Prepare for Python2-Python3 straddle.

- Remove PY25 cruft.

- Selectively apply cleanups from 'python-modernize':

  - New exception syntax.
  - Use 'six' to handle module renames.
  - Use 'six' to handle text / binary stuff.

This PR covers most of the work from #66 which falls inside `python`
(rather than the Python code generation stuff in 'src').

15 files changed, 144 insertions, 198 deletions

diff --git a/python/google/protobuf/__init__.py b/python/google/protobuf/__init__.py
index ec3b0934..03eb74ef 100755
--- a/python/google/protobuf/__init__.py
+++ b/python/google/protobuf/__init__.py
@@ -28,8 +28,6 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-# Needs to stay compatible with Python 2.5 due to GAE.
-#
 # Copyright 2007 Google Inc. All Rights Reserved.
 
 __version__ = '3.0.0-pre'
diff --git a/python/google/protobuf/descriptor.py b/python/google/protobuf/descriptor.py
index af571b7c..e7acdacd 100755
--- a/python/google/protobuf/descriptor.py
+++ b/python/google/protobuf/descriptor.py
@@ -28,8 +28,6 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-# Needs to stay compatible with Python 2.5 due to GAE.
-#
 # Copyright 2007 Google Inc. All Rights Reserved.
 
 """Descriptors essentially contain exactly the information found in a .proto
@@ -846,5 +844,5 @@ def MakeDescriptor(desc_proto, package='', build_file_if_cpp=True):
 
   desc_name = '.'.join(full_message_name)
   return Descriptor(desc_proto.name, desc_name, None, None, fields,
-                    nested_types.values(), enum_types.values(), [],
+                    list(nested_types.values()), list(enum_types.values()), [],
                     options=desc_proto.options)
diff --git a/python/google/protobuf/descriptor_pool.py b/python/google/protobuf/descriptor_pool.py
index bcac513a..9a751bac 100644
--- a/python/google/protobuf/descriptor_pool.py
+++ b/python/google/protobuf/descriptor_pool.py
@@ -57,8 +57,6 @@ directly instead of this class.
 
 __author__ = 'matthewtoia@google.com (Matt Toia)'
 
-import sys
-
 from google.protobuf import descriptor
 from google.protobuf import descriptor_database
 from google.protobuf import text_encoding
@@ -175,8 +173,7 @@ class DescriptorPool(object):
 
     try:
       file_proto = self._internal_db.FindFileByName(file_name)
-    except KeyError:
-      _, error, _ = sys.exc_info()  #PY25 compatible for GAE.
+    except KeyError as error:
       if self._descriptor_db:
         file_proto = self._descriptor_db.FindFileByName(file_name)
       else:
@@ -211,8 +208,7 @@ class DescriptorPool(object):
 
     try:
       file_proto = self._internal_db.FindFileContainingSymbol(symbol)
-    except KeyError:
-      _, error, _ = sys.exc_info()  #PY25 compatible for GAE.
+    except KeyError as error:
       if self._descriptor_db:
         file_proto = self._descriptor_db.FindFileContainingSymbol(symbol)
       else:
@@ -282,9 +278,9 @@ class DescriptorPool(object):
       # file proto.
       for dependency in built_deps:
         scope.update(self._ExtractSymbols(
-            dependency.message_types_by_name.values()))
+            list(dependency.message_types_by_name.values())))
         scope.update((_PrefixWithDot(enum.full_name), enum)
-                     for enum in dependency.enum_types_by_name.values())
+                     for enum in list(dependency.enum_types_by_name.values()))
 
       for message_type in file_proto.message_type:
         message_desc = self._ConvertMessageDescriptor(
diff --git a/python/google/protobuf/internal/cpp_message.py b/python/google/protobuf/internal/cpp_message.py
index 0313cb0b..031b525d 100755
--- a/python/google/protobuf/internal/cpp_message.py
+++ b/python/google/protobuf/internal/cpp_message.py
@@ -34,8 +34,12 @@ Descriptor objects at runtime backed by the protocol buffer C++ API.
 
 __author__ = 'petar@google.com (Petar Petrov)'
 
-import copy_reg
+import collections
 import operator
+
+import six
+import six.moves.copyreg
+
 from google.protobuf.internal import _net_proto2___python
 from google.protobuf.internal import enum_type_wrapper
 from google.protobuf import message
@@ -146,7 +150,7 @@ class RepeatedScalarContainer(object):
   def __eq__(self, other):
     if self is other:
       return True
-    if not operator.isSequenceType(other):
+    if not isinstance(other, collections.Sequence):
       raise TypeError(
           'Can only compare repeated scalar fields against sequences.')
     # We are presumably comparing against some other sequence type.
@@ -259,7 +263,7 @@ class RepeatedCompositeContainer(object):
       index_key = lambda i: key(self[i])
 
     # Sort the list of current indexes by the underlying object.
-    indexes = range(len(self))
+    indexes = list(range(len(self)))
     indexes.sort(cmp=cmp, key=index_key, reverse=reverse)
 
     # Apply the transposition.
@@ -385,7 +389,7 @@ def InitMessage(message_descriptor, cls):
   _AddInitMethod(message_descriptor, cls)
   _AddMessageMethods(message_descriptor, cls)
   _AddPropertiesForExtensions(message_descriptor, cls)
-  copy_reg.pickle(cls, lambda obj: (cls, (), obj.__getstate__()))
+  six.moves.copyreg.pickle(cls, lambda obj: (cls, (), obj.__getstate__()))
 
 
 def _AddDescriptors(message_descriptor, dictionary):
@@ -400,7 +404,7 @@ def _AddDescriptors(message_descriptor, dictionary):
     dictionary['__descriptors'][field.name] = GetFieldDescriptor(
         field.full_name)
 
-  dictionary['__slots__'] = list(dictionary['__descriptors'].iterkeys()) + [
+  dictionary['__slots__'] = list(dictionary['__descriptors'].keys())) + [
       '_cmsg', '_owner', '_composite_fields', 'Extensions', '_HACK_REFCOUNTS']
 
 
@@ -420,7 +424,7 @@ def _AddEnumValues(message_descriptor, dictionary):
 def _AddClassAttributesForNestedExtensions(message_descriptor, dictionary):
   """Adds class attributes for the nested extensions."""
   extension_dict = message_descriptor.extensions_by_name
-  for extension_name, extension_field in extension_dict.iteritems():
+  for extension_name, extension_field in extension_dict.items():
     assert extension_name not in dictionary
     dictionary[extension_name] = extension_field
 
@@ -474,7 +478,7 @@ def _AddInitMethod(message_descriptor, cls):
       self._HACK_REFCOUNTS = self
     self._composite_fields = {}
 
-    for field_name, field_value in kwargs.iteritems():
+    for field_name, field_value in kwargs.items():
       field_cdescriptor = self.__descriptors.get(field_name, None)
       if not field_cdescriptor:
         raise ValueError('Protocol message has no "%s" field.' % field_name)
@@ -538,7 +542,7 @@ def _AddMessageMethods(message_descriptor, cls):
 
   def Clear(self):
     cmessages_to_release = []
-    for field_name, child_field in self._composite_fields.iteritems():
+    for field_name, child_field in self._composite_fields.items():
       child_cdescriptor = self.__descriptors[field_name]
       # TODO(anuraag): Support clearing repeated message fields as well.
       if (child_cdescriptor.label != _LABEL_REPEATED and
@@ -631,7 +635,7 @@ def _AddMessageMethods(message_descriptor, cls):
     return text_format.MessageToString(self, as_utf8=True).decode('utf-8')
 
   # Attach the local methods to the message class.
-  for key, value in locals().copy().iteritems():
+  for key, value in locals().copy().items():
     if key not in ('key', 'value', '__builtins__', '__name__', '__doc__'):
       setattr(cls, key, value)
 
@@ -658,6 +662,6 @@ def _AddMessageMethods(message_descriptor, cls):
 def _AddPropertiesForExtensions(message_descriptor, cls):
   """Adds properties for all fields in this protocol message type."""
   extension_dict = message_descriptor.extensions_by_name
-  for extension_name, extension_field in extension_dict.iteritems():
+  for extension_name, extension_field in extension_dict.items():
     constant_name = extension_name.upper() + '_FIELD_NUMBER'
     setattr(cls, constant_name, extension_field.number)
diff --git a/python/google/protobuf/internal/decoder.py b/python/google/protobuf/internal/decoder.py
index a4b90608..6b72adef 100755
--- a/python/google/protobuf/internal/decoder.py
+++ b/python/google/protobuf/internal/decoder.py
@@ -28,8 +28,6 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-#PY25 compatible for GAE.
-#
 # Copyright 2009 Google Inc. All Rights Reserved.
 
 """Code for decoding protocol buffer primitives.
@@ -85,8 +83,9 @@ we repeatedly read a tag, look up the corresponding decoder, and invoke it.
 __author__ = 'kenton@google.com (Kenton Varda)'
 
 import struct
-import sys  ##PY25
-_PY2 = sys.version_info[0] < 3  ##PY25
+
+import six
+
 from google.protobuf.internal import encoder
 from google.protobuf.internal import wire_format
 from google.protobuf import message
@@ -114,14 +113,11 @@ def _VarintDecoder(mask, result_type):
   decoder returns a (value, new_pos) pair.
   """
 
-  local_ord = ord
-  py2 = _PY2  ##PY25
-##!PY25  py2 = str is bytes
   def DecodeVarint(buffer, pos):
     result = 0
     shift = 0
     while 1:
-      b = local_ord(buffer[pos]) if py2 else buffer[pos]
+      b = six.indexbytes(buffer, pos)
       result |= ((b & 0x7f) << shift)
       pos += 1
       if not (b & 0x80):
@@ -137,14 +133,11 @@ def _VarintDecoder(mask, result_type):
 def _SignedVarintDecoder(mask, result_type):
   """Like _VarintDecoder() but decodes signed values."""
 
-  local_ord = ord
-  py2 = _PY2  ##PY25
-##!PY25  py2 = str is bytes
   def DecodeVarint(buffer, pos):
     result = 0
     shift = 0
     while 1:
-      b = local_ord(buffer[pos]) if py2 else buffer[pos]
+      b = six.indexbytes(buffer, pos)
       result |= ((b & 0x7f) << shift)
       pos += 1
       if not (b & 0x80):
@@ -164,8 +157,8 @@ def _SignedVarintDecoder(mask, result_type):
 # alternate implementations where the distinction is more significant
 # (e.g. the C++ implementation) simpler.
 
-_DecodeVarint = _VarintDecoder((1 << 64) - 1, long)
-_DecodeSignedVarint = _SignedVarintDecoder((1 << 64) - 1, long)
+_DecodeVarint = _VarintDecoder((1 << 64) - 1, int)
+_DecodeSignedVarint = _SignedVarintDecoder((1 << 64) - 1, int)
 
 # Use these versions for values which must be limited to 32 bits.
 _DecodeVarint32 = _VarintDecoder((1 << 32) - 1, int)
@@ -183,10 +176,8 @@ def ReadTag(buffer, pos):
   use that, but not in Python.
   """
 
-  py2 = _PY2  ##PY25
-##!PY25  py2 = str is bytes
   start = pos
-  while (ord(buffer[pos]) if py2 else buffer[pos]) & 0x80:
+  while six.indexbytes(buffer, pos) & 0x80:
     pos += 1
   pos += 1
   return (buffer[start:pos], pos)
@@ -301,7 +292,6 @@ def _FloatDecoder():
   """
 
   local_unpack = struct.unpack
-  b = (lambda x:x) if _PY2 else lambda x:x.encode('latin1')  ##PY25
 
   def InnerDecode(buffer, pos):
     # We expect a 32-bit value in little-endian byte order.  Bit 1 is the sign
@@ -312,17 +302,12 @@ def _FloatDecoder():
     # If this value has all its exponent bits set, then it's non-finite.
     # In Python 2.4, struct.unpack will convert it to a finite 64-bit value.
     # To avoid that, we parse it specially.
-    if ((float_bytes[3:4] in b('\x7F\xFF'))  ##PY25
-##!PY25    if ((float_bytes[3:4] in b'\x7F\xFF')
-        and (float_bytes[2:3] >= b('\x80'))):  ##PY25
-##!PY25        and (float_bytes[2:3] >= b'\x80')):
+    if (float_bytes[3:4] in b'\x7F\xFF' and float_bytes[2:3] >= b'\x80'):
       # If at least one significand bit is set...
-      if float_bytes[0:3] != b('\x00\x00\x80'):  ##PY25
-##!PY25      if float_bytes[0:3] != b'\x00\x00\x80':
+      if float_bytes[0:3] != b'\x00\x00\x80':
         return (_NAN, new_pos)
       # If sign bit is set...
-      if float_bytes[3:4] == b('\xFF'):  ##PY25
-##!PY25      if float_bytes[3:4] == b'\xFF':
+      if float_bytes[3:4] == b'\xFF':
         return (_NEG_INF, new_pos)
       return (_POS_INF, new_pos)
 
@@ -341,7 +326,6 @@ def _DoubleDecoder():
   """
 
   local_unpack = struct.unpack
-  b = (lambda x:x) if _PY2 else lambda x:x.encode('latin1')  ##PY25
 
   def InnerDecode(buffer, pos):
     # We expect a 64-bit value in little-endian byte order.  Bit 1 is the sign
@@ -352,12 +336,9 @@ def _DoubleDecoder():
     # If this value has all its exponent bits set and at least one significand
     # bit set, it's not a number.  In Python 2.4, struct.unpack will treat it
     # as inf or -inf.  To avoid that, we treat it specially.
-##!PY25    if ((double_bytes[7:8] in b'\x7F\xFF')
-##!PY25        and (double_bytes[6:7] >= b'\xF0')
-##!PY25        and (double_bytes[0:7] != b'\x00\x00\x00\x00\x00\x00\xF0')):
-    if ((double_bytes[7:8] in b('\x7F\xFF'))  ##PY25
-        and (double_bytes[6:7] >= b('\xF0'))  ##PY25
-        and (double_bytes[0:7] != b('\x00\x00\x00\x00\x00\x00\xF0'))):  ##PY25
+    if ((double_bytes[7:8] in b'\x7F\xFF')
+        and (double_bytes[6:7] >= b'\xF0')
+        and (double_bytes[0:7] != b'\x00\x00\x00\x00\x00\x00\xF0')):
       return (_NAN, new_pos)
 
     # Note that we expect someone up-stack to catch struct.error and convert
@@ -480,12 +461,12 @@ def StringDecoder(field_number, is_repeated, is_packed, key, new_default):
   """Returns a decoder for a string field."""
 
   local_DecodeVarint = _DecodeVarint
-  local_unicode = unicode
+  local_unicode = six.text_type
 
   def _ConvertToUnicode(byte_str):
     try:
       return local_unicode(byte_str, 'utf-8')
-    except UnicodeDecodeError, e:
+    except UnicodeDecodeError as e:
       # add more information to the error message and re-raise it.
       e.reason = '%s in field: %s' % (e, key.full_name)
       raise
diff --git a/python/google/protobuf/internal/encoder.py b/python/google/protobuf/internal/encoder.py
index 38a5138a..21ed2ed7 100755
--- a/python/google/protobuf/internal/encoder.py
+++ b/python/google/protobuf/internal/encoder.py
@@ -28,8 +28,6 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-#PY25 compatible for GAE.
-#
 # Copyright 2009 Google Inc. All Rights Reserved.
 
 """Code for encoding protocol message primitives.
@@ -71,8 +69,9 @@ sizer rather than when calling them.  In particular:
 __author__ = 'kenton@google.com (Kenton Varda)'
 
 import struct
-import sys  ##PY25
-_PY2 = sys.version_info[0] < 3  ##PY25
+
+import six
+
 from google.protobuf.internal import wire_format
 
 
@@ -346,16 +345,14 @@ def MessageSetItemSizer(field_number):
 def _VarintEncoder():
   """Return an encoder for a basic varint value (does not include tag)."""
 
-  local_chr = _PY2 and chr or (lambda x: bytes((x,)))  ##PY25
-##!PY25  local_chr = chr if bytes is str else lambda x: bytes((x,))
   def EncodeVarint(write, value):
     bits = value & 0x7f
     value >>= 7
     while value:
-      write(local_chr(0x80|bits))
+      write(six.int2byte(0x80|bits))
       bits = value & 0x7f
       value >>= 7
-    return write(local_chr(bits))
+    return write(six.int2byte(bits))
 
   return EncodeVarint
 
@@ -364,18 +361,16 @@ def _SignedVarintEncoder():
   """Return an encoder for a basic signed varint value (does not include
   tag)."""
 
-  local_chr = _PY2 and chr or (lambda x: bytes((x,)))  ##PY25
-##!PY25  local_chr = chr if bytes is str else lambda x: bytes((x,))
   def EncodeSignedVarint(write, value):
     if value < 0:
       value += (1 << 64)
     bits = value & 0x7f
     value >>= 7
     while value:
-      write(local_chr(0x80|bits))
+      write(six.int2byte(0x80|bits))
       bits = value & 0x7f
       value >>= 7
-    return write(local_chr(bits))
+    return write(six.int2byte(bits))
 
   return EncodeSignedVarint
 
@@ -390,8 +385,7 @@ def _VarintBytes(value):
 
   pieces = []
   _EncodeVarint(pieces.append, value)
-  return "".encode("latin1").join(pieces)  ##PY25
-##!PY25  return b"".join(pieces)
+  return b"".join(pieces)
 
 
 def TagBytes(field_number, wire_type):
@@ -529,33 +523,26 @@ def _FloatingPointEncoder(wire_type, format):
       format:  The format string to pass to struct.pack().
   """
 
-  b = _PY2 and (lambda x:x) or (lambda x:x.encode('latin1'))  ##PY25
   value_size = struct.calcsize(format)
   if value_size == 4:
     def EncodeNonFiniteOrRaise(write, value):
       # Remember that the serialized form uses little-endian byte order.
       if value == _POS_INF:
-        write(b('\x00\x00\x80\x7F'))  ##PY25
-##!PY25        write(b'\x00\x00\x80\x7F')
+        write(b'\x00\x00\x80\x7F')
       elif value == _NEG_INF:
-        write(b('\x00\x00\x80\xFF'))  ##PY25
-##!PY25        write(b'\x00\x00\x80\xFF')
+        write(b'\x00\x00\x80\xFF')
       elif value != value:           # NaN
-        write(b('\x00\x00\xC0\x7F'))  ##PY25
-##!PY25        write(b'\x00\x00\xC0\x7F')
+        write(b'\x00\x00\xC0\x7F')
       else:
         raise
   elif value_size == 8:
     def EncodeNonFiniteOrRaise(write, value):
       if value == _POS_INF:
-        write(b('\x00\x00\x00\x00\x00\x00\xF0\x7F'))  ##PY25
-##!PY25        write(b'\x00\x00\x00\x00\x00\x00\xF0\x7F')
+        write(b'\x00\x00\x00\x00\x00\x00\xF0\x7F')
       elif value == _NEG_INF:
-        write(b('\x00\x00\x00\x00\x00\x00\xF0\xFF'))  ##PY25
-##!PY25        write(b'\x00\x00\x00\x00\x00\x00\xF0\xFF')
+        write(b'\x00\x00\x00\x00\x00\x00\xF0\xFF')
       elif value != value:                         # NaN
-        write(b('\x00\x00\x00\x00\x00\x00\xF8\x7F'))  ##PY25
-##!PY25        write(b'\x00\x00\x00\x00\x00\x00\xF8\x7F')
+        write(b'\x00\x00\x00\x00\x00\x00\xF8\x7F')
       else:
         raise
   else:
@@ -631,10 +618,8 @@ DoubleEncoder   = _FloatingPointEncoder(wire_format.WIRETYPE_FIXED64, '<d')
 def BoolEncoder(field_number, is_repeated, is_packed):
   """Returns an encoder for a boolean field."""
 
-##!PY25  false_byte = b'\x00'
-##!PY25  true_byte = b'\x01'
-  false_byte = '\x00'.encode('latin1')  ##PY25
-  true_byte = '\x01'.encode('latin1')  ##PY25
+  false_byte = b'\x00'
+  true_byte = b'\x01'
   if is_packed:
     tag_bytes = TagBytes(field_number, wire_format.WIRETYPE_LENGTH_DELIMITED)
     local_EncodeVarint = _EncodeVarint
@@ -770,8 +755,7 @@ def MessageSetItemEncoder(field_number):
       }
     }
   """
-  start_bytes = "".encode("latin1").join([  ##PY25
-##!PY25  start_bytes = b"".join([
+  start_bytes = b"".join([
       TagBytes(1, wire_format.WIRETYPE_START_GROUP),
       TagBytes(2, wire_format.WIRETYPE_VARINT),
       _VarintBytes(field_number),
diff --git a/python/google/protobuf/internal/generator_test.py b/python/google/protobuf/internal/generator_test.py
index 03361e66..ac9808d1 100755
--- a/python/google/protobuf/internal/generator_test.py
+++ b/python/google/protobuf/internal/generator_test.py
@@ -294,7 +294,7 @@ class GeneratorTest(basetest.TestCase):
     self.assertSameElements(
         nested_names,
         [field.name for field in desc.oneofs[0].fields])
-    for field_name, field_desc in desc.fields_by_name.iteritems():
+    for field_name, field_desc in desc.fields_by_name.items():
       if field_name in nested_names:
         self.assertIs(desc.oneofs[0], field_desc.containing_oneof)
       else:
diff --git a/python/google/protobuf/internal/message_factory_test.py b/python/google/protobuf/internal/message_factory_test.py
index fcf13410..f4617b85 100644
--- a/python/google/protobuf/internal/message_factory_test.py
+++ b/python/google/protobuf/internal/message_factory_test.py
@@ -107,14 +107,14 @@ class MessageFactoryTest(basetest.TestCase):
       self.assertContainsSubset(
           ['google.protobuf.python.internal.Factory2Message',
            'google.protobuf.python.internal.Factory1Message'],
-          messages.keys())
+          list(messages.keys()))
       self._ExerciseDynamicClass(
           messages['google.protobuf.python.internal.Factory2Message'])
       self.assertContainsSubset(
           ['google.protobuf.python.internal.Factory2Message.one_more_field',
            'google.protobuf.python.internal.another_field'],
-          (messages['google.protobuf.python.internal.Factory1Message']
-           ._extensions_by_name.keys()))
+          (list(messages['google.protobuf.python.internal.Factory1Message']
+           ._extensions_by_name.keys())))
       factory_msg1 = messages['google.protobuf.python.internal.Factory1Message']
       msg1 = messages['google.protobuf.python.internal.Factory1Message']()
       ext1 = factory_msg1._extensions_by_name[
diff --git a/python/google/protobuf/internal/python_message.py b/python/google/protobuf/internal/python_message.py
index 6fda6ae0..58c65db9 100755
--- a/python/google/protobuf/internal/python_message.py
+++ b/python/google/protobuf/internal/python_message.py
@@ -28,8 +28,6 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-# Keep it Python2.5 compatible for GAE.
-#
 # Copyright 2007 Google Inc. All Rights Reserved.
 #
 # This code is meant to work on Python 2.4 and above only.
@@ -54,19 +52,14 @@ this file*.
 
 __author__ = 'robinson@google.com (Will Robinson)'
 
+from io import BytesIO
 import sys
-if sys.version_info[0] < 3:
-  try:
-    from cStringIO import StringIO as BytesIO
-  except ImportError:
-    from StringIO import StringIO as BytesIO
-  import copy_reg as copyreg
-else:
-  from io import BytesIO
-  import copyreg
 import struct
 import weakref
 
+import six
+import six.moves.copyreg as copyreg
+
 # We use "as" to avoid name collisions with variables.
 from google.protobuf.internal import containers
 from google.protobuf.internal import decoder
@@ -237,7 +230,7 @@ def _AttachFieldHelpers(cls, field_descriptor):
 
 def _AddClassAttributesForNestedExtensions(descriptor, dictionary):
   extension_dict = descriptor.extensions_by_name
-  for extension_name, extension_field in extension_dict.iteritems():
+  for extension_name, extension_field in extension_dict.items():
     assert extension_name not in dictionary
     dictionary[extension_name] = extension_field
 
@@ -314,7 +307,7 @@ def _ReraiseTypeErrorWithFieldName(message_name, field_name):
     exc = TypeError('%s for field %s.%s' % (str(exc), message_name, field_name))
 
   # re-raise possibly-amended exception with original traceback:
-  raise type(exc), exc, sys.exc_info()[2]
+  six.reraise(type(exc), exc, sys.exc_info()[2])
 
 
 def _AddInitMethod(message_descriptor, cls):
@@ -334,7 +327,7 @@ def _AddInitMethod(message_descriptor, cls):
     self._is_present_in_parent = False
     self._listener = message_listener_mod.NullMessageListener()
     self._listener_for_children = _Listener(self)
-    for field_name, field_value in kwargs.iteritems():
+    for field_name, field_value in kwargs.items():
       field = _GetFieldByName(message_descriptor, field_name)
       if field is None:
         raise TypeError("%s() got an unexpected keyword argument '%s'" %
@@ -563,7 +556,7 @@ def _AddPropertiesForNonRepeatedCompositeField(field, cls):
 def _AddPropertiesForExtensions(descriptor, cls):
   """Adds properties for all fields in this protocol message type."""
   extension_dict = descriptor.extensions_by_name
-  for extension_name, extension_field in extension_dict.iteritems():
+  for extension_name, extension_field in extension_dict.items():
     constant_name = extension_name.upper() + "_FIELD_NUMBER"
     setattr(cls, constant_name, extension_field.number)
 
@@ -618,7 +611,7 @@ def _AddListFieldsMethod(message_descriptor, cls):
   """Helper for _AddMessageMethods()."""
 
   def ListFields(self):
-    all_fields = [item for item in self._fields.iteritems() if _IsPresent(item)]
+    all_fields = [item for item in self._fields.items() if _IsPresent(item)]
     all_fields.sort(key = lambda item: item[0].number)
     return all_fields
 
@@ -863,7 +856,7 @@ def _AddMergeFromStringMethod(message_descriptor, cls):
     except (IndexError, TypeError):
       # Now ord(buf[p:p+1]) == ord('') gets TypeError.
       raise message_mod.DecodeError('Truncated message.')
-    except struct.error, e:
+    except struct.error as e:
       raise message_mod.DecodeError(e)
     return length   # Return this for legacy reasons.
   cls.MergeFromString = MergeFromString
@@ -963,7 +956,7 @@ def _AddIsInitializedMethod(message_descriptor, cls):
           name = field.name
 
         if field.label == _FieldDescriptor.LABEL_REPEATED:
-          for i in xrange(len(value)):
+          for i in range(len(value)):
             element = value[i]
             prefix = "%s[%d]." % (name, i)
             sub_errors = element.FindInitializationErrors()
@@ -993,7 +986,7 @@ def _AddMergeFromMethod(cls):
 
     fields = self._fields
 
-    for field, value in msg._fields.iteritems():
+    for field, value in msg._fields.items():
       if field.label == LABEL_REPEATED:
         field_value = fields.get(field)
         if field_value is None:
diff --git a/python/google/protobuf/internal/reflection_test.py b/python/google/protobuf/internal/reflection_test.py
index 6b24b092..52a08d7d 100755
--- a/python/google/protobuf/internal/reflection_test.py
+++ b/python/google/protobuf/internal/reflection_test.py
@@ -40,6 +40,8 @@ import gc
 import operator
 import struct
 
+import six
+
 from google.apputils import basetest
 from google.protobuf import unittest_import_pb2
 from google.protobuf import unittest_mset_pb2
@@ -467,7 +469,7 @@ class ReflectionTest(basetest.TestCase):
     proto.repeated_string.extend(['foo', 'bar'])
     proto.repeated_string.extend([])
     proto.repeated_string.append('baz')
-    proto.repeated_string.extend(str(x) for x in xrange(2))
+    proto.repeated_string.extend(str(x) for x in range(2))
     proto.optional_int32 = 21
     proto.repeated_bool  # Access but don't set anything; should not be listed.
     self.assertEqual(
@@ -620,14 +622,18 @@ class ReflectionTest(basetest.TestCase):
     if struct.calcsize('L') == 4:
       # Python only has signed ints, so 32-bit python can't fit an uint32
       # in an int.
-      TestGetAndDeserialize('optional_uint32', 1 << 31, long)
+      TestGetAndDeserialize('optional_uint32', 1 << 31, int)
     else:
       # 64-bit python can fit uint32 inside an int
       TestGetAndDeserialize('optional_uint32', 1 << 31, int)
-    TestGetAndDeserialize('optional_int64', 1 << 30, long)
-    TestGetAndDeserialize('optional_int64', 1 << 60, long)
-    TestGetAndDeserialize('optional_uint64', 1 << 30, long)
-    TestGetAndDeserialize('optional_uint64', 1 << 60, long)
+    try:
+      integer_64 = long
+    except NameError: # Python3
+      integer_64 = int
+    TestGetAndDeserialize('optional_int64', 1 << 30, integer_64)
+    TestGetAndDeserialize('optional_int64', 1 << 60, integer_64)
+    TestGetAndDeserialize('optional_uint64', 1 << 30, integer_64)
+    TestGetAndDeserialize('optional_uint64', 1 << 60, integer_64)
 
   def testSingleScalarBoundsChecking(self):
     def TestMinAndMaxIntegers(field_name, expected_min, expected_max):
@@ -753,18 +759,18 @@ class ReflectionTest(basetest.TestCase):
 
   def testEnum_KeysAndValues(self):
     self.assertEqual(['FOREIGN_FOO', 'FOREIGN_BAR', 'FOREIGN_BAZ'],
-                     unittest_pb2.ForeignEnum.keys())
+                     list(unittest_pb2.ForeignEnum.keys()))
     self.assertEqual([4, 5, 6],
-                     unittest_pb2.ForeignEnum.values())
+                     list(unittest_pb2.ForeignEnum.values()))
     self.assertEqual([('FOREIGN_FOO', 4), ('FOREIGN_BAR', 5),
                       ('FOREIGN_BAZ', 6)],
-                     unittest_pb2.ForeignEnum.items())
+                     list(unittest_pb2.ForeignEnum.items()))
 
     proto = unittest_pb2.TestAllTypes()
-    self.assertEqual(['FOO', 'BAR', 'BAZ', 'NEG'], proto.NestedEnum.keys())
-    self.assertEqual([1, 2, 3, -1], proto.NestedEnum.values())
+    self.assertEqual(['FOO', 'BAR', 'BAZ', 'NEG'], list(proto.NestedEnum.keys()))
+    self.assertEqual([1, 2, 3, -1], list(proto.NestedEnum.values()))
     self.assertEqual([('FOO', 1), ('BAR', 2), ('BAZ', 3), ('NEG', -1)],
-                     proto.NestedEnum.items())
+                     list(proto.NestedEnum.items()))
 
   def testRepeatedScalars(self):
     proto = unittest_pb2.TestAllTypes()
@@ -803,7 +809,7 @@ class ReflectionTest(basetest.TestCase):
     self.assertEqual([5, 25, 20, 15, 30], proto.repeated_int32[:])
 
     # Test slice assignment with an iterator
-    proto.repeated_int32[1:4] = (i for i in xrange(3))
+    proto.repeated_int32[1:4] = (i for i in range(3))
     self.assertEqual([5, 0, 1, 2, 30], proto.repeated_int32)
 
     # Test slice assignment.
@@ -1006,9 +1012,8 @@ class ReflectionTest(basetest.TestCase):
         containing_type=None, nested_types=[], enum_types=[],
         fields=[foo_field_descriptor], extensions=[],
         options=descriptor_pb2.MessageOptions())
-    class MyProtoClass(message.Message):
+    class MyProtoClass(six.with_metaclass(reflection.GeneratedProtocolMessageType, message.Message)):
       DESCRIPTOR = mydescriptor
-      __metaclass__ = reflection.GeneratedProtocolMessageType
     myproto_instance = MyProtoClass()
     self.assertEqual(0, myproto_instance.foo_field)
     self.assertTrue(not myproto_instance.HasField('foo_field'))
@@ -1048,14 +1053,13 @@ class ReflectionTest(basetest.TestCase):
     new_field.label = descriptor_pb2.FieldDescriptorProto.LABEL_REPEATED
 
     desc = descriptor.MakeDescriptor(desc_proto)
-    self.assertTrue(desc.fields_by_name.has_key('name'))
-    self.assertTrue(desc.fields_by_name.has_key('year'))
-    self.assertTrue(desc.fields_by_name.has_key('automatic'))
-    self.assertTrue(desc.fields_by_name.has_key('price'))
-    self.assertTrue(desc.fields_by_name.has_key('owners'))
-
-    class CarMessage(message.Message):
-      __metaclass__ = reflection.GeneratedProtocolMessageType
+    self.assertTrue('name' in desc.fields_by_name)
+    self.assertTrue('year' in desc.fields_by_name)
+    self.assertTrue('automatic' in desc.fields_by_name)
+    self.assertTrue('price' in desc.fields_by_name)
+    self.assertTrue('owners' in desc.fields_by_name)
+
+    class CarMessage(six.with_metaclass(reflection.GeneratedProtocolMessageType, message.Message)):
       DESCRIPTOR = desc
 
     prius = CarMessage()
@@ -1661,14 +1665,14 @@ class ReflectionTest(basetest.TestCase):
                       setattr, proto, 'optional_bytes', u'unicode object')
 
     # Check that the default value is of python's 'unicode' type.
-    self.assertEqual(type(proto.optional_string), unicode)
+    self.assertEqual(type(proto.optional_string), six.text_type)
 
-    proto.optional_string = unicode('Testing')
+    proto.optional_string = six.text_type('Testing')
     self.assertEqual(proto.optional_string, str('Testing'))
 
     # Assign a value of type 'str' which can be encoded in UTF-8.
     proto.optional_string = str('Testing')
-    self.assertEqual(proto.optional_string, unicode('Testing'))
+    self.assertEqual(proto.optional_string, six.text_type('Testing'))
 
     # Try to assign a 'bytes' object which contains non-UTF-8.
     self.assertRaises(ValueError,
@@ -1715,7 +1719,7 @@ class ReflectionTest(basetest.TestCase):
     bytes_read = message2.MergeFromString(raw.item[0].message)
     self.assertEqual(len(raw.item[0].message), bytes_read)
 
-    self.assertEqual(type(message2.str), unicode)
+    self.assertEqual(type(message2.str), six.text_type)
     self.assertEqual(message2.str, test_utf8)
 
     # The pure Python API throws an exception on MergeFromString(),
@@ -1739,7 +1743,7 @@ class ReflectionTest(basetest.TestCase):
   def testBytesInTextFormat(self):
     proto = unittest_pb2.TestAllTypes(optional_bytes=b'\x00\x7f\x80\xff')
     self.assertEqual(u'optional_bytes: "\\000\\177\\200\\377"\n',
-                     unicode(proto))
+                     six.text_type(proto))
 
   def testEmptyNestedMessage(self):
     proto = unittest_pb2.TestAllTypes()
@@ -2307,7 +2311,7 @@ class SerializationTest(basetest.TestCase):
     test_util.SetAllFields(first_proto)
     serialized = first_proto.SerializeToString()
 
-    for truncation_point in xrange(len(serialized) + 1):
+    for truncation_point in range(len(serialized) + 1):
       try:
         second_proto = unittest_pb2.TestAllTypes()
         unknown_fields = unittest_pb2.TestEmptyMessage()
@@ -2908,8 +2912,7 @@ class ClassAPITest(basetest.TestCase):
     msg_descriptor = descriptor.MakeDescriptor(
         file_descriptor.message_type[0])
 
-    class MessageClass(message.Message):
-      __metaclass__ = reflection.GeneratedProtocolMessageType
+    class MessageClass(six.with_metaclass(reflection.GeneratedProtocolMessageType, message.Message)):
       DESCRIPTOR = msg_descriptor
     msg = MessageClass()
     msg_str = (
diff --git a/python/google/protobuf/internal/text_format_test.py b/python/google/protobuf/internal/text_format_test.py
index 55e3c2c8..3455a000 100755
--- a/python/google/protobuf/internal/text_format_test.py
+++ b/python/google/protobuf/internal/text_format_test.py
@@ -36,9 +36,10 @@ __author__ = 'kenton@google.com (Kenton Varda)'
 
 import re
 
+import six
+
 from google.apputils import basetest
 from google.protobuf import text_format
-from google.protobuf.internal import api_implementation
 from google.protobuf.internal import test_util
 from google.protobuf import unittest_pb2
 from google.protobuf import unittest_mset_pb2
@@ -143,7 +144,7 @@ class TextFormatTest(basetest.TestCase):
         'repeated_string: "\\303\\274\\352\\234\\237"\n')
 
   def testPrintExoticUnicodeSubclass(self):
-    class UnicodeSub(unicode):
+    class UnicodeSub(six.text_type):
       pass
     message = unittest_pb2.TestAllTypes()
     message.repeated_string.append(UnicodeSub(u'\u00fc\ua71f'))
diff --git a/python/google/protobuf/internal/type_checkers.py b/python/google/protobuf/internal/type_checkers.py
index 118725da..8d10fbe0 100755
--- a/python/google/protobuf/internal/type_checkers.py
+++ b/python/google/protobuf/internal/type_checkers.py
@@ -28,8 +28,6 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-#PY25 compatible for GAE.
-#
 # Copyright 2008 Google Inc. All Rights Reserved.
 
 """Provides type checking routines.
@@ -49,9 +47,8 @@ TYPE_TO_DESERIALIZE_METHOD: A dictionary with field types and deserialization
 
 __author__ = 'robinson@google.com (Will Robinson)'
 
-import sys  ##PY25
-if sys.version < '2.6': bytes = str  ##PY25
-from google.protobuf.internal import api_implementation
+import six
+
 from google.protobuf.internal import decoder
 from google.protobuf.internal import encoder
 from google.protobuf.internal import wire_format
@@ -111,9 +108,9 @@ class IntValueChecker(object):
   """Checker used for integer fields.  Performs type-check and range check."""
 
   def CheckValue(self, proposed_value):
-    if not isinstance(proposed_value, (int, long)):
+    if not isinstance(proposed_value, six.integer_types):
       message = ('%.1024r has type %s, but expected one of: %s' %
-                 (proposed_value, type(proposed_value), (int, long)))
+                 (proposed_value, type(proposed_value), six.integer_types))
       raise TypeError(message)
     if not self._MIN <= proposed_value <= self._MAX:
       raise ValueError('Value out of range: %d' % proposed_value)
@@ -132,9 +129,9 @@ class EnumValueChecker(object):
     self._enum_type = enum_type
 
   def CheckValue(self, proposed_value):
-    if not isinstance(proposed_value, (int, long)):
+    if not isinstance(proposed_value, six.integer_types):
       message = ('%.1024r has type %s, but expected one of: %s' %
-                 (proposed_value, type(proposed_value), (int, long)))
+                 (proposed_value, type(proposed_value), six.integer_types))
       raise TypeError(message)
     if proposed_value not in self._enum_type.values_by_number:
       raise ValueError('Unknown enum value: %d' % proposed_value)
@@ -149,9 +146,9 @@ class UnicodeValueChecker(object):
   """
 
   def CheckValue(self, proposed_value):
-    if not isinstance(proposed_value, (bytes, unicode)):
+    if not isinstance(proposed_value, (bytes, six.text_type)):
       message = ('%.1024r has type %s, but expected one of: %s' %
-                 (proposed_value, type(proposed_value), (bytes, unicode)))
+                 (proposed_value, type(proposed_value), (bytes, six.text_type)))
       raise TypeError(message)
 
     # If the value is of type 'bytes' make sure that it is valid UTF-8 data.
@@ -183,13 +180,13 @@ class Uint32ValueChecker(IntValueChecker):
 class Int64ValueChecker(IntValueChecker):
   _MIN = -(1 << 63)
   _MAX = (1 << 63) - 1
-  _TYPE = long
+  _TYPE = int
 
 
 class Uint64ValueChecker(IntValueChecker):
   _MIN = 0
   _MAX = (1 << 64) - 1
-  _TYPE = long
+  _TYPE = int
 
 
 # Type-checkers for all scalar CPPTYPEs.
@@ -199,9 +196,9 @@ _VALUE_CHECKERS = {
     _FieldDescriptor.CPPTYPE_UINT32: Uint32ValueChecker(),
     _FieldDescriptor.CPPTYPE_UINT64: Uint64ValueChecker(),
     _FieldDescriptor.CPPTYPE_DOUBLE: TypeChecker(
-        float, int, long),
+        float, int, int),
     _FieldDescriptor.CPPTYPE_FLOAT: TypeChecker(
-        float, int, long),
+        float, int, int),
     _FieldDescriptor.CPPTYPE_BOOL: TypeChecker(bool, int),
     _FieldDescriptor.CPPTYPE_STRING: TypeChecker(bytes),
     }
diff --git a/python/google/protobuf/message_factory.py b/python/google/protobuf/message_factory.py
index 7fd7bec0..36062a56 100644
--- a/python/google/protobuf/message_factory.py
+++ b/python/google/protobuf/message_factory.py
@@ -28,8 +28,6 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-#PY25 compatible for GAE.
-#
 # Copyright 2012 Google Inc. All Rights Reserved.
 
 """Provides a factory class for generating dynamic messages.
@@ -43,7 +41,6 @@ my_proto_instance = message_classes['some.proto.package.MessageName']()
 
 __author__ = 'matthewtoia@google.com (Matt Toia)'
 
-import sys  ##PY25
 from google.protobuf import descriptor_database
 from google.protobuf import descriptor_pool
 from google.protobuf import message
@@ -75,8 +72,7 @@ class MessageFactory(object):
     """
     if descriptor.full_name not in self._classes:
       descriptor_name = descriptor.name
-      if sys.version_info[0] < 3:  ##PY25
-##!PY25      if str is bytes:  # PY2
+      if str is bytes:  # PY2
         descriptor_name = descriptor.name.encode('ascii', 'ignore')
       result_class = reflection.GeneratedProtocolMessageType(
           descriptor_name,
@@ -111,7 +107,7 @@ class MessageFactory(object):
     result = {}
     for file_name in files:
       file_desc = self.pool.FindFileByName(file_name)
-      for name, msg in file_desc.message_types_by_name.iteritems():
+      for name, msg in file_desc.message_types_by_name.items():
         if file_desc.package:
           full_name = '.'.join([file_desc.package, name])
         else:
@@ -128,7 +124,7 @@ class MessageFactory(object):
       # ignore the registration if the original was the same, or raise
       # an error if they were different.
 
-      for name, extension in file_desc.extensions_by_name.iteritems():
+      for name, extension in file_desc.extensions_by_name.items():
         if extension.containing_type.full_name not in self._classes:
           self.GetPrototype(extension.containing_type)
         extended_class = self._classes[extension.containing_type.full_name]
diff --git a/python/google/protobuf/text_encoding.py b/python/google/protobuf/text_encoding.py
index 2d86a67c..a0728e3c 100644
--- a/python/google/protobuf/text_encoding.py
+++ b/python/google/protobuf/text_encoding.py
@@ -27,16 +27,13 @@
 # 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.
-
-#PY25 compatible for GAE.
-#
 """Encoding related utilities."""
-
 import re
-import sys  ##PY25
+
+import six
 
 # Lookup table for utf8
-_cescape_utf8_to_str = [chr(i) for i in xrange(0, 256)]
+_cescape_utf8_to_str = [chr(i) for i in range(0, 256)]
 _cescape_utf8_to_str[9] = r'\t'  # optional escape
 _cescape_utf8_to_str[10] = r'\n'  # optional escape
 _cescape_utf8_to_str[13] = r'\r'  # optional escape
@@ -46,9 +43,9 @@ _cescape_utf8_to_str[34] = r'\"'  # necessary escape
 _cescape_utf8_to_str[92] = r'\\'  # necessary escape
 
 # Lookup table for non-utf8, with necessary escapes at (o >= 127 or o < 32)
-_cescape_byte_to_str = ([r'\%03o' % i for i in xrange(0, 32)] +
-                        [chr(i) for i in xrange(32, 127)] +
-                        [r'\%03o' % i for i in xrange(127, 256)])
+_cescape_byte_to_str = ([r'\%03o' % i for i in range(0, 32)] +
+                        [chr(i) for i in range(32, 127)] +
+                        [r'\%03o' % i for i in range(127, 256)])
 _cescape_byte_to_str[9] = r'\t'  # optional escape
 _cescape_byte_to_str[10] = r'\n'  # optional escape
 _cescape_byte_to_str[13] = r'\r'  # optional escape
@@ -75,7 +72,7 @@ def CEscape(text, as_utf8):
   """
   # PY3 hack: make Ord work for str and bytes:
   # //platforms/networking/data uses unicode here, hence basestring.
-  Ord = ord if isinstance(text, basestring) else lambda x: x
+  Ord = ord if isinstance(text, six.string_types) else lambda x: x
   if as_utf8:
     return ''.join(_cescape_utf8_to_str[Ord(c)] for c in text)
   return ''.join(_cescape_byte_to_str[Ord(c)] for c in text)
@@ -100,8 +97,7 @@ def CUnescape(text):
   # allow single-digit hex escapes (like '\xf').
   result = _CUNESCAPE_HEX.sub(ReplaceHex, text)
 
-  if sys.version_info[0] < 3:  ##PY25
-##!PY25  if str is bytes:  # PY2
+  if str is bytes:  # PY2
     return result.decode('string_escape')
   result = ''.join(_cescape_highbit_to_str[ord(c)] for c in result)
   return (result.encode('ascii')  # Make it bytes to allow decode.
diff --git a/python/google/protobuf/text_format.py b/python/google/protobuf/text_format.py
index fb54c50c..87b5c222 100755
--- a/python/google/protobuf/text_format.py
+++ b/python/google/protobuf/text_format.py
@@ -28,8 +28,6 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-#PY25 compatible for GAE.
-#
 # Copyright 2007 Google Inc. All Rights Reserved.
 
 """Contains routines for printing protocol messages in text format."""
@@ -39,6 +37,8 @@ __author__ = 'kenton@google.com (Kenton Varda)'
 import cStringIO
 import re
 
+import six
+
 from google.protobuf.internal import type_checkers
 from google.protobuf import descriptor
 from google.protobuf import text_encoding
@@ -195,7 +195,7 @@ def PrintFieldValue(field, value, out, indent=0, as_utf8=False,
       out.write(str(value))
   elif field.cpp_type == descriptor.FieldDescriptor.CPPTYPE_STRING:
     out.write('\"')
-    if isinstance(value, unicode):
+    if isinstance(value, six.text_type):
       out_value = value.encode('utf-8')
     else:
       out_value = value
@@ -505,7 +505,7 @@ class _Tokenizer(object):
   def _PopLine(self):
     while len(self._current_line) <= self._column:
       try:
-        self._current_line = self._lines.next()
+        self._current_line = next(self._lines)
       except StopIteration:
         self._current_line = ''
         self._more_lines = False
@@ -575,7 +575,7 @@ class _Tokenizer(object):
     """
     try:
       result = ParseInteger(self.token, is_signed=True, is_long=False)
-    except ValueError, e:
+    except ValueError as e:
       raise self._ParseError(str(e))
     self.NextToken()
     return result
@@ -591,7 +591,7 @@ class _Tokenizer(object):
     """
     try:
       result = ParseInteger(self.token, is_signed=False, is_long=False)
-    except ValueError, e:
+    except ValueError as e:
       raise self._ParseError(str(e))
     self.NextToken()
     return result
@@ -607,7 +607,7 @@ class _Tokenizer(object):
     """
     try:
       result = ParseInteger(self.token, is_signed=True, is_long=True)
-    except ValueError, e:
+    except ValueError as e:
       raise self._ParseError(str(e))
     self.NextToken()
     return result
@@ -623,7 +623,7 @@ class _Tokenizer(object):
     """
     try:
       result = ParseInteger(self.token, is_signed=False, is_long=True)
-    except ValueError, e:
+    except ValueError as e:
       raise self._ParseError(str(e))
     self.NextToken()
     return result
@@ -639,7 +639,7 @@ class _Tokenizer(object):
     """
     try:
       result = ParseFloat(self.token)
-    except ValueError, e:
+    except ValueError as e:
       raise self._ParseError(str(e))
     self.NextToken()
     return result
@@ -655,7 +655,7 @@ class _Tokenizer(object):
     """
     try:
       result = ParseBool(self.token)
-    except ValueError, e:
+    except ValueError as e:
       raise self._ParseError(str(e))
     self.NextToken()
     return result
@@ -671,8 +671,8 @@ class _Tokenizer(object):
     """
     the_bytes = self.ConsumeByteString()
     try:
-      return unicode(the_bytes, 'utf-8')
-    except UnicodeDecodeError, e:
+      return six.text_type(the_bytes, 'utf-8')
+    except UnicodeDecodeError as e:
       raise self._StringParseError(e)
 
   def ConsumeByteString(self):
@@ -687,8 +687,7 @@ class _Tokenizer(object):
     the_list = [self._ConsumeSingleByteString()]
     while self.token and self.token[0] in ('\'', '"'):
       the_list.append(self._ConsumeSingleByteString())
-    return ''.encode('latin1').join(the_list)  ##PY25
-##!PY25    return b''.join(the_list)
+    return b''.join(the_list)
 
   def _ConsumeSingleByteString(self):
     """Consume one token of a string literal.
@@ -706,7 +705,7 @@ class _Tokenizer(object):
 
     try:
       result = text_encoding.CUnescape(text[1:-1])
-    except ValueError, e:
+    except ValueError as e:
       raise self._ParseError(str(e))
     self.NextToken()
     return result
@@ -714,7 +713,7 @@ class _Tokenizer(object):
   def ConsumeEnum(self, field):
     try:
       result = ParseEnum(field, self.token)
-    except ValueError, e:
+    except ValueError as e:
       raise self._ParseError(str(e))
     self.NextToken()
     return result
@@ -779,7 +778,7 @@ def ParseInteger(text, is_signed=False, is_long=False):
     # alternate implementations where the distinction is more significant
     # (e.g. the C++ implementation) simpler.
     if is_long:
-      result = long(text, 0)
+      result = int(text, 0)
     else:
       result = int(text, 0)
   except ValueError: