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diff --git a/python/google/protobuf/internal/output_stream.py b/python/google/protobuf/internal/output_stream.py
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+++ b/python/google/protobuf/internal/output_stream.py
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+# Protocol Buffers - Google's data interchange format
+# Copyright 2008 Google Inc.
+# http://code.google.com/p/protobuf/
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""OutputStream is the primitive interface for sticking bits on the wire.
+
+All protocol buffer serialization can be expressed in terms of
+the OutputStream primitives provided here.
+"""
+
+__author__ = 'robinson@google.com (Will Robinson)'
+
+import array
+import struct
+from google.protobuf import message
+from google.protobuf.internal import wire_format
+
+
+
+# Note that much of this code is ported from //net/proto/ProtocolBuffer, and
+# that the interface is strongly inspired by CodedOutputStream from the C++
+# proto2 implementation.
+
+
+class OutputStream(object):
+
+  """Contains all logic for writing bits, and ToString() to get the result."""
+
+  def __init__(self):
+    self._buffer = array.array('B')
+
+  def AppendRawBytes(self, raw_bytes):
+    """Appends raw_bytes to our internal buffer."""
+    self._buffer.fromstring(raw_bytes)
+
+  def AppendLittleEndian32(self, unsigned_value):
+    """Appends an unsigned 32-bit integer to the internal buffer,
+    in little-endian byte order.
+    """
+    if not 0 <= unsigned_value <= wire_format.UINT32_MAX:
+      raise message.EncodeError(
+          'Unsigned 32-bit out of range: %d' % unsigned_value)
+    self._buffer.fromstring(struct.pack(
+        wire_format.FORMAT_UINT32_LITTLE_ENDIAN, unsigned_value))
+
+  def AppendLittleEndian64(self, unsigned_value):
+    """Appends an unsigned 64-bit integer to the internal buffer,
+    in little-endian byte order.
+    """
+    if not 0 <= unsigned_value <= wire_format.UINT64_MAX:
+      raise message.EncodeError(
+          'Unsigned 64-bit out of range: %d' % unsigned_value)
+    self._buffer.fromstring(struct.pack(
+        wire_format.FORMAT_UINT64_LITTLE_ENDIAN, unsigned_value))
+
+  def AppendVarint32(self, value):
+    """Appends a signed 32-bit integer to the internal buffer,
+    encoded as a varint.  (Note that a negative varint32 will
+    always require 10 bytes of space.)
+    """
+    if not wire_format.INT32_MIN <= value <= wire_format.INT32_MAX:
+      raise message.EncodeError('Value out of range: %d' % value)
+    self.AppendVarint64(value)
+
+  def AppendVarUInt32(self, value):
+    """Appends an unsigned 32-bit integer to the internal buffer,
+    encoded as a varint.
+    """
+    if not 0 <= value <= wire_format.UINT32_MAX:
+      raise message.EncodeError('Value out of range: %d' % value)
+    self.AppendVarUInt64(value)
+
+  def AppendVarint64(self, value):
+    """Appends a signed 64-bit integer to the internal buffer,
+    encoded as a varint.
+    """
+    if not wire_format.INT64_MIN <= value <= wire_format.INT64_MAX:
+      raise message.EncodeError('Value out of range: %d' % value)
+    if value < 0:
+      value += (1 << 64)
+    self.AppendVarUInt64(value)
+
+  def AppendVarUInt64(self, unsigned_value):
+    """Appends an unsigned 64-bit integer to the internal buffer,
+    encoded as a varint.
+    """
+    if not 0 <= unsigned_value <= wire_format.UINT64_MAX:
+      raise message.EncodeError('Value out of range: %d' % unsigned_value)
+    while True:
+      bits = unsigned_value & 0x7f
+      unsigned_value >>= 7
+      if unsigned_value:
+        bits |= 0x80
+      self._buffer.append(bits)
+      if not unsigned_value:
+        break
+
+  def ToString(self):
+    """Returns a string containing the bytes in our internal buffer."""
+    return self._buffer.tostring()