diff --git a/python/google/protobuf/message.py b/python/google/protobuf/message.py
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+# 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.
+
+# TODO(robinson): We should just make these methods all "pure-virtual" and move
+# all implementation out, into reflection.py for now.
+
+
+"""Contains an abstract base class for protocol messages."""
+
+__author__ = 'robinson@google.com (Will Robinson)'
+
+class Error(Exception): pass
+class DecodeError(Error): pass
+class EncodeError(Error): pass
+
+
+class Message(object):
+
+  """Abstract base class for protocol messages.
+
+  Protocol message classes are almost always generated by the protocol
+  compiler.  These generated types subclass Message and implement the methods
+  shown below.
+
+  TODO(robinson): Link to an HTML document here.
+
+  TODO(robinson): Document that instances of this class will also
+  have an Extensions attribute with __getitem__ and __setitem__.
+  Again, not sure how to best convey this.
+
+  TODO(robinson): Document that the class must also have a static
+    RegisterExtension(extension_field) method.
+    Not sure how to best express at this point.
+  """
+
+  # TODO(robinson): Document these fields and methods.
+
+  __slots__ = []
+
+  DESCRIPTOR = None
+
+  def __deepcopy__(self, memo=None):
+    clone = type(self)()
+    clone.MergeFrom(self)
+    return clone
+
+  def __eq__(self, other_msg):
+    """Recursively compares two messages by value and structure."""
+    raise NotImplementedError
+
+  def __ne__(self, other_msg):
+    # Can't just say self != other_msg, since that would infinitely recurse. :)
+    return not self == other_msg
+
+  def __hash__(self):
+    raise TypeError('unhashable object')
+
+  def __str__(self):
+    """Outputs a human-readable representation of the message."""
+    raise NotImplementedError
+
+  def __unicode__(self):
+    """Outputs a human-readable representation of the message."""
+    raise NotImplementedError
+
+  def MergeFrom(self, other_msg):
+    """Merges the contents of the specified message into current message.
+
+    This method merges the contents of the specified message into the current
+    message. Singular fields that are set in the specified message overwrite
+    the corresponding fields in the current message. Repeated fields are
+    appended. Singular sub-messages and groups are recursively merged.
+
+    Args:
+      other_msg: Message to merge into the current message.
+    """
+    raise NotImplementedError
+
+  def CopyFrom(self, other_msg):
+    """Copies the content of the specified message into the current message.
+
+    The method clears the current message and then merges the specified
+    message using MergeFrom.
+
+    Args:
+      other_msg: Message to copy into the current one.
+    """
+    if self is other_msg:
+      return
+    self.Clear()
+    self.MergeFrom(other_msg)
+
+  def Clear(self):
+    """Clears all data that was set in the message."""
+    raise NotImplementedError
+
+  def SetInParent(self):
+    """Mark this as present in the parent.
+
+    This normally happens automatically when you assign a field of a
+    sub-message, but sometimes you want to make the sub-message
+    present while keeping it empty.  If you find yourself using this,
+    you may want to reconsider your design."""
+    raise NotImplementedError
+
+  def IsInitialized(self):
+    """Checks if the message is initialized.
+
+    Returns:
+      The method returns True if the message is initialized (i.e. all of its
+      required fields are set).
+    """
+    raise NotImplementedError
+
+  # TODO(robinson): MergeFromString() should probably return None and be
+  # implemented in terms of a helper that returns the # of bytes read.  Our
+  # deserialization routines would use the helper when recursively
+  # deserializing, but the end user would almost always just want the no-return
+  # MergeFromString().
+
+  def MergeFromString(self, serialized):
+    """Merges serialized protocol buffer data into this message.
+
+    When we find a field in |serialized| that is already present
+    in this message:
+      - If it's a "repeated" field, we append to the end of our list.
+      - Else, if it's a scalar, we overwrite our field.
+      - Else, (it's a nonrepeated composite), we recursively merge
+        into the existing composite.
+
+    TODO(robinson): Document handling of unknown fields.
+
+    Args:
+      serialized: Any object that allows us to call buffer(serialized)
+        to access a string of bytes using the buffer interface.
+
+    TODO(robinson): When we switch to a helper, this will return None.
+
+    Returns:
+      The number of bytes read from |serialized|.
+      For non-group messages, this will always be len(serialized),
+      but for messages which are actually groups, this will
+      generally be less than len(serialized), since we must
+      stop when we reach an END_GROUP tag.  Note that if
+      we *do* stop because of an END_GROUP tag, the number
+      of bytes returned does not include the bytes
+      for the END_GROUP tag information.
+    """
+    raise NotImplementedError
+
+  def ParseFromString(self, serialized):
+    """Parse serialized protocol buffer data into this message.
+
+    Like MergeFromString(), except we clear the object first and
+    do not return the value that MergeFromString returns.
+    """
+    self.Clear()
+    self.MergeFromString(serialized)
+
+  def SerializeToString(self):
+    """Serializes the protocol message to a binary string.
+
+    Returns:
+      A binary string representation of the message if all of the required
+      fields in the message are set (i.e. the message is initialized).
+
+    Raises:
+      message.EncodeError if the message isn't initialized.
+    """
+    raise NotImplementedError
+
+  def SerializePartialToString(self):
+    """Serializes the protocol message to a binary string.
+
+    This method is similar to SerializeToString but doesn't check if the
+    message is initialized.
+
+    Returns:
+      A string representation of the partial message.
+    """
+    raise NotImplementedError
+
+  # TODO(robinson): Decide whether we like these better
+  # than auto-generated has_foo() and clear_foo() methods
+  # on the instances themselves.  This way is less consistent
+  # with C++, but it makes reflection-type access easier and
+  # reduces the number of magically autogenerated things.
+  #
+  # TODO(robinson): Be sure to document (and test) exactly
+  # which field names are accepted here.  Are we case-sensitive?
+  # What do we do with fields that share names with Python keywords
+  # like 'lambda' and 'yield'?
+  #
+  # nnorwitz says:
+  # """
+  # Typically (in python), an underscore is appended to names that are
+  # keywords. So they would become lambda_ or yield_.
+  # """
+  def ListFields(self):
+    """Returns a list of (FieldDescriptor, value) tuples for all
+    fields in the message which are not empty.  A singular field is non-empty
+    if HasField() would return true, and a repeated field is non-empty if
+    it contains at least one element.  The fields are ordered by field
+    number"""
+    raise NotImplementedError
+
+  def HasField(self, field_name):
+    """Checks if a certain field is set for the message, or if any field inside
+    a oneof group is set.  Note that if the field_name is not defined in the
+    message descriptor, ValueError will be raised."""
+    raise NotImplementedError
+
+  def ClearField(self, field_name):
+    """Clears the contents of a given field, or the field set inside a oneof
+    group.  If the name neither refers to a defined field or oneof group,
+    ValueError is raised."""
+    raise NotImplementedError
+
+  def WhichOneof(self, oneof_group):
+    """Returns the name of the field that is set inside a oneof group, or
+    None if no field is set.  If no group with the given name exists, ValueError
+    will be raised."""
+    raise NotImplementedError
+
+  def HasExtension(self, extension_handle):
+    raise NotImplementedError
+
+  def ClearExtension(self, extension_handle):
+    raise NotImplementedError
+
+  def ByteSize(self):
+    """Returns the serialized size of this message.
+    Recursively calls ByteSize() on all contained messages.
+    """
+    raise NotImplementedError
+
+  def _SetListener(self, message_listener):
+    """Internal method used by the protocol message implementation.
+    Clients should not call this directly.
+
+    Sets a listener that this message will call on certain state transitions.
+
+    The purpose of this method is to register back-edges from children to
+    parents at runtime, for the purpose of setting "has" bits and
+    byte-size-dirty bits in the parent and ancestor objects whenever a child or
+    descendant object is modified.
+
+    If the client wants to disconnect this Message from the object tree, she
+    explicitly sets callback to None.
+
+    If message_listener is None, unregisters any existing listener.  Otherwise,
+    message_listener must implement the MessageListener interface in
+    internal/message_listener.py, and we discard any listener registered
+    via a previous _SetListener() call.
+    """
+    raise NotImplementedError
+
+  def __getstate__(self):
+    """Support the pickle protocol."""
+    return dict(serialized=self.SerializePartialToString())
+
+  def __setstate__(self, state):
+    """Support the pickle protocol."""
+    self.__init__()
+    self.ParseFromString(state['serialized'])