diff --git a/froofle/protobuf/reflection.py b/froofle/protobuf/reflection.py
new file mode 100644
index 0000000..e2abff0
--- /dev/null
+++ b/froofle/protobuf/reflection.py
@@ -0,0 +1,1653 @@
+# Protocol Buffers - Google's data interchange format
+# Copyright 2008 Google Inc.  All rights reserved.
+# http://code.google.com/p/protobuf/
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+#     * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#     * Redistributions in binary form must reproduce the above
+# copyright notice, this list of conditions and the following disclaimer
+# in the documentation and/or other materials provided with the
+# distribution.
+#     * Neither the name of Google Inc. nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+# This code is meant to work on Python 2.4 and above only.
+#
+# TODO(robinson): Helpers for verbose, common checks like seeing if a
+# descriptor's cpp_type is CPPTYPE_MESSAGE.
+
+"""Contains a metaclass and helper functions used to create
+protocol message classes from Descriptor objects at runtime.
+
+Recall that a metaclass is the "type" of a class.
+(A class is to a metaclass what an instance is to a class.)
+
+In this case, we use the GeneratedProtocolMessageType metaclass
+to inject all the useful functionality into the classes
+output by the protocol compiler at compile-time.
+
+The upshot of all this is that the real implementation
+details for ALL pure-Python protocol buffers are *here in
+this file*.
+"""
+
+__author__ = 'robinson@google.com (Will Robinson)'
+
+import heapq
+import threading
+import weakref
+# We use "as" to avoid name collisions with variables.
+from froofle.protobuf.internal import decoder
+from froofle.protobuf.internal import encoder
+from froofle.protobuf.internal import message_listener as message_listener_mod
+from froofle.protobuf.internal import type_checkers
+from froofle.protobuf.internal import wire_format
+from froofle.protobuf import descriptor as descriptor_mod
+from froofle.protobuf import message as message_mod
+
+_FieldDescriptor = descriptor_mod.FieldDescriptor
+
+
+class GeneratedProtocolMessageType(type):
+
+  """Metaclass for protocol message classes created at runtime from Descriptors.
+
+  We add implementations for all methods described in the Message class.  We
+  also create properties to allow getting/setting all fields in the protocol
+  message.  Finally, we create slots to prevent users from accidentally
+  "setting" nonexistent fields in the protocol message, which then wouldn't get
+  serialized / deserialized properly.
+
+  The protocol compiler currently uses this metaclass to create protocol
+  message classes at runtime.  Clients can also manually create their own
+  classes at runtime, as in this example:
+
+  mydescriptor = Descriptor(.....)
+  class MyProtoClass(Message):
+    __metaclass__ = GeneratedProtocolMessageType
+    DESCRIPTOR = mydescriptor
+  myproto_instance = MyProtoClass()
+  myproto.foo_field = 23
+  ...
+  """
+
+  # Must be consistent with the protocol-compiler code in
+  # proto2/compiler/internal/generator.*.
+  _DESCRIPTOR_KEY = 'DESCRIPTOR'
+
+  def __new__(cls, name, bases, dictionary):
+    """Custom allocation for runtime-generated class types.
+
+    We override __new__ because this is apparently the only place
+    where we can meaningfully set __slots__ on the class we're creating(?).
+    (The interplay between metaclasses and slots is not very well-documented).
+
+    Args:
+      name: Name of the class (ignored, but required by the
+        metaclass protocol).
+      bases: Base classes of the class we're constructing.
+        (Should be message.Message).  We ignore this field, but
+        it's required by the metaclass protocol
+      dictionary: The class dictionary of the class we're
+        constructing.  dictionary[_DESCRIPTOR_KEY] must contain
+        a Descriptor object describing this protocol message
+        type.
+
+    Returns:
+      Newly-allocated class.
+    """
+    descriptor = dictionary[GeneratedProtocolMessageType._DESCRIPTOR_KEY]
+    _AddSlots(descriptor, dictionary)
+    _AddClassAttributesForNestedExtensions(descriptor, dictionary)
+    superclass = super(GeneratedProtocolMessageType, cls)
+    return superclass.__new__(cls, name, bases, dictionary)
+
+  def __init__(cls, name, bases, dictionary):
+    """Here we perform the majority of our work on the class.
+    We add enum getters, an __init__ method, implementations
+    of all Message methods, and properties for all fields
+    in the protocol type.
+
+    Args:
+      name: Name of the class (ignored, but required by the
+        metaclass protocol).
+      bases: Base classes of the class we're constructing.
+        (Should be message.Message).  We ignore this field, but
+        it's required by the metaclass protocol
+      dictionary: The class dictionary of the class we're
+        constructing.  dictionary[_DESCRIPTOR_KEY] must contain
+        a Descriptor object describing this protocol message
+        type.
+    """
+    descriptor = dictionary[GeneratedProtocolMessageType._DESCRIPTOR_KEY]
+    # We act as a "friend" class of the descriptor, setting
+    # its _concrete_class attribute the first time we use a
+    # given descriptor to initialize a concrete protocol message
+    # class.
+    concrete_class_attr_name = '_concrete_class'
+    if not hasattr(descriptor, concrete_class_attr_name):
+      setattr(descriptor, concrete_class_attr_name, cls)
+    cls._known_extensions = []
+    _AddEnumValues(descriptor, cls)
+    _AddInitMethod(descriptor, cls)
+    _AddPropertiesForFields(descriptor, cls)
+    _AddStaticMethods(cls)
+    _AddMessageMethods(descriptor, cls)
+    _AddPrivateHelperMethods(cls)
+    superclass = super(GeneratedProtocolMessageType, cls)
+    superclass.__init__(cls, name, bases, dictionary)
+
+
+# Stateless helpers for GeneratedProtocolMessageType below.
+# Outside clients should not access these directly.
+#
+# I opted not to make any of these methods on the metaclass, to make it more
+# clear that I'm not really using any state there and to keep clients from
+# thinking that they have direct access to these construction helpers.
+
+
+def _PropertyName(proto_field_name):
+  """Returns the name of the public property attribute which
+  clients can use to get and (in some cases) set the value
+  of a protocol message field.
+
+  Args:
+    proto_field_name: The protocol message field name, exactly
+      as it appears (or would appear) in a .proto file.
+  """
+  # TODO(robinson): Escape Python keywords (e.g., yield), and test this support.
+  # nnorwitz makes my day by writing:
+  # """
+  # FYI.  See the keyword module in the stdlib. This could be as simple as:
+  #
+  # if keyword.iskeyword(proto_field_name):
+  #   return proto_field_name + "_"
+  # return proto_field_name
+  # """
+  return proto_field_name
+
+
+def _ValueFieldName(proto_field_name):
+  """Returns the name of the (internal) instance attribute which objects
+  should use to store the current value for a given protocol message field.
+
+  Args:
+    proto_field_name: The protocol message field name, exactly
+      as it appears (or would appear) in a .proto file.
+  """
+  return '_value_' + proto_field_name
+
+
+def _HasFieldName(proto_field_name):
+  """Returns the name of the (internal) instance attribute which
+  objects should use to store a boolean telling whether this field
+  is explicitly set or not.
+
+  Args:
+    proto_field_name: The protocol message field name, exactly
+      as it appears (or would appear) in a .proto file.
+  """
+  return '_has_' + proto_field_name
+
+
+def _AddSlots(message_descriptor, dictionary):
+  """Adds a __slots__ entry to dictionary, containing the names of all valid
+  attributes for this message type.
+
+  Args:
+    message_descriptor: A Descriptor instance describing this message type.
+    dictionary: Class dictionary to which we'll add a '__slots__' entry.
+  """
+  field_names = [_ValueFieldName(f.name) for f in message_descriptor.fields]
+  field_names.extend(_HasFieldName(f.name) for f in message_descriptor.fields
+                     if f.label != _FieldDescriptor.LABEL_REPEATED)
+  field_names.extend(('Extensions',
+                      '_cached_byte_size',
+                      '_cached_byte_size_dirty',
+                      '_called_transition_to_nonempty',
+                      '_listener',
+                      '_lock', '__weakref__'))
+  dictionary['__slots__'] = field_names
+
+
+def _AddClassAttributesForNestedExtensions(descriptor, dictionary):
+  extension_dict = descriptor.extensions_by_name
+  for extension_name, extension_field in extension_dict.iteritems():
+    assert extension_name not in dictionary
+    dictionary[extension_name] = extension_field
+
+
+def _AddEnumValues(descriptor, cls):
+  """Sets class-level attributes for all enum fields defined in this message.
+
+  Args:
+    descriptor: Descriptor object for this message type.
+    cls: Class we're constructing for this message type.
+  """
+  for enum_type in descriptor.enum_types:
+    for enum_value in enum_type.values:
+      setattr(cls, enum_value.name, enum_value.number)
+
+
+def _DefaultValueForField(message, field):
+  """Returns a default value for a field.
+
+  Args:
+    message: Message instance containing this field, or a weakref proxy
+      of same.
+    field: FieldDescriptor object for this field.
+
+  Returns: A default value for this field.  May refer back to |message|
+    via a weak reference.
+  """
+  # TODO(robinson): Only the repeated fields need a reference to 'message' (so
+  # that they can set the 'has' bit on the containing Message when someone
+  # append()s a value).  We could special-case this, and avoid an extra
+  # function call on __init__() and Clear() for non-repeated fields.
+
+  # TODO(robinson): Find a better place for the default value assertion in this
+  # function.  No need to repeat them every time the client calls Clear('foo').
+  # (We should probably just assert these things once and as early as possible,
+  # by tightening checking in the descriptor classes.)
+  if field.label == _FieldDescriptor.LABEL_REPEATED:
+    if field.default_value != []:
+      raise ValueError('Repeated field default value not empty list: %s' % (
+          field.default_value))
+    listener = _Listener(message, None)
+    if field.cpp_type == _FieldDescriptor.CPPTYPE_MESSAGE:
+      # We can't look at _concrete_class yet since it might not have
+      # been set.  (Depends on order in which we initialize the classes).
+      return _RepeatedCompositeFieldContainer(listener, field.message_type)
+    else:
+      return _RepeatedScalarFieldContainer(
+          listener, type_checkers.GetTypeChecker(field.cpp_type, field.type))
+
+  if field.cpp_type == _FieldDescriptor.CPPTYPE_MESSAGE:
+    assert field.default_value is None
+
+  return field.default_value
+
+
+def _AddInitMethod(message_descriptor, cls):
+  """Adds an __init__ method to cls."""
+  fields = message_descriptor.fields
+  def init(self):
+    self._cached_byte_size = 0
+    self._cached_byte_size_dirty = False
+    self._listener = message_listener_mod.NullMessageListener()
+    self._called_transition_to_nonempty = False
+    # TODO(robinson): We should only create a lock if we really need one
+    # in this class.
+    self._lock = threading.Lock()
+    for field in fields:
+      default_value = _DefaultValueForField(self, field)
+      python_field_name = _ValueFieldName(field.name)
+      setattr(self, python_field_name, default_value)
+      if field.label != _FieldDescriptor.LABEL_REPEATED:
+        setattr(self, _HasFieldName(field.name), False)
+    self.Extensions = _ExtensionDict(self, cls._known_extensions)
+
+  init.__module__ = None
+  init.__doc__ = None
+  cls.__init__ = init
+
+
+def _AddPropertiesForFields(descriptor, cls):
+  """Adds properties for all fields in this protocol message type."""
+  for field in descriptor.fields:
+    _AddPropertiesForField(field, cls)
+
+
+def _AddPropertiesForField(field, cls):
+  """Adds a public property for a protocol message field.
+  Clients can use this property to get and (in the case
+  of non-repeated scalar fields) directly set the value
+  of a protocol message field.
+
+  Args:
+    field: A FieldDescriptor for this field.
+    cls: The class we're constructing.
+  """
+  # Catch it if we add other types that we should
+  # handle specially here.
+  assert _FieldDescriptor.MAX_CPPTYPE == 10
+
+  if field.label == _FieldDescriptor.LABEL_REPEATED:
+    _AddPropertiesForRepeatedField(field, cls)
+  elif field.cpp_type == _FieldDescriptor.CPPTYPE_MESSAGE:
+    _AddPropertiesForNonRepeatedCompositeField(field, cls)
+  else:
+    _AddPropertiesForNonRepeatedScalarField(field, cls)
+
+
+def _AddPropertiesForRepeatedField(field, cls):
+  """Adds a public property for a "repeated" protocol message field.  Clients
+  can use this property to get the value of the field, which will be either a
+  _RepeatedScalarFieldContainer or _RepeatedCompositeFieldContainer (see
+  below).
+
+  Note that when clients add values to these containers, we perform
+  type-checking in the case of repeated scalar fields, and we also set any
+  necessary "has" bits as a side-effect.
+
+  Args:
+    field: A FieldDescriptor for this field.
+    cls: The class we're constructing.
+  """
+  proto_field_name = field.name
+  python_field_name = _ValueFieldName(proto_field_name)
+  property_name = _PropertyName(proto_field_name)
+
+  def getter(self):
+    return getattr(self, python_field_name)
+  getter.__module__ = None
+  getter.__doc__ = 'Getter for %s.' % proto_field_name
+
+  # We define a setter just so we can throw an exception with a more
+  # helpful error message.
+  def setter(self, new_value):
+    raise AttributeError('Assignment not allowed to repeated field '
+                         '"%s" in protocol message object.' % proto_field_name)
+
+  doc = 'Magic attribute generated for "%s" proto field.' % proto_field_name
+  setattr(cls, property_name, property(getter, setter, doc=doc))
+
+
+def _AddPropertiesForNonRepeatedScalarField(field, cls):
+  """Adds a public property for a nonrepeated, scalar protocol message field.
+  Clients can use this property to get and directly set the value of the field.
+  Note that when the client sets the value of a field by using this property,
+  all necessary "has" bits are set as a side-effect, and we also perform
+  type-checking.
+
+  Args:
+    field: A FieldDescriptor for this field.
+    cls: The class we're constructing.
+  """
+  proto_field_name = field.name
+  python_field_name = _ValueFieldName(proto_field_name)
+  has_field_name = _HasFieldName(proto_field_name)
+  property_name = _PropertyName(proto_field_name)
+  type_checker = type_checkers.GetTypeChecker(field.cpp_type, field.type)
+
+  def getter(self):
+    return getattr(self, python_field_name)
+  getter.__module__ = None
+  getter.__doc__ = 'Getter for %s.' % proto_field_name
+  def setter(self, new_value):
+    type_checker.CheckValue(new_value)
+    setattr(self, has_field_name, True)
+    self._MarkByteSizeDirty()
+    self._MaybeCallTransitionToNonemptyCallback()
+    setattr(self, python_field_name, new_value)
+  setter.__module__ = None
+  setter.__doc__ = 'Setter for %s.' % proto_field_name
+
+  # Add a property to encapsulate the getter/setter.
+  doc = 'Magic attribute generated for "%s" proto field.' % proto_field_name
+  setattr(cls, property_name, property(getter, setter, doc=doc))
+
+
+def _AddPropertiesForNonRepeatedCompositeField(field, cls):
+  """Adds a public property for a nonrepeated, composite protocol message field.
+  A composite field is a "group" or "message" field.
+
+  Clients can use this property to get the value of the field, but cannot
+  assign to the property directly.
+
+  Args:
+    field: A FieldDescriptor for this field.
+    cls: The class we're constructing.
+  """
+  # TODO(robinson): Remove duplication with similar method
+  # for non-repeated scalars.
+  proto_field_name = field.name
+  python_field_name = _ValueFieldName(proto_field_name)
+  has_field_name = _HasFieldName(proto_field_name)
+  property_name = _PropertyName(proto_field_name)
+  message_type = field.message_type
+
+  def getter(self):
+    # TODO(robinson): Appropriately scary note about double-checked locking.
+    field_value = getattr(self, python_field_name)
+    if field_value is None:
+      self._lock.acquire()
+      try:
+        field_value = getattr(self, python_field_name)
+        if field_value is None:
+          field_class = message_type._concrete_class
+          field_value = field_class()
+          field_value._SetListener(_Listener(self, has_field_name))
+          setattr(self, python_field_name, field_value)
+      finally:
+        self._lock.release()
+    return field_value
+  getter.__module__ = None
+  getter.__doc__ = 'Getter for %s.' % proto_field_name
+
+  # We define a setter just so we can throw an exception with a more
+  # helpful error message.
+  def setter(self, new_value):
+    raise AttributeError('Assignment not allowed to composite field '
+                         '"%s" in protocol message object.' % proto_field_name)
+
+  # Add a property to encapsulate the getter.
+  doc = 'Magic attribute generated for "%s" proto field.' % proto_field_name
+  setattr(cls, property_name, property(getter, setter, doc=doc))
+
+
+def _AddStaticMethods(cls):
+  # TODO(robinson): This probably needs to be thread-safe(?)
+  def RegisterExtension(extension_handle):
+    extension_handle.containing_type = cls.DESCRIPTOR
+    cls._known_extensions.append(extension_handle)
+  cls.RegisterExtension = staticmethod(RegisterExtension)
+
+
+def _AddListFieldsMethod(message_descriptor, cls):
+  """Helper for _AddMessageMethods()."""
+
+  # Ensure that we always list in ascending field-number order.
+  # For non-extension fields, we can do the sort once, here, at import-time.
+  # For extensions, we sort on each ListFields() call, though
+  # we could do better if we have to.
+  fields = sorted(message_descriptor.fields, key=lambda f: f.number)
+  has_field_names = (_HasFieldName(f.name) for f in fields)
+  value_field_names = (_ValueFieldName(f.name) for f in fields)
+  triplets = zip(has_field_names, value_field_names, fields)
+
+  def ListFields(self):
+    # We need to list all extension and non-extension fields
+    # together, in sorted order by field number.
+
+    # Step 0: Get an iterator over all "set" non-extension fields,
+    # sorted by field number.
+    # This iterator yields (field_number, field_descriptor, value) tuples.
+    def SortedSetFieldsIter():
+      # Note that triplets is already sorted by field number.
+      for has_field_name, value_field_name, field_descriptor in triplets:
+        if field_descriptor.label == _FieldDescriptor.LABEL_REPEATED:
+          value = getattr(self, _ValueFieldName(field_descriptor.name))
+          if len(value) > 0:
+            yield (field_descriptor.number, field_descriptor, value)
+        elif getattr(self, _HasFieldName(field_descriptor.name)):
+          value = getattr(self, _ValueFieldName(field_descriptor.name))
+          yield (field_descriptor.number, field_descriptor, value)
+    sorted_fields = SortedSetFieldsIter()
+
+    # Step 1: Get an iterator over all "set" extension fields,
+    # sorted by field number.
+    # This iterator ALSO yields (field_number, field_descriptor, value) tuples.
+    # TODO(robinson): It's not necessary to repeat this with each
+    # serialization call.  We can do better.
+    sorted_extension_fields = sorted(
+        [(f.number, f, v) for f, v in self.Extensions._ListSetExtensions()])
+
+    # Step 2: Create a composite iterator that merges the extension-
+    # and non-extension fields, and that still yields fields in
+    # sorted order.
+    all_set_fields = _ImergeSorted(sorted_fields, sorted_extension_fields)
+
+    # Step 3: Strip off the field numbers and return.
+    return [field[1:] for field in all_set_fields]
+
+  cls.ListFields = ListFields
+
+def _AddHasFieldMethod(cls):
+  """Helper for _AddMessageMethods()."""
+  def HasField(self, field_name):
+    try:
+      return getattr(self, _HasFieldName(field_name))
+    except AttributeError:
+      raise ValueError('Protocol message has no "%s" field.' % field_name)
+  cls.HasField = HasField
+
+
+def _AddClearFieldMethod(cls):
+  """Helper for _AddMessageMethods()."""
+  def ClearField(self, field_name):
+    try:
+      field = self.DESCRIPTOR.fields_by_name[field_name]
+    except KeyError:
+      raise ValueError('Protocol message has no "%s" field.' % field_name)
+    proto_field_name = field.name
+    python_field_name = _ValueFieldName(proto_field_name)
+    has_field_name = _HasFieldName(proto_field_name)
+    default_value = _DefaultValueForField(self, field)
+    if field.label == _FieldDescriptor.LABEL_REPEATED:
+      self._MarkByteSizeDirty()
+    else:
+      if field.cpp_type == _FieldDescriptor.CPPTYPE_MESSAGE:
+        old_field_value = getattr(self, python_field_name)
+        if old_field_value is not None:
+          # Snip the old object out of the object tree.
+          old_field_value._SetListener(None)
+      if getattr(self, has_field_name):
+        setattr(self, has_field_name, False)
+        # Set dirty bit on ourself and parents only if
+        # we're actually changing state.
+        self._MarkByteSizeDirty()
+    setattr(self, python_field_name, default_value)
+  cls.ClearField = ClearField
+
+
+def _AddClearExtensionMethod(cls):
+  """Helper for _AddMessageMethods()."""
+  def ClearExtension(self, extension_handle):
+    self.Extensions._ClearExtension(extension_handle)
+  cls.ClearExtension = ClearExtension
+
+
+def _AddClearMethod(cls):
+  """Helper for _AddMessageMethods()."""
+  def Clear(self):
+    # Clear fields.
+    fields = self.DESCRIPTOR.fields
+    for field in fields:
+      self.ClearField(field.name)
+    # Clear extensions.
+    extensions = self.Extensions._ListSetExtensions()
+    for extension in extensions:
+      self.ClearExtension(extension[0])
+  cls.Clear = Clear
+
+
+def _AddHasExtensionMethod(cls):
+  """Helper for _AddMessageMethods()."""
+  def HasExtension(self, extension_handle):
+    return self.Extensions._HasExtension(extension_handle)
+  cls.HasExtension = HasExtension
+
+
+def _AddEqualsMethod(message_descriptor, cls):
+  """Helper for _AddMessageMethods()."""
+  def __eq__(self, other):
+    if self is other:
+      return True
+
+    # Compare all fields contained directly in this message.
+    for field_descriptor in message_descriptor.fields:
+      label = field_descriptor.label
+      property_name = _PropertyName(field_descriptor.name)
+      # Non-repeated field equality requires matching "has" bits as well
+      # as having an equal value.
+      if label != _FieldDescriptor.LABEL_REPEATED:
+        self_has = self.HasField(property_name)
+        other_has = other.HasField(property_name)
+        if self_has != other_has:
+          return False
+        if not self_has:
+          # If the "has" bit for this field is False, we must stop here.
+          # Otherwise we will recurse forever on recursively-defined protos.
+          continue
+      if getattr(self, property_name) != getattr(other, property_name):
+        return False
+
+    # Compare the extensions present in both messages.
+    return self.Extensions == other.Extensions
+  cls.__eq__ = __eq__
+
+
+def _AddSetListenerMethod(cls):
+  """Helper for _AddMessageMethods()."""
+  def SetListener(self, listener):
+    if listener is None:
+      self._listener = message_listener_mod.NullMessageListener()
+    else:
+      self._listener = listener
+  cls._SetListener = SetListener
+
+
+def _BytesForNonRepeatedElement(value, field_number, field_type):
+  """Returns the number of bytes needed to serialize a non-repeated element.
+  The returned byte count includes space for tag information and any
+  other additional space associated with serializing value.
+
+  Args:
+    value: Value we're serializing.
+    field_number: Field number of this value.  (Since the field number
+      is stored as part of a varint-encoded tag, this has an impact
+      on the total bytes required to serialize the value).
+    field_type: The type of the field.  One of the TYPE_* constants
+      within FieldDescriptor.
+  """
+  try:
+    fn = type_checkers.TYPE_TO_BYTE_SIZE_FN[field_type]
+    return fn(field_number, value)
+  except KeyError:
+    raise message_mod.EncodeError('Unrecognized field type: %d' % field_type)
+
+
+def _AddByteSizeMethod(message_descriptor, cls):
+  """Helper for _AddMessageMethods()."""
+
+  def BytesForField(message, field, value):
+    """Returns the number of bytes required to serialize a single field
+    in message.  The field may be repeated or not, composite or not.
+
+    Args:
+      message: The Message instance containing a field of the given type.
+      field: A FieldDescriptor describing the field of interest.
+      value: The value whose byte size we're interested in.
+
+    Returns: The number of bytes required to serialize the current value
+      of "field" in "message", including space for tags and any other
+      necessary information.
+    """
+
+    if _MessageSetField(field):
+      return wire_format.MessageSetItemByteSize(field.number, value)
+
+    field_number, field_type = field.number, field.type
+
+    # Repeated fields.
+    if field.label == _FieldDescriptor.LABEL_REPEATED:
+      elements = value
+    else:
+      elements = [value]
+
+    size = sum(_BytesForNonRepeatedElement(element, field_number, field_type)
+               for element in elements)
+    return size
+
+  fields = message_descriptor.fields
+  has_field_names = (_HasFieldName(f.name) for f in fields)
+  zipped = zip(has_field_names, fields)
+
+  def ByteSize(self):
+    if not self._cached_byte_size_dirty:
+      return self._cached_byte_size
+
+    size = 0
+    # Hardcoded fields first.
+    for has_field_name, field in zipped:
+      if (field.label == _FieldDescriptor.LABEL_REPEATED
+          or getattr(self, has_field_name)):
+        value = getattr(self, _ValueFieldName(field.name))
+        size += BytesForField(self, field, value)
+    # Extensions next.
+    for field, value in self.Extensions._ListSetExtensions():
+      size += BytesForField(self, field, value)
+
+    self._cached_byte_size = size
+    self._cached_byte_size_dirty = False
+    return size
+  cls.ByteSize = ByteSize
+
+
+def _MessageSetField(field_descriptor):
+  """Checks if a field should be serialized using the message set wire format.
+
+  Args:
+    field_descriptor: Descriptor of the field.
+
+  Returns:
+    True if the field should be serialized using the message set wire format,
+    false otherwise.
+  """
+  return (field_descriptor.is_extension and
+          field_descriptor.label != _FieldDescriptor.LABEL_REPEATED and
+          field_descriptor.cpp_type == _FieldDescriptor.CPPTYPE_MESSAGE and
+          field_descriptor.containing_type.GetOptions().message_set_wire_format)
+
+
+def _SerializeValueToEncoder(value, field_number, field_descriptor, encoder):
+  """Appends the serialization of a single value to encoder.
+
+  Args:
+    value: Value to serialize.
+    field_number: Field number of this value.
+    field_descriptor: Descriptor of the field to serialize.
+    encoder: encoder.Encoder object to which we should serialize this value.
+  """
+  if _MessageSetField(field_descriptor):
+    encoder.AppendMessageSetItem(field_number, value)
+    return
+
+  try:
+    method = type_checkers.TYPE_TO_SERIALIZE_METHOD[field_descriptor.type]
+    method(encoder, field_number, value)
+  except KeyError:
+    raise message_mod.EncodeError('Unrecognized field type: %d' %
+                                  field_descriptor.type)
+
+
+def _ImergeSorted(*streams):
+  """Merges N sorted iterators into a single sorted iterator.
+  Each element in streams must be an iterable that yields
+  its elements in sorted order, and the elements contained
+  in each stream must all be comparable.
+
+  There may be repeated elements in the component streams or
+  across the streams; the repeated elements will all be repeated
+  in the merged iterator as well.
+
+  I believe that the heapq module at HEAD in the Python
+  sources has a method like this, but for now we roll our own.
+  """
+  iters = [iter(stream) for stream in streams]
+  heap = []
+  for index, it in enumerate(iters):
+    try:
+      heap.append((it.next(), index))
+    except StopIteration:
+      pass
+  heapq.heapify(heap)
+
+  while heap:
+    smallest_value, idx = heap[0]
+    yield smallest_value
+    try:
+      next_element = iters[idx].next()
+      heapq.heapreplace(heap, (next_element, idx))
+    except StopIteration:
+      heapq.heappop(heap)
+
+
+def _AddSerializeToStringMethod(message_descriptor, cls):
+  """Helper for _AddMessageMethods()."""
+
+  def SerializeToString(self):
+    # Check if the message has all of its required fields set.
+    errors = []
+    if not _InternalIsInitialized(self, errors):
+      raise message_mod.EncodeError('\n'.join(errors))
+    return self.SerializePartialToString()
+  cls.SerializeToString = SerializeToString
+
+
+def _AddSerializePartialToStringMethod(message_descriptor, cls):
+  """Helper for _AddMessageMethods()."""
+  Encoder = encoder.Encoder
+
+  def SerializePartialToString(self):
+    encoder = Encoder()
+    # We need to serialize all extension and non-extension fields
+    # together, in sorted order by field number.
+    for field_descriptor, field_value in self.ListFields():
+      if field_descriptor.label == _FieldDescriptor.LABEL_REPEATED:
+        repeated_value = field_value
+      else:
+        repeated_value = [field_value]
+      for element in repeated_value:
+        _SerializeValueToEncoder(element, field_descriptor.number,
+                                 field_descriptor, encoder)
+    return encoder.ToString()
+  cls.SerializePartialToString = SerializePartialToString
+
+
+def _WireTypeForFieldType(field_type):
+  """Given a field type, returns the expected wire type."""
+  try:
+    return type_checkers.FIELD_TYPE_TO_WIRE_TYPE[field_type]
+  except KeyError:
+    raise message_mod.DecodeError('Unknown field type: %d' % field_type)
+
+
+def _RecursivelyMerge(field_number, field_type, decoder, message):
+  """Decodes a message from decoder into message.
+  message is either a group or a nested message within some containing
+  protocol message.  If it's a group, we use the group protocol to
+  deserialize, and if it's a nested message, we use the nested-message
+  protocol.
+
+  Args:
+    field_number: The field number of message in its enclosing protocol buffer.
+    field_type: The field type of message.  Must be either TYPE_MESSAGE
+      or TYPE_GROUP.
+    decoder: Decoder to read from.
+    message: Message to deserialize into.
+  """
+  if field_type == _FieldDescriptor.TYPE_MESSAGE:
+    decoder.ReadMessageInto(message)
+  elif field_type == _FieldDescriptor.TYPE_GROUP:
+    decoder.ReadGroupInto(field_number, message)
+  else:
+    raise message_mod.DecodeError('Unexpected field type: %d' % field_type)
+
+
+def _DeserializeScalarFromDecoder(field_type, decoder):
+  """Deserializes a scalar of the requested type from decoder.  field_type must
+  be a scalar (non-group, non-message) FieldDescriptor.FIELD_* constant.
+  """
+  try:
+    method = type_checkers.TYPE_TO_DESERIALIZE_METHOD[field_type]
+    return method(decoder)
+  except KeyError:
+    raise message_mod.DecodeError('Unrecognized field type: %d' % field_type)
+
+
+def _SkipField(field_number, wire_type, decoder):
+  """Skips a field with the specified wire type.
+
+  Args:
+    field_number: Tag number of the field to skip.
+    wire_type: Wire type of the field to skip.
+    decoder: Decoder used to deserialize the messsage. It must be positioned
+      just after reading the the tag and wire type of the field.
+  """
+  if wire_type == wire_format.WIRETYPE_VARINT:
+    decoder.ReadUInt64()
+  elif wire_type == wire_format.WIRETYPE_FIXED64:
+    decoder.ReadFixed64()
+  elif wire_type == wire_format.WIRETYPE_LENGTH_DELIMITED:
+    decoder.SkipBytes(decoder.ReadInt32())
+  elif wire_type == wire_format.WIRETYPE_START_GROUP:
+    _SkipGroup(field_number, decoder)
+  elif wire_type == wire_format.WIRETYPE_END_GROUP:
+    pass
+  elif wire_type == wire_format.WIRETYPE_FIXED32:
+    decoder.ReadFixed32()
+  else:
+    raise message_mod.DecodeError('Unexpected wire type: %d' % wire_type)
+
+
+def _SkipGroup(group_number, decoder):
+  """Skips a nested group from the decoder.
+
+  Args:
+    group_number: Tag number of the group to skip.
+    decoder: Decoder used to deserialize the message. It must be positioned
+      exactly at the beginning of the message that should be skipped.
+  """
+  while True:
+    field_number, wire_type = decoder.ReadFieldNumberAndWireType()
+    if (wire_type == wire_format.WIRETYPE_END_GROUP and
+        field_number == group_number):
+      return
+    _SkipField(field_number, wire_type, decoder)
+
+
+def _DeserializeMessageSetItem(message, decoder):
+  """Deserializes a message using the message set wire format.
+
+  Args:
+    message: Message to be parsed to.
+    decoder: The decoder to be used to deserialize encoded data. Note that the
+      decoder should be positioned just after reading the START_GROUP tag that
+      began the messageset item.
+  """
+  field_number, wire_type = decoder.ReadFieldNumberAndWireType()
+  if wire_type != wire_format.WIRETYPE_VARINT or field_number != 2:
+    raise message_mod.DecodeError(
+        'Incorrect message set wire format. '
+        'wire_type: %d, field_number: %d' % (wire_type, field_number))
+
+  type_id = decoder.ReadInt32()
+  field_number, wire_type = decoder.ReadFieldNumberAndWireType()
+  if wire_type != wire_format.WIRETYPE_LENGTH_DELIMITED or field_number != 3:
+    raise message_mod.DecodeError(
+        'Incorrect message set wire format. '
+        'wire_type: %d, field_number: %d' % (wire_type, field_number))
+
+  extension_dict = message.Extensions
+  extensions_by_number = extension_dict._AllExtensionsByNumber()
+  if type_id not in extensions_by_number:
+    _SkipField(field_number, wire_type, decoder)
+    return
+
+  field_descriptor = extensions_by_number[type_id]
+  value = extension_dict[field_descriptor]
+  decoder.ReadMessageInto(value)
+  # Read the END_GROUP tag.
+  field_number, wire_type = decoder.ReadFieldNumberAndWireType()
+  if wire_type != wire_format.WIRETYPE_END_GROUP or field_number != 1:
+    raise message_mod.DecodeError(
+        'Incorrect message set wire format. '
+        'wire_type: %d, field_number: %d' % (wire_type, field_number))
+
+
+def _DeserializeOneEntity(message_descriptor, message, decoder):
+  """Deserializes the next wire entity from decoder into message.
+  The next wire entity is either a scalar or a nested message,
+  and may also be an element in a repeated field (the wire encoding
+  is the same).
+
+  Args:
+    message_descriptor: A Descriptor instance describing all fields
+      in message.
+    message: The Message instance into which we're decoding our fields.
+    decoder: The Decoder we're using to deserialize encoded data.
+
+  Returns: The number of bytes read from decoder during this method.
+  """
+  initial_position = decoder.Position()
+  field_number, wire_type = decoder.ReadFieldNumberAndWireType()
+  extension_dict = message.Extensions
+  extensions_by_number = extension_dict._AllExtensionsByNumber()
+  if field_number in message_descriptor.fields_by_number:
+    # Non-extension field.
+    field_descriptor = message_descriptor.fields_by_number[field_number]
+    value = getattr(message, _PropertyName(field_descriptor.name))
+    def nonextension_setter_fn(scalar):
+      setattr(message, _PropertyName(field_descriptor.name), scalar)
+    scalar_setter_fn = nonextension_setter_fn
+  elif field_number in extensions_by_number:
+    # Extension field.
+    field_descriptor = extensions_by_number[field_number]
+    value = extension_dict[field_descriptor]
+    def extension_setter_fn(scalar):
+      extension_dict[field_descriptor] = scalar
+    scalar_setter_fn = extension_setter_fn
+  elif wire_type == wire_format.WIRETYPE_END_GROUP:
+    # We assume we're being parsed as the group that's ended.
+    return 0
+  elif (wire_type == wire_format.WIRETYPE_START_GROUP and
+        field_number == 1 and
+        message_descriptor.GetOptions().message_set_wire_format):
+    # A Message Set item.
+    _DeserializeMessageSetItem(message, decoder)
+    return decoder.Position() - initial_position
+  else:
+    _SkipField(field_number, wire_type, decoder)
+    return decoder.Position() - initial_position
+
+  # If we reach this point, we've identified the field as either
+  # hardcoded or extension, and set |field_descriptor|, |scalar_setter_fn|,
+  # and |value| appropriately.  Now actually deserialize the thing.
+  #
+  # field_descriptor: Describes the field we're deserializing.
+  # value: The value currently stored in the field to deserialize.
+  #   Used only if the field is composite and/or repeated.
+  # scalar_setter_fn: A function F such that F(scalar) will
+  #   set a nonrepeated scalar value for this field.  Used only
+  #   if this field is a nonrepeated scalar.
+
+  field_number = field_descriptor.number
+  field_type = field_descriptor.type
+  expected_wire_type = _WireTypeForFieldType(field_type)
+  if wire_type != expected_wire_type:
+    # Need to fill in uninterpreted_bytes.  Work for the next CL.
+    raise RuntimeError('TODO(robinson): Wiretype mismatches not handled.')
+
+  property_name = _PropertyName(field_descriptor.name)
+  label = field_descriptor.label
+  cpp_type = field_descriptor.cpp_type
+
+  # Nonrepeated scalar.  Just set the field directly.
+  if (label != _FieldDescriptor.LABEL_REPEATED
+      and cpp_type != _FieldDescriptor.CPPTYPE_MESSAGE):
+    scalar_setter_fn(_DeserializeScalarFromDecoder(field_type, decoder))
+    return decoder.Position() - initial_position
+
+  # Nonrepeated composite.  Recursively deserialize.
+  if label != _FieldDescriptor.LABEL_REPEATED:
+    composite = value
+    _RecursivelyMerge(field_number, field_type, decoder, composite)
+    return decoder.Position() - initial_position
+
+  # Now we know we're dealing with a repeated field of some kind.
+  element_list = value
+
+  if cpp_type != _FieldDescriptor.CPPTYPE_MESSAGE:
+    # Repeated scalar.
+    element_list.append(_DeserializeScalarFromDecoder(field_type, decoder))
+    return decoder.Position() - initial_position
+  else:
+    # Repeated composite.
+    composite = element_list.add()
+    _RecursivelyMerge(field_number, field_type, decoder, composite)
+    return decoder.Position() - initial_position
+
+
+def _FieldOrExtensionValues(message, field_or_extension):
+  """Retrieves the list of values for the specified field or extension.
+
+  The target field or extension can be optional, required or repeated, but it
+  must have value(s) set. The assumption is that the target field or extension
+  is set (e.g. _HasFieldOrExtension holds true).
+
+  Args:
+    message: Message which contains the target field or extension.
+    field_or_extension: Field or extension for which the list of values is
+      required. Must be an instance of FieldDescriptor.
+
+  Returns:
+    A list of values for the specified field or extension. This list will only
+    contain a single element if the field is non-repeated.
+  """
+  if field_or_extension.is_extension:
+    value = message.Extensions[field_or_extension]
+  else:
+    value = getattr(message, _ValueFieldName(field_or_extension.name))
+  if field_or_extension.label != _FieldDescriptor.LABEL_REPEATED:
+    return [value]
+  else:
+    # In this case value is a list or repeated values.
+    return value
+
+
+def _HasFieldOrExtension(message, field_or_extension):
+  """Checks if a message has the specified field or extension set.
+
+  The field or extension specified can be optional, required or repeated. If
+  it is repeated, this function returns True. Otherwise it checks the has bit
+  of the field or extension.
+
+  Args:
+    message: Message which contains the target field or extension.
+    field_or_extension: Field or extension to check. This must be a
+      FieldDescriptor instance.
+
+  Returns:
+    True if the message has a value set for the specified field or extension,
+    or if the field or extension is repeated.
+  """
+  if field_or_extension.label == _FieldDescriptor.LABEL_REPEATED:
+    return True
+  if field_or_extension.is_extension:
+    return message.HasExtension(field_or_extension)
+  else:
+    return message.HasField(field_or_extension.name)
+
+
+def _IsFieldOrExtensionInitialized(message, field, errors=None):
+  """Checks if a message field or extension is initialized.
+
+  Args:
+    message: The message which contains the field or extension.
+    field: Field or extension to check. This must be a FieldDescriptor instance.
+    errors: Errors will be appended to it, if set to a meaningful value.
+
+  Returns:
+    True if the field/extension can be considered initialized.
+  """
+  # If the field is required and is not set, it isn't initialized.
+  if field.label == _FieldDescriptor.LABEL_REQUIRED:
+    if not _HasFieldOrExtension(message, field):
+      if errors is not None:
+        errors.append('Required field %s is not set.' % field.full_name)
+      return False
+
+  # If the field is optional and is not set, or if it
+  # isn't a submessage then the field is initialized.
+  if field.label == _FieldDescriptor.LABEL_OPTIONAL:
+    if not _HasFieldOrExtension(message, field):
+      return True
+  if field.cpp_type != _FieldDescriptor.CPPTYPE_MESSAGE:
+    return True
+
+  # The field is set and is either a single or a repeated submessage.
+  messages = _FieldOrExtensionValues(message, field)
+  # If all submessages in this field are initialized, the field is
+  # considered initialized.
+  for message in messages:
+    if not _InternalIsInitialized(message, errors):
+      return False
+  return True
+
+
+def _InternalIsInitialized(message, errors=None):
+  """Checks if all required fields of a message are set.
+
+  Args:
+    message: The message to check.
+    errors: If set, initialization errors will be appended to it.
+
+  Returns:
+    True iff the specified message has all required fields set.
+  """
+  fields_and_extensions = []
+  fields_and_extensions.extend(message.DESCRIPTOR.fields)
+  fields_and_extensions.extend(
+      [extension[0] for extension in message.Extensions._ListSetExtensions()])
+  for field_or_extension in fields_and_extensions:
+    if not _IsFieldOrExtensionInitialized(message, field_or_extension, errors):
+      return False
+  return True
+
+
+def _AddMergeFromStringMethod(message_descriptor, cls):
+  """Helper for _AddMessageMethods()."""
+  Decoder = decoder.Decoder
+  def MergeFromString(self, serialized):
+    decoder = Decoder(serialized)
+    byte_count = 0
+    while not decoder.EndOfStream():
+      bytes_read = _DeserializeOneEntity(message_descriptor, self, decoder)
+      if not bytes_read:
+        break
+      byte_count += bytes_read
+    return byte_count
+  cls.MergeFromString = MergeFromString
+
+
+def _AddIsInitializedMethod(cls):
+  """Adds the IsInitialized method to the protocol message class."""
+  cls.IsInitialized = _InternalIsInitialized
+
+
+def _MergeFieldOrExtension(destination_msg, field, value):
+  """Merges a specified message field into another message."""
+  property_name = _PropertyName(field.name)
+  is_extension = field.is_extension
+
+  if not is_extension:
+    destination = getattr(destination_msg, property_name)
+  elif (field.label == _FieldDescriptor.LABEL_REPEATED or
+        field.cpp_type == _FieldDescriptor.CPPTYPE_MESSAGE):
+    destination = destination_msg.Extensions[field]
+
+  # Case 1 - a composite field.
+  if field.cpp_type == _FieldDescriptor.CPPTYPE_MESSAGE:
+    if field.label == _FieldDescriptor.LABEL_REPEATED:
+      for v in value:
+        destination.add().MergeFrom(v)
+    else:
+      destination.MergeFrom(value)
+    return
+
+  # Case 2 - a repeated field.
+  if field.label == _FieldDescriptor.LABEL_REPEATED:
+    for v in value:
+      destination.append(v)
+    return
+
+  # Case 3 - a singular field.
+  if is_extension:
+    destination_msg.Extensions[field] = value
+  else:
+    setattr(destination_msg, property_name, value)
+
+
+def _AddMergeFromMethod(cls):
+  def MergeFrom(self, msg):
+    assert msg is not self
+    for field in msg.ListFields():
+      _MergeFieldOrExtension(self, field[0], field[1])
+  cls.MergeFrom = MergeFrom
+
+
+def _AddMessageMethods(message_descriptor, cls):
+  """Adds implementations of all Message methods to cls."""
+  _AddListFieldsMethod(message_descriptor, cls)
+  _AddHasFieldMethod(cls)
+  _AddClearFieldMethod(cls)
+  _AddClearExtensionMethod(cls)
+  _AddClearMethod(cls)
+  _AddHasExtensionMethod(cls)
+  _AddEqualsMethod(message_descriptor, cls)
+  _AddSetListenerMethod(cls)
+  _AddByteSizeMethod(message_descriptor, cls)
+  _AddSerializeToStringMethod(message_descriptor, cls)
+  _AddSerializePartialToStringMethod(message_descriptor, cls)
+  _AddMergeFromStringMethod(message_descriptor, cls)
+  _AddIsInitializedMethod(cls)
+  _AddMergeFromMethod(cls)
+
+
+def _AddPrivateHelperMethods(cls):
+  """Adds implementation of private helper methods to cls."""
+
+  def MaybeCallTransitionToNonemptyCallback(self):
+    """Calls self._listener.TransitionToNonempty() the first time this
+    method is called.  On all subsequent calls, this is a no-op.
+    """
+    if not self._called_transition_to_nonempty:
+      self._listener.TransitionToNonempty()
+      self._called_transition_to_nonempty = True
+  cls._MaybeCallTransitionToNonemptyCallback = (
+      MaybeCallTransitionToNonemptyCallback)
+
+  def MarkByteSizeDirty(self):
+    """Sets the _cached_byte_size_dirty bit to true,
+    and propagates this to our listener iff this was a state change.
+    """
+    if not self._cached_byte_size_dirty:
+      self._cached_byte_size_dirty = True
+      self._listener.ByteSizeDirty()
+  cls._MarkByteSizeDirty = MarkByteSizeDirty
+
+
+class _Listener(object):
+
+  """MessageListener implementation that a parent message registers with its
+  child message.
+
+  In order to support semantics like:
+
+    foo.bar.baz = 23
+    assert foo.HasField('bar')
+
+  ...child objects must have back references to their parents.
+  This helper class is at the heart of this support.
+  """
+
+  def __init__(self, parent_message, has_field_name):
+    """Args:
+      parent_message: The message whose _MaybeCallTransitionToNonemptyCallback()
+        and _MarkByteSizeDirty() methods we should call when we receive
+        TransitionToNonempty() and ByteSizeDirty() messages.
+      has_field_name: The name of the "has" field that we should set in
+        the parent message when we receive a TransitionToNonempty message,
+        or None if there's no "has" field to set.  (This will be the case
+        for child objects in "repeated" fields).
+    """
+    # This listener establishes a back reference from a child (contained) object
+    # to its parent (containing) object.  We make this a weak reference to avoid
+    # creating cyclic garbage when the client finishes with the 'parent' object
+    # in the tree.
+    if isinstance(parent_message, weakref.ProxyType):
+      self._parent_message_weakref = parent_message
+    else:
+      self._parent_message_weakref = weakref.proxy(parent_message)
+    self._has_field_name = has_field_name
+
+  def TransitionToNonempty(self):
+    try:
+      if self._has_field_name is not None:
+        setattr(self._parent_message_weakref, self._has_field_name, True)
+      # Propagate the signal to our parents iff this is the first field set.
+      self._parent_message_weakref._MaybeCallTransitionToNonemptyCallback()
+    except ReferenceError:
+      # We can get here if a client has kept a reference to a child object,
+      # and is now setting a field on it, but the child's parent has been
+      # garbage-collected.  This is not an error.
+      pass
+
+  def ByteSizeDirty(self):
+    try:
+      self._parent_message_weakref._MarkByteSizeDirty()
+    except ReferenceError:
+      # Same as above.
+      pass
+
+
+# TODO(robinson): Move elsewhere?
+# TODO(robinson): Provide a clear() method here in addition to ClearField()?
+class _RepeatedScalarFieldContainer(object):
+
+  """Simple, type-checked, list-like container for holding repeated scalars."""
+
+  # Minimizes memory usage and disallows assignment to other attributes.
+  __slots__ = ['_message_listener', '_type_checker', '_values']
+
+  def __init__(self, message_listener, type_checker):
+    """
+    Args:
+      message_listener: A MessageListener implementation.
+        The _RepeatedScalarFieldContaininer will call this object's
+        TransitionToNonempty() method when it transitions from being empty to
+        being nonempty.
+      type_checker: A _ValueChecker instance to run on elements inserted
+        into this container.
+    """
+    self._message_listener = message_listener
+    self._type_checker = type_checker
+    self._values = []
+
+  def append(self, elem):
+    self._type_checker.CheckValue(elem)
+    self._values.append(elem)
+    self._message_listener.ByteSizeDirty()
+    if len(self._values) == 1:
+      self._message_listener.TransitionToNonempty()
+
+  def remove(self, elem):
+    self._values.remove(elem)
+    self._message_listener.ByteSizeDirty()
+
+  # List-like __getitem__() support also makes us iterable (via "iter(foo)"
+  # or implicitly via "for i in mylist:") for free.
+  def __getitem__(self, key):
+    return self._values[key]
+
+  def __setitem__(self, key, value):
+    # No need to call TransitionToNonempty(), since if we're able to
+    # set the element at this index, we were already nonempty before
+    # this method was called.
+    self._message_listener.ByteSizeDirty()
+    self._type_checker.CheckValue(value)
+    self._values[key] = value
+
+  def __len__(self):
+    return len(self._values)
+
+  def __eq__(self, other):
+    if self is other:
+      return True
+    # Special case for the same type which should be common and fast.
+    if isinstance(other, self.__class__):
+      return other._values == self._values
+    # We are presumably comparing against some other sequence type.
+    return other == self._values
+
+  def __ne__(self, other):
+    # Can't use != here since it would infinitely recurse.
+    return not self == other
+
+
+# TODO(robinson): Move elsewhere?
+# TODO(robinson): Provide a clear() method here in addition to ClearField()?
+# TODO(robinson): Unify common functionality with
+# _RepeatedScalarFieldContaininer?
+class _RepeatedCompositeFieldContainer(object):
+
+  """Simple, list-like container for holding repeated composite fields."""
+
+  # Minimizes memory usage and disallows assignment to other attributes.
+  __slots__ = ['_values', '_message_descriptor', '_message_listener']
+
+  def __init__(self, message_listener, message_descriptor):
+    """Note that we pass in a descriptor instead of the generated directly,
+    since at the time we construct a _RepeatedCompositeFieldContainer we
+    haven't yet necessarily initialized the type that will be contained in the
+    container.
+
+    Args:
+      message_listener: A MessageListener implementation.
+        The _RepeatedCompositeFieldContainer will call this object's
+        TransitionToNonempty() method when it transitions from being empty to
+        being nonempty.
+      message_descriptor: A Descriptor instance describing the protocol type
+        that should be present in this container.  We'll use the
+        _concrete_class field of this descriptor when the client calls add().
+    """
+    self._message_listener = message_listener
+    self._message_descriptor = message_descriptor
+    self._values = []
+
+  def add(self):
+    new_element = self._message_descriptor._concrete_class()
+    new_element._SetListener(self._message_listener)
+    self._values.append(new_element)
+    self._message_listener.ByteSizeDirty()
+    self._message_listener.TransitionToNonempty()
+    return new_element
+
+  def __delitem__(self, key):
+    self._message_listener.ByteSizeDirty()
+    del self._values[key]
+
+  # List-like __getitem__() support also makes us iterable (via "iter(foo)"
+  # or implicitly via "for i in mylist:") for free.
+  def __getitem__(self, key):
+    return self._values[key]
+
+  def __len__(self):
+    return len(self._values)
+
+  def __eq__(self, other):
+    if self is other:
+      return True
+    if not isinstance(other, self.__class__):
+      raise TypeError('Can only compare repeated composite fields against '
+                      'other repeated composite fields.')
+    return self._values == other._values
+
+  def __ne__(self, other):
+    # Can't use != here since it would infinitely recurse.
+    return not self == other
+
+  # TODO(robinson): Implement, document, and test slicing support.
+
+
+# TODO(robinson): Move elsewhere?  This file is getting pretty ridiculous...
+# TODO(robinson): Unify error handling of "unknown extension" crap.
+# TODO(robinson): There's so much similarity between the way that
+# extensions behave and the way that normal fields behave that it would
+# be really nice to unify more code.  It's not immediately obvious
+# how to do this, though, and I'd rather get the full functionality
+# implemented (and, crucially, get all the tests and specs fleshed out
+# and passing), and then come back to this thorny unification problem.
+# TODO(robinson): Support iteritems()-style iteration over all
+# extensions with the "has" bits turned on?
+class _ExtensionDict(object):
+
+  """Dict-like container for supporting an indexable "Extensions"
+  field on proto instances.
+
+  Note that in all cases we expect extension handles to be
+  FieldDescriptors.
+  """
+
+  class _ExtensionListener(object):
+
+    """Adapts an _ExtensionDict to behave as a MessageListener."""
+
+    def __init__(self, extension_dict, handle_id):
+      self._extension_dict = extension_dict
+      self._handle_id = handle_id
+
+    def TransitionToNonempty(self):
+      self._extension_dict._SubmessageTransitionedToNonempty(self._handle_id)
+
+    def ByteSizeDirty(self):
+      self._extension_dict._SubmessageByteSizeBecameDirty()
+
+  # TODO(robinson): Somewhere, we need to blow up if people
+  # try to register two extensions with the same field number.
+  # (And we need a test for this of course).
+
+  def __init__(self, extended_message, known_extensions):
+    """extended_message: Message instance for which we are the Extensions dict.
+      known_extensions: Iterable of known extension handles.
+        These must be FieldDescriptors.
+    """
+    # We keep a weak reference to extended_message, since
+    # it has a reference to this instance in turn.
+    self._extended_message = weakref.proxy(extended_message)
+    # We make a deep copy of known_extensions to avoid any
+    # thread-safety concerns, since the argument passed in
+    # is the global (class-level) dict of known extensions for
+    # this type of message, which could be modified at any time
+    # via a RegisterExtension() call.
+    #
+    # This dict maps from handle id to handle (a FieldDescriptor).
+    #
+    # XXX
+    # TODO(robinson): This isn't good enough.  The client could
+    # instantiate an object in module A, then afterward import
+    # module B and pass the instance to B.Foo().  If B imports
+    # an extender of this proto and then tries to use it, B
+    # will get a KeyError, even though the extension *is* registered
+    # at the time of use.
+    # XXX
+    self._known_extensions = dict((id(e), e) for e in known_extensions)
+    # Read lock around self._values, which may be modified by multiple
+    # concurrent readers in the conceptually "const" __getitem__ method.
+    # So, we grab this lock in every "read-only" method to ensure
+    # that concurrent read access is safe without external locking.
+    self._lock = threading.Lock()
+    # Maps from extension handle ID to current value of that extension.
+    self._values = {}
+    # Maps from extension handle ID to a boolean "has" bit, but only
+    # for non-repeated extension fields.
+    keys = (id for id, extension in self._known_extensions.iteritems()
+            if extension.label != _FieldDescriptor.LABEL_REPEATED)
+    self._has_bits = dict.fromkeys(keys, False)
+
+  def __getitem__(self, extension_handle):
+    """Returns the current value of the given extension handle."""
+    # We don't care as much about keeping critical sections short in the
+    # extension support, since it's presumably much less of a common case.
+    self._lock.acquire()
+    try:
+      handle_id = id(extension_handle)
+      if handle_id not in self._known_extensions:
+        raise KeyError('Extension not known to this class')
+      if handle_id not in self._values:
+        self._AddMissingHandle(extension_handle, handle_id)
+      return self._values[handle_id]
+    finally:
+      self._lock.release()
+
+  def __eq__(self, other):
+    # We have to grab read locks since we're accessing _values
+    # in a "const" method.  See the comment in the constructor.
+    if self is other:
+      return True
+    self._lock.acquire()
+    try:
+      other._lock.acquire()
+      try:
+        if self._has_bits != other._has_bits:
+          return False
+        # If there's a "has" bit, then only compare values where it is true.
+        for k, v in self._values.iteritems():
+          if self._has_bits.get(k, False) and v != other._values[k]:
+            return False
+        return True
+      finally:
+        other._lock.release()
+    finally:
+      self._lock.release()
+
+  def __ne__(self, other):
+    return not self == other
+
+  # Note that this is only meaningful for non-repeated, scalar extension
+  # fields.  Note also that we may have to call
+  # MaybeCallTransitionToNonemptyCallback() when we do successfully set a field
+  # this way, to set any necssary "has" bits in the ancestors of the extended
+  # message.
+  def __setitem__(self, extension_handle, value):
+    """If extension_handle specifies a non-repeated, scalar extension
+    field, sets the value of that field.
+    """
+    handle_id = id(extension_handle)
+    if handle_id not in self._known_extensions:
+      raise KeyError('Extension not known to this class')
+    field = extension_handle  # Just shorten the name.
+    if (field.label == _FieldDescriptor.LABEL_OPTIONAL
+        and field.cpp_type != _FieldDescriptor.CPPTYPE_MESSAGE):
+      # It's slightly wasteful to lookup the type checker each time,
+      # but we expect this to be a vanishingly uncommon case anyway.
+      type_checker = type_checkers.GetTypeChecker(field.cpp_type, field.type)
+      type_checker.CheckValue(value)
+      self._values[handle_id] = value
+      self._has_bits[handle_id] = True
+      self._extended_message._MarkByteSizeDirty()
+      self._extended_message._MaybeCallTransitionToNonemptyCallback()
+    else:
+      raise TypeError('Extension is repeated and/or a composite type.')
+
+  def _AddMissingHandle(self, extension_handle, handle_id):
+    """Helper internal to ExtensionDict."""
+    # Special handling for non-repeated message extensions, which (like
+    # normal fields of this kind) are initialized lazily.
+    # REQUIRES: _lock already held.
+    cpp_type = extension_handle.cpp_type
+    label = extension_handle.label
+    if (cpp_type == _FieldDescriptor.CPPTYPE_MESSAGE
+        and label != _FieldDescriptor.LABEL_REPEATED):
+      self._AddMissingNonRepeatedCompositeHandle(extension_handle, handle_id)
+    else:
+      self._values[handle_id] = _DefaultValueForField(
+          self._extended_message, extension_handle)
+
+  def _AddMissingNonRepeatedCompositeHandle(self, extension_handle, handle_id):
+    """Helper internal to ExtensionDict."""
+    # REQUIRES: _lock already held.
+    value = extension_handle.message_type._concrete_class()
+    value._SetListener(_ExtensionDict._ExtensionListener(self, handle_id))
+    self._values[handle_id] = value
+
+  def _SubmessageTransitionedToNonempty(self, handle_id):
+    """Called when a submessage with a given handle id first transitions to
+    being nonempty.  Called by _ExtensionListener.
+    """
+    assert handle_id in self._has_bits
+    self._has_bits[handle_id] = True
+    self._extended_message._MaybeCallTransitionToNonemptyCallback()
+
+  def _SubmessageByteSizeBecameDirty(self):
+    """Called whenever a submessage's cached byte size becomes invalid
+    (goes from being "clean" to being "dirty").  Called by _ExtensionListener.
+    """
+    self._extended_message._MarkByteSizeDirty()
+
+  # We may wish to widen the public interface of Message.Extensions
+  # to expose some of this private functionality in the future.
+  # For now, we make all this functionality module-private and just
+  # implement what we need for serialization/deserialization,
+  # HasField()/ClearField(), etc.
+
+  def _HasExtension(self, extension_handle):
+    """Method for internal use by this module.
+    Returns true iff we "have" this extension in the sense of the
+    "has" bit being set.
+    """
+    handle_id = id(extension_handle)
+    # Note that this is different from the other checks.
+    if handle_id not in self._has_bits:
+      raise KeyError('Extension not known to this class, or is repeated field.')
+    return self._has_bits[handle_id]
+
+  # Intentionally pretty similar to ClearField() above.
+  def _ClearExtension(self, extension_handle):
+    """Method for internal use by this module.
+    Clears the specified extension, unsetting its "has" bit.
+    """
+    handle_id = id(extension_handle)
+    if handle_id not in self._known_extensions:
+      raise KeyError('Extension not known to this class')
+    default_value = _DefaultValueForField(self._extended_message,
+                                          extension_handle)
+    if extension_handle.label == _FieldDescriptor.LABEL_REPEATED:
+      self._extended_message._MarkByteSizeDirty()
+    else:
+      cpp_type = extension_handle.cpp_type
+      if cpp_type == _FieldDescriptor.CPPTYPE_MESSAGE:
+        if handle_id in self._values:
+          # Future modifications to this object shouldn't set any
+          # "has" bits here.
+          self._values[handle_id]._SetListener(None)
+      if self._has_bits[handle_id]:
+        self._has_bits[handle_id] = False
+        self._extended_message._MarkByteSizeDirty()
+    if handle_id in self._values:
+      del self._values[handle_id]
+
+  def _ListSetExtensions(self):
+    """Method for internal use by this module.
+
+    Returns an sequence of all extensions that are currently "set"
+    in this extension dict.  A "set" extension is a repeated extension,
+    or a non-repeated extension with its "has" bit set.
+
+    The returned sequence contains (field_descriptor, value) pairs,
+    where value is the current value of the extension with the given
+    field descriptor.
+
+    The sequence values are in arbitrary order.
+    """
+    self._lock.acquire()  # Read-only methods must lock around self._values.
+    try:
+      set_extensions = []
+      for handle_id, value in self._values.iteritems():
+        handle = self._known_extensions[handle_id]
+        if (handle.label == _FieldDescriptor.LABEL_REPEATED
+            or self._has_bits[handle_id]):
+          set_extensions.append((handle, value))
+      return set_extensions
+    finally:
+      self._lock.release()
+
+  def _AllExtensionsByNumber(self):
+    """Method for internal use by this module.
+
+    Returns: A dict mapping field_number to (handle, field_descriptor),
+      for *all* registered extensions for this dict.
+    """
+    # TODO(robinson): Precompute and store this away.  Note that we'll have to
+    # be careful when we move away from having _known_extensions as a
+    # deep-copied member of this object.
+    return dict((f.number, f) for f in self._known_extensions.itervalues())