg);
+}
+
 PyObject* FromString(PyTypeObject* cls, PyObject* serialized) {
   PyObject* py_cmsg = PyObject_CallObject(
       reinterpret_cast<PyObject*>(cls), NULL);
@@ -1955,7 +2145,8 @@ static PyObject* AddDescriptors(PyObject* cls, PyObject* descriptor) {
   // which was built previously.
   for (int i = 0; i < message_descriptor->enum_type_count(); ++i) {
     const EnumDescriptor* enum_descriptor = message_descriptor->enum_type(i);
-    ScopedPyObjectPtr enum_type(PyEnumDescriptor_New(enum_descriptor));
+    ScopedPyObjectPtr enum_type(
+        PyEnumDescriptor_FromDescriptor(enum_descriptor));
     if (enum_type == NULL) {
       return NULL;
      }
@@ -1993,7 +2184,7 @@ static PyObject* AddDescriptors(PyObject* cls, PyObject* descriptor) {
   // which was defined previously.
   for (int i = 0; i < message_descriptor->extension_count(); ++i) {
     const google::protobuf::FieldDescriptor* field = message_descriptor->extension(i);
-    ScopedPyObjectPtr extension_field(PyFieldDescriptor_New(field));
+    ScopedPyObjectPtr extension_field(PyFieldDescriptor_FromDescriptor(field));
     if (extension_field == NULL) {
       return NULL;
     }
@@ -2097,26 +2288,6 @@ PyObject* SetState(CMessage* self, PyObject* state) {
 }
 
 // CMessage static methods:
-PyObject* _GetMessageDescriptor(PyObject* unused, PyObject* arg) {
-  return cdescriptor_pool::FindMessageByName(GetDescriptorPool(), arg);
-}
-
-PyObject* _GetFieldDescriptor(PyObject* unused, PyObject* arg) {
-  return cdescriptor_pool::FindFieldByName(GetDescriptorPool(), arg);
-}
-
-PyObject* _GetExtensionDescriptor(PyObject* unused, PyObject* arg) {
-  return cdescriptor_pool::FindExtensionByName(GetDescriptorPool(), arg);
-}
-
-PyObject* _GetEnumDescriptor(PyObject* unused, PyObject* arg) {
-  return cdescriptor_pool::FindEnumTypeByName(GetDescriptorPool(), arg);
-}
-
-PyObject* _GetOneofDescriptor(PyObject* unused, PyObject* arg) {
-  return cdescriptor_pool::FindOneofByName(GetDescriptorPool(), arg);
-}
-
 PyObject* _CheckCalledFromGeneratedFile(PyObject* unused,
                                         PyObject* unused_arg) {
   if (!_CalledFromGeneratedFile(1)) {
@@ -2188,21 +2359,6 @@ static PyMethodDef Methods[] = {
     "or None if no field is set." },
 
   // Static Methods.
-  { "_BuildFile", (PyCFunction)Python_BuildFile, METH_O | METH_STATIC,
-    "Registers a new protocol buffer file in the global C++ descriptor pool." },
-  { "_GetMessageDescriptor", (PyCFunction)_GetMessageDescriptor,
-    METH_O | METH_STATIC, "Finds a message descriptor in the message pool." },
-  { "_GetFieldDescriptor", (PyCFunction)_GetFieldDescriptor,
-    METH_O | METH_STATIC, "Finds a field descriptor in the message pool." },
-  { "_GetExtensionDescriptor", (PyCFunction)_GetExtensionDescriptor,
-    METH_O | METH_STATIC,
-    "Finds a extension descriptor in the message pool." },
-  { "_GetEnumDescriptor", (PyCFunction)_GetEnumDescriptor,
-    METH_O | METH_STATIC,
-    "Finds an enum descriptor in the message pool." },
-  { "_GetOneofDescriptor", (PyCFunction)_GetOneofDescriptor,
-    METH_O | METH_STATIC,
-    "Finds an oneof descriptor in the message pool." },
   { "_CheckCalledFromGeneratedFile", (PyCFunction)_CheckCalledFromGeneratedFile,
     METH_NOARGS | METH_STATIC,
     "Raises TypeError if the caller is not in a _pb2.py file."},
@@ -2234,6 +2390,31 @@ PyObject* GetAttr(CMessage* self, PyObject* name) {
         reinterpret_cast<PyObject*>(self), name);
   }
 
+  if (field_descriptor->is_map()) {
+    PyObject* py_container = NULL;
+    const Descriptor* entry_type = field_descriptor->message_type();
+    const FieldDescriptor* value_type = entry_type->FindFieldByName("value");
+    if (value_type->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
+      PyObject* value_class = cdescriptor_pool::GetMessageClass(
+          GetDescriptorPool(), value_type->message_type());
+      if (value_class == NULL) {
+        return NULL;
+      }
+      py_container = message_map_container::NewContainer(self, field_descriptor,
+                                                         value_class);
+    } else {
+      py_container = scalar_map_container::NewContainer(self, field_descriptor);
+    }
+    if (py_container == NULL) {
+      return NULL;
+    }
+    if (!SetCompositeField(self, name, py_container)) {
+      Py_DECREF(py_container);
+      return NULL;
+    }
+    return py_container;
+  }
+
   if (field_descriptor->label() == FieldDescriptor::LABEL_REPEATED) {
     PyObject* py_container = NULL;
     if (field_descriptor->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
@@ -2267,7 +2448,7 @@ PyObject* GetAttr(CMessage* self, PyObject* name) {
     return sub_message;
   }
 
-  return InternalGetScalar(self, field_descriptor);
+  return InternalGetScalar(self->message, field_descriptor);
 }
 
 int SetAttr(CMessage* self, PyObject* name, PyObject* value) {
@@ -2304,9 +2485,7 @@ int SetAttr(CMessage* self, PyObject* name, PyObject* value) {
 
 PyTypeObject CMessage_Type = {
   PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  // Keep the fully qualified _message symbol in a line for opensource.
-  "google.protobuf.pyext._message."
-  "CMessage",                          // tp_name
+  FULL_MODULE_NAME ".CMessage",        // tp_name
   sizeof(CMessage),                    // tp_basicsize
   0,                                   //  tp_itemsize
   (destructor)cmessage::Dealloc,       //  tp_dealloc
@@ -2401,7 +2580,7 @@ void InitGlobals() {
   k_extensions_by_name = PyString_FromString("_extensions_by_name");
   k_extensions_by_number = PyString_FromString("_extensions_by_number");
 
-  message_factory = new DynamicMessageFactory(GetDescriptorPool()->pool);
+  message_factory = new DynamicMessageFactory();
   message_factory->SetDelegateToGeneratedFactory(true);
 }
 
@@ -2469,6 +2648,61 @@ bool InitProto2MessageModule(PyObject *m) {
       reinterpret_cast<PyObject*>(
           &RepeatedCompositeContainer_Type));
 
+  // ScalarMapContainer_Type derives from our MutableMapping type.
+  PyObject* containers =
+      PyImport_ImportModule("google.protobuf.internal.containers");
+  if (containers == NULL) {
+    return false;
+  }
+
+  PyObject* mutable_mapping =
+      PyObject_GetAttrString(containers, "MutableMapping");
+  Py_DECREF(containers);
+
+  if (mutable_mapping == NULL) {
+    return false;
+  }
+
+  if (!PyObject_TypeCheck(mutable_mapping, &PyType_Type)) {
+    Py_DECREF(mutable_mapping);
+    return false;
+  }
+
+  ScalarMapContainer_Type.tp_base =
+      reinterpret_cast<PyTypeObject*>(mutable_mapping);
+
+  if (PyType_Ready(&ScalarMapContainer_Type) < 0) {
+    return false;
+  }
+
+  PyModule_AddObject(m, "ScalarMapContainer",
+                     reinterpret_cast<PyObject*>(&ScalarMapContainer_Type));
+
+  if (PyType_Ready(&ScalarMapIterator_Type) < 0) {
+    return false;
+  }
+
+  PyModule_AddObject(m, "ScalarMapIterator",
+                     reinterpret_cast<PyObject*>(&ScalarMapIterator_Type));
+
+  Py_INCREF(mutable_mapping);
+  MessageMapContainer_Type.tp_base =
+      reinterpret_cast<PyTypeObject*>(mutable_mapping);
+
+  if (PyType_Ready(&MessageMapContainer_Type) < 0) {
+    return false;
+  }
+
+  PyModule_AddObject(m, "MessageMapContainer",
+                     reinterpret_cast<PyObject*>(&MessageMapContainer_Type));
+
+  if (PyType_Ready(&MessageMapIterator_Type) < 0) {
+    return false;
+  }
+
+  PyModule_AddObject(m, "MessageMapIterator",
+                     reinterpret_cast<PyObject*>(&MessageMapIterator_Type));
+
   ExtensionDict_Type.tp_hash = PyObject_HashNotImplemented;
   if (PyType_Ready(&ExtensionDict_Type) < 0) {
     return false;
@@ -2478,6 +2712,12 @@ bool InitProto2MessageModule(PyObject *m) {
       m, "ExtensionDict",
       reinterpret_cast<PyObject*>(&ExtensionDict_Type));
 
+  // Expose the DescriptorPool used to hold all descriptors added from generated
+  // pb2.py files.
+  Py_INCREF(GetDescriptorPool());  // PyModule_AddObject steals a reference.
+  PyModule_AddObject(
+      m, "default_pool", reinterpret_cast<PyObject*>(GetDescriptorPool()));
+
   // 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 2f2da795..7360b207 100644
--- a/python/google/protobuf/pyext/message.h
+++ b/python/google/protobuf/pyext/message.h
@@ -120,7 +120,7 @@ CMessage* NewEmptyMessage(PyObject* type, const Descriptor* descriptor);
 // A new message will be created if this is a read-only default instance.
 //
 // Corresponds to reflection api method ReleaseMessage.
-int ReleaseSubMessage(Message* message,
+int ReleaseSubMessage(CMessage* self,
                       const FieldDescriptor* field_descriptor,
                       CMessage* child_cmessage);
 
@@ -144,7 +144,7 @@ PyObject* InternalGetSubMessage(
 // by slice will be removed from cmessage_list by this function.
 //
 // Corresponds to reflection api method RemoveLast.
-int InternalDeleteRepeatedField(Message* message,
+int InternalDeleteRepeatedField(CMessage* self,
                                 const FieldDescriptor* field_descriptor,
                                 PyObject* slice, PyObject* cmessage_list);
 
@@ -153,10 +153,15 @@ int InternalSetScalar(CMessage* self,
                       const FieldDescriptor* field_descriptor,
                       PyObject* value);
 
+// Sets the specified scalar value to the message.  Requires it is not a Oneof.
+int InternalSetNonOneofScalar(Message* message,
+                              const FieldDescriptor* field_descriptor,
+                              PyObject* arg);
+
 // Retrieves the specified scalar value from the message.
 //
 // Returns a new python reference.
-PyObject* InternalGetScalar(CMessage* self,
+PyObject* InternalGetScalar(const Message* message,
                             const FieldDescriptor* field_descriptor);
 
 // Clears the message, removing all contained data. Extension dictionary and
@@ -279,7 +284,7 @@ extern PyObject* kint64min_py;
 extern PyObject* kint64max_py;
 extern PyObject* kuint64max_py;
 
-#define C(str) const_cast<char*>(str)
+#define FULL_MODULE_NAME "google.protobuf.pyext._message"
 
 void FormatTypeError(PyObject* arg, char* expected_types);
 template<class T>
diff --git a/python/google/protobuf/pyext/message_map_container.cc b/python/google/protobuf/pyext/message_map_container.cc
new file mode 100644
index 00000000..ab8d8fb9
--- /dev/null
+++ b/python/google/protobuf/pyext/message_map_container.cc
@@ -0,0 +1,540 @@
+// 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/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;
+  }
+
+  PyObject* obj = PyType_GenericAlloc(&MessageMapContainer_Type, 0);
+  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);
+
+  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, 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, 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 != 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);
+
+  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.
+  size_t 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 PyMappingMethods MpMethods = {
+  Length,    // mp_length
+  GetItem,   // mp_subscript
+  SetItem,   // mp_ass_subscript
+};
+
+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
+
+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
+  &message_map_container::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
+};
+
+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/message_map_container.h b/python/google/protobuf/pyext/message_map_container.h
new file mode 100644
index 00000000..4ca0aecc
--- /dev/null
+++ b/python/google/protobuf/pyext/message_map_container.h
@@ -0,0 +1,117 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef GOOGLE_PROTOBUF_PYTHON_CPP_MESSAGE_MAP_CONTAINER_H__
+#define GOOGLE_PROTOBUF_PYTHON_CPP_MESSAGE_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;
+
+using internal::shared_ptr;
+
+namespace python {
+
+struct CMessage;
+
+struct MessageMapContainer {
+  PyObject_HEAD;
+
+  // This is the top-level C++ Message object that owns the whole
+  // proto tree.  Every Python MessageMapContainer 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;
+
+  // 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;
+
+  // 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;
+};
+
+extern PyTypeObject MessageMapContainer_Type;
+extern PyTypeObject MessageMapIterator_Type;
+
+namespace message_map_container {
+
+// Builds a MessageMapContainer 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);
+
+// Set the owner field of self and any children of self.
+void SetOwner(MessageMapContainer* self,
+              const shared_ptr<Message>& new_owner);
+
+}  // namespace message_map_container
+}  // namespace python
+}  // namespace protobuf
+
+}  // namespace google
+#endif  // GOOGLE_PROTOBUF_PYTHON_CPP_MESSAGE_MAP_CONTAINER_H__
diff --git a/python/google/protobuf/pyext/repeated_composite_container.cc b/python/google/protobuf/pyext/repeated_composite_container.cc
index 0fe98e73..86b75d0f 100644
--- a/python/google/protobuf/pyext/repeated_composite_container.cc
+++ b/python/google/protobuf/pyext/repeated_composite_container.cc
@@ -367,8 +367,8 @@ int AssignSubscript(RepeatedCompositeContainer* self,
   }
 
   // Delete from the underlying Message, if any.
-  if (self->message != NULL) {
-    if (cmessage::InternalDeleteRepeatedField(self->message,
+  if (self->parent != NULL) {
+    if (cmessage::InternalDeleteRepeatedField(self->parent,
                                               self->parent_field_descriptor,
                                               slice,
                                               self->child_messages) < 0) {
@@ -572,47 +572,35 @@ static PyObject* Pop(RepeatedCompositeContainer* self,
   return item;
 }
 
-// The caller takes ownership of the returned Message.
-Message* ReleaseLast(const FieldDescriptor* field,
-                     const Descriptor* type,
-                     Message* message) {
+// Release field of parent message and transfer the ownership to target.
+void ReleaseLastTo(CMessage* parent,
+                   const FieldDescriptor* field,
+                   CMessage* target) {
+  GOOGLE_CHECK_NOTNULL(parent);
   GOOGLE_CHECK_NOTNULL(field);
-  GOOGLE_CHECK_NOTNULL(type);
-  GOOGLE_CHECK_NOTNULL(message);
+  GOOGLE_CHECK_NOTNULL(target);
 
-  Message* released_message = message->GetReflection()->ReleaseLast(
-      message, field);
+  shared_ptr<Message> released_message(
+      parent->message->GetReflection()->ReleaseLast(parent->message, field));
   // TODO(tibell): Deal with proto1.
 
   // ReleaseMessage will return NULL which differs from
   // child_cmessage->message, if the field does not exist.  In this case,
   // the latter points to the default instance via a const_cast<>, so we
   // have to reset it to a new mutable object since we are taking ownership.
-  if (released_message == NULL) {
+  if (released_message.get() == NULL) {
     const Message* prototype =
-        cmessage::GetMessageFactory()->GetPrototype(type);
+        cmessage::GetMessageFactory()->GetPrototype(
+            target->message->GetDescriptor());
     GOOGLE_CHECK_NOTNULL(prototype);
-    return prototype->New();
-  } else {
-    return released_message;
+    released_message.reset(prototype->New());
   }
-}
 
-// Release field of message and transfer the ownership to cmessage.
-void ReleaseLastTo(const FieldDescriptor* field,
-                   Message* message,
-                   CMessage* cmessage) {
-  GOOGLE_CHECK_NOTNULL(field);
-  GOOGLE_CHECK_NOTNULL(message);
-  GOOGLE_CHECK_NOTNULL(cmessage);
-
-  shared_ptr<Message> released_message(
-      ReleaseLast(field, cmessage->message->GetDescriptor(), message));
-  cmessage->parent = NULL;
-  cmessage->parent_field_descriptor = NULL;
-  cmessage->message = released_message.get();
-  cmessage->read_only = false;
-  cmessage::SetOwner(cmessage, released_message);
+  target->parent = NULL;
+  target->parent_field_descriptor = NULL;
+  target->message = released_message.get();
+  target->read_only = false;
+  cmessage::SetOwner(target, released_message);
 }
 
 // Called to release a container using
@@ -635,7 +623,7 @@ int Release(RepeatedCompositeContainer* self) {
   for (Py_ssize_t i = size - 1; i >= 0; --i) {
     CMessage* child_cmessage = reinterpret_cast<CMessage*>(
         PyList_GET_ITEM(self->child_messages, i));
-    ReleaseLastTo(field, message, child_cmessage);
+    ReleaseLastTo(self->parent, field, child_cmessage);
   }
 
   // Detach from containing message.
@@ -732,9 +720,7 @@ static PyMethodDef Methods[] = {
 
 PyTypeObject RepeatedCompositeContainer_Type = {
   PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  // Keep the fully qualified _message symbol in a line for opensource.
-  "google.protobuf.pyext._message."
-  "RepeatedCompositeContainer",        // tp_name
+  FULL_MODULE_NAME ".RepeatedCompositeContainer",  // tp_name
   sizeof(RepeatedCompositeContainer),  // tp_basicsize
   0,                                   //  tp_itemsize
   (destructor)repeated_composite_container::Dealloc,  //  tp_dealloc
diff --git a/python/google/protobuf/pyext/repeated_composite_container.h b/python/google/protobuf/pyext/repeated_composite_container.h
index ce7cee0f..e0f21360 100644
--- a/python/google/protobuf/pyext/repeated_composite_container.h
+++ b/python/google/protobuf/pyext/repeated_composite_container.h
@@ -161,13 +161,13 @@ int SetOwner(RepeatedCompositeContainer* self,
              const shared_ptr<Message>& new_owner);
 
 // Removes the last element of the repeated message field 'field' on
-// the Message 'message', and transfers the ownership of the released
-// Message to 'cmessage'.
+// the Message 'parent', and transfers the ownership of the released
+// Message to 'target'.
 //
 // Corresponds to reflection api method ReleaseMessage.
-void ReleaseLastTo(const FieldDescriptor* field,
-                   Message* message,
-                   CMessage* cmessage);
+void ReleaseLastTo(CMessage* parent,
+                   const FieldDescriptor* field,
+                   CMessage* target);
 
 }  // namespace repeated_composite_container
 }  // namespace python
diff --git a/python/google/protobuf/pyext/repeated_scalar_container.cc b/python/google/protobuf/pyext/repeated_scalar_container.cc
index 110a4c85..fd196836 100644
--- a/python/google/protobuf/pyext/repeated_scalar_container.cc
+++ b/python/google/protobuf/pyext/repeated_scalar_container.cc
@@ -102,7 +102,7 @@ static int AssignItem(RepeatedScalarContainer* self,
 
   if (arg == NULL) {
     ScopedPyObjectPtr py_index(PyLong_FromLong(index));
-    return cmessage::InternalDeleteRepeatedField(message, field_descriptor,
+    return cmessage::InternalDeleteRepeatedField(self->parent, field_descriptor,
                                                  py_index, NULL);
   }
 
@@ -470,7 +470,7 @@ static int AssSubscript(RepeatedScalarContainer* self,
 
   if (value == NULL) {
     return cmessage::InternalDeleteRepeatedField(
-        message, field_descriptor, slice, NULL);
+        self->parent, field_descriptor, slice, NULL);
   }
 
   if (!create_list) {
@@ -769,9 +769,7 @@ static PyMethodDef Methods[] = {
 
 PyTypeObject RepeatedScalarContainer_Type = {
   PyVarObject_HEAD_INIT(&PyType_Type, 0)
-  // Keep the fully qualified _message symbol in a line for opensource.
-  "google.protobuf.pyext._message."
-  "RepeatedScalarContainer",           // tp_name
+  FULL_MODULE_NAME ".RepeatedScalarContainer",  // tp_name
   sizeof(RepeatedScalarContainer),     // tp_basicsize
   0,                                   //  tp_itemsize
   (destructor)repeated_scalar_container::Dealloc,  //  tp_dealloc
diff --git a/python/google/protobuf/pyext/scalar_map_container.cc b/python/google/protobuf/pyext/scalar_map_container.cc
new file mode 100644
index 00000000..6f731d27
--- /dev/null
+++ b/python/google/protobuf/pyext/scalar_map_container.cc
@@ -0,0 +1,514 @@
+// 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/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;
+  }
+
+  ScopedPyObjectPtr obj(PyType_GenericAlloc(&ScalarMapContainer_Type, 0));
+  if (obj.get() == NULL) {
+    return PyErr_Format(PyExc_RuntimeError,
+                        "Could not allocate new container.");
+  }
+
+  ScalarMapContainer* 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");
+
+  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, 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 != 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 (int 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 PyMappingMethods MpMethods = {
+  Length,    // mp_length
+  GetItem,   // mp_subscript
+  SetItem,   // mp_ass_subscript
+};
+
+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
+
+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
+  &scalar_map_container::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
+};
+
+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
new file mode 100644
index 00000000..254e6e98
--- /dev/null
+++ b/python/google/protobuf/pyext/scalar_map_container.h
@@ -0,0 +1,110 @@
+// 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;
+
+using internal::shared_ptr;
+
+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;
+};
+
+extern PyTypeObject ScalarMapContainer_Type;
+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__