diff --git a/sonnet/examples/module_with_build_args.py b/sonnet/examples/module_with_build_args.py
index e647cf2e..dfe3d73b 100644
--- a/sonnet/examples/module_with_build_args.py
+++ b/sonnet/examples/module_with_build_args.py
@@ -72,7 +72,7 @@ def main(unused_argv):
 
   with tf.Session() as sess:
     sess.run(tf.global_variables_initializer())
-    for _ in xrange(100):
+    for _ in range(100):
       sess.run(train_step)
     # Check that evaluating train_model_outputs twice returns the same value.
     train_outputs, train_outputs_2 = sess.run([train_model_outputs,
diff --git a/sonnet/examples/rnn_shakespeare.py b/sonnet/examples/rnn_shakespeare.py
index b4a98769..2fb9a19a 100644
--- a/sonnet/examples/rnn_shakespeare.py
+++ b/sonnet/examples/rnn_shakespeare.py
@@ -151,7 +151,7 @@ def generate_string(self, initial_logits, initial_state, sequence_length):
     current_state = initial_state
 
     generated_letters = []
-    for _ in xrange(sequence_length):
+    for _ in range(sequence_length):
       # Sample a character index from distribution.
       char_index = tf.squeeze(tf.multinomial(current_logits, 1))
       char_one_hot = tf.one_hot(char_index, self._output_size, 1.0, 0.0)
@@ -270,7 +270,7 @@ def train(num_training_iterations, report_interval,
 
     start_iteration = sess.run(global_step)
 
-    for train_iteration in xrange(start_iteration, num_training_iterations):
+    for train_iteration in range(start_iteration, num_training_iterations):
       if (train_iteration + 1) % report_interval == 0:
         train_loss_v, valid_loss_v, _ = sess.run(
             (train_loss, valid_loss, train_step))
diff --git a/sonnet/python/modules/base.py b/sonnet/python/modules/base.py
index 84084a31..c18dd061 100644
--- a/sonnet/python/modules/base.py
+++ b/sonnet/python/modules/base.py
@@ -26,7 +26,6 @@
 
 import abc
 import collections
-import types
 # Dependency imports
 import six
 from sonnet.python.modules import util
@@ -130,7 +129,7 @@ def __init__(self, name=None):
       ValueError: If name is not specified.
     """
 
-    if name is None or not isinstance(name, types.StringTypes):
+    if name is None or not isinstance(name, six.string_types):
       raise ValueError("Name must be a string.")
 
     self._connected_subgraphs = []
diff --git a/sonnet/python/modules/base_test.py b/sonnet/python/modules/base_test.py
index 0242ae79..83358e19 100644
--- a/sonnet/python/modules/base_test.py
+++ b/sonnet/python/modules/base_test.py
@@ -22,6 +22,7 @@
 from functools import partial
 
 import numpy as np
+import six
 from sonnet.python.modules import base
 import tensorflow as tf
 
@@ -62,8 +63,10 @@ def testInitializerKeys(self):
     self.assertEqual(keys, {"foo", "bar"})
     keys = ModuleWithNoInitializerKeys.get_possible_initializer_keys()
     self.assertEqual(keys, set())
+    msg = ("missing 1 required positional argument" if six.PY3
+           else "takes exactly 2 arguments")
     self.assertRaisesRegexp(
-        TypeError, "takes exactly 2 arguments",
+        TypeError, msg,
         ModuleWithCustomInitializerKeys.get_possible_initializer_keys)
     keys = ModuleWithCustomInitializerKeys.get_possible_initializer_keys(True)
     self.assertEqual(keys, {"foo"})
@@ -146,7 +149,7 @@ def testFunctionType(self):
     with self.assertRaises(TypeError) as cm:
       base.Module(build="not_a_function")
 
-    self.assertEqual(cm.exception.message, "Input 'build' must be callable.")
+    self.assertEqual(str(cm.exception), "Input 'build' must be callable.")
 
   def testSharing(self):
     batch_size = 3
diff --git a/sonnet/python/modules/basic.py b/sonnet/python/modules/basic.py
index 91ef8e4a..3ac2ec1e 100644
--- a/sonnet/python/modules/basic.py
+++ b/sonnet/python/modules/basic.py
@@ -27,6 +27,7 @@
 # Dependency imports
 
 import numpy as np
+from six.moves import xrange  # pylint: disable=redefined-builtin
 from sonnet.python.modules import base
 from sonnet.python.modules import util
 from sonnet.python.ops import nest
diff --git a/sonnet/python/modules/basic_rnn_test.py b/sonnet/python/modules/basic_rnn_test.py
index f076f1bf..14e97507 100644
--- a/sonnet/python/modules/basic_rnn_test.py
+++ b/sonnet/python/modules/basic_rnn_test.py
@@ -22,6 +22,7 @@
 
 # Dependency imports
 import numpy as np
+from six.moves import xrange  # pylint: disable=redefined-builtin
 import sonnet as snt
 from sonnet.testing import parameterized
 import tensorflow as tf
diff --git a/sonnet/python/modules/basic_test.py b/sonnet/python/modules/basic_test.py
index 815a751b..b1c32bda 100644
--- a/sonnet/python/modules/basic_test.py
+++ b/sonnet/python/modules/basic_test.py
@@ -23,6 +23,7 @@
 # Dependency imports
 
 import numpy as np
+from six.moves import xrange  # pylint: disable=redefined-builtin
 import sonnet as snt
 from sonnet.testing import parameterized
 import tensorflow as tf
diff --git a/sonnet/python/modules/batch_norm.py b/sonnet/python/modules/batch_norm.py
index eb0cc586..cf8723ad 100644
--- a/sonnet/python/modules/batch_norm.py
+++ b/sonnet/python/modules/batch_norm.py
@@ -306,7 +306,7 @@ def _infer_fused_data_format(self, input_batch):
       # Reduce over the second dimension.
       return "NCHW"
     else:
-      raise ValueError("Invalid axis option {:s}. This does not correspond to"
+      raise ValueError("Invalid axis option {}. This does not correspond to"
                        " either the NHWC format (0, 1, 2) or the NCHW "
                        "(0, 2, 3).".format(axis))
 
@@ -439,23 +439,23 @@ def _build(self, input_batch, is_training=True, test_local_stats=True):
     if self._axis is not None:
       if len(self._axis) > len(input_shape):
         raise base.IncompatibleShapeError(
-            "Too many indices specified in axis: len({:s}) > len({:s}).".format(
+            "Too many indices specified in axis: len({}) > len({}).".format(
                 self._axis, input_shape))
 
       if max(self._axis) >= len(input_shape):
         raise base.IncompatibleShapeError(
             "One or more index in axis is too large for "
-            "input shape: {:s} >= {:d}.".format(self._axis, len(input_shape)))
+            "input shape: {} >= {:d}.".format(self._axis, len(input_shape)))
 
       if min(self._axis) < 0:
         raise base.IncompatibleShapeError(
-            "Indices in axis must be non-negative: {:s} < 0.".format(
+            "Indices in axis must be non-negative: {} < 0.".format(
                 self._axis))
 
       axis = self._axis
     else:
       # Reduce over all dimensions except the last.
-      axis = range(len(input_shape))[:-1]
+      axis = tuple(range(len(input_shape))[:-1])
 
     # See following for important note on accuracy for dtype=tf.float16
     # https://github.com/tensorflow/tensorflow/blob/master/tensorflow/python/ops/nn_impl.py#L63
diff --git a/sonnet/python/modules/block_matrix.py b/sonnet/python/modules/block_matrix.py
index 1e459d9e..b4d5f90e 100644
--- a/sonnet/python/modules/block_matrix.py
+++ b/sonnet/python/modules/block_matrix.py
@@ -19,6 +19,7 @@
 from __future__ import print_function
 
 # Dependency imports
+from six.moves import xrange  # pylint: disable=redefined-builtin
 from sonnet.python.modules import base
 import tensorflow as tf
 
diff --git a/sonnet/python/modules/conv_test.py b/sonnet/python/modules/conv_test.py
index f4d2d52a..56ff907e 100644
--- a/sonnet/python/modules/conv_test.py
+++ b/sonnet/python/modules/conv_test.py
@@ -753,7 +753,7 @@ def testMaskErrorInvalidRank(self):
     with self.assertRaises(snt.Error) as cm:
       snt.Conv2D(output_channels=4, kernel_shape=3, mask=mask)
     self.assertEqual(
-        cm.exception.message,
+        str(cm.exception),
         "Invalid mask rank: {}".format(mask.ndim))
 
   def testMaskErrorInvalidType(self):
@@ -763,7 +763,7 @@ def testMaskErrorInvalidType(self):
     with self.assertRaises(TypeError) as cm:
       snt.Conv2D(output_channels=4, kernel_shape=3, mask=mask)
     self.assertEqual(
-        cm.exception.message, "Invalid type for mask: {}".format(type(mask)))
+        str(cm.exception), "Invalid type for mask: {}".format(type(mask)))
 
   def testMaskErrorIncompatibleRank2(self):
     """Errors are thrown for incompatible rank 2 mask."""
@@ -772,8 +772,8 @@ def testMaskErrorIncompatibleRank2(self):
     x = tf.constant(0.0, shape=(2, 8, 8, 6))
     with self.assertRaises(snt.Error) as cm:
       snt.Conv2D(output_channels=4, kernel_shape=5, mask=mask)(x)
-    self.assertEqual(
-        cm.exception.message, "Invalid mask shape: {}".format(mask.shape))
+    self.assertTrue(str(cm.exception).startswith(
+        "Invalid mask shape: {}".format(mask.shape)))
 
   def testMaskErrorIncompatibleRank4(self):
     """Errors are thrown for incompatible rank 4 mask."""
@@ -782,8 +782,8 @@ def testMaskErrorIncompatibleRank4(self):
     x = tf.constant(0.0, shape=(2, 8, 8, 6))
     with self.assertRaises(snt.Error) as cm:
       snt.Conv2D(output_channels=4, kernel_shape=5, mask=mask)(x)
-    self.assertEqual(
-        cm.exception.message, "Invalid mask shape: {}".format(mask.shape))
+    self.assertTrue(str(cm.exception).startswith(
+        "Invalid mask shape: {}".format(mask.shape)))
 
 
 class Conv2DTransposeTest(parameterized.ParameterizedTestCase,
diff --git a/sonnet/python/modules/nets/convnet.py b/sonnet/python/modules/nets/convnet.py
index ec61769f..8393491e 100644
--- a/sonnet/python/modules/nets/convnet.py
+++ b/sonnet/python/modules/nets/convnet.py
@@ -20,6 +20,7 @@
 
 import collections
 
+from six.moves import xrange  # pylint: disable=redefined-builtin
 from sonnet.python.modules import base
 from sonnet.python.modules import batch_norm
 from sonnet.python.modules import conv
diff --git a/sonnet/python/modules/nets/mlp.py b/sonnet/python/modules/nets/mlp.py
index c9febd29..3f2f4cab 100644
--- a/sonnet/python/modules/nets/mlp.py
+++ b/sonnet/python/modules/nets/mlp.py
@@ -20,6 +20,7 @@
 
 import collections
 
+from six.moves import xrange  # pylint: disable=redefined-builtin
 from sonnet.python.modules import base
 from sonnet.python.modules import basic
 from sonnet.python.modules import util
diff --git a/sonnet/python/modules/rnn_core.py b/sonnet/python/modules/rnn_core.py
index 965a8a5f..fdf0d653 100644
--- a/sonnet/python/modules/rnn_core.py
+++ b/sonnet/python/modules/rnn_core.py
@@ -29,6 +29,7 @@
 # Dependency imports
 
 import six
+from six.moves import xrange  # pylint: disable=redefined-builtin
 from sonnet.python.modules import base
 from sonnet.python.modules import basic
 import tensorflow as tf
diff --git a/sonnet/python/modules/sequential_test.py b/sonnet/python/modules/sequential_test.py
index d6256042..118a1967 100644
--- a/sonnet/python/modules/sequential_test.py
+++ b/sonnet/python/modules/sequential_test.py
@@ -19,6 +19,7 @@
 from __future__ import print_function
 
 # Dependency imports
+import six
 import sonnet as snt
 import tensorflow as tf
 
@@ -60,7 +61,10 @@ def module1(a, b):
     def module2(a, b, c):
       return a, b, c
 
-    err_str = r"module2\(\) takes exactly 3 arguments \(2 given\)"
+    if six.PY3:
+      err_str = r"module2\(\) missing 1 required positional argument: 'c'"
+    else:
+      err_str = r"module2\(\) takes exactly 3 arguments \(2 given\)"
     with self.assertRaisesRegexp(TypeError, err_str):
       _, _ = snt.Sequential([module1, module2], name="seq2")(1, 2)
 
diff --git a/sonnet/python/modules/spatial_transformer.py b/sonnet/python/modules/spatial_transformer.py
index 74dabbaf..a92088b9 100644
--- a/sonnet/python/modules/spatial_transformer.py
+++ b/sonnet/python/modules/spatial_transformer.py
@@ -23,6 +23,7 @@
 
 # Dependency imports
 import numpy as np
+from six.moves import xrange  # pylint: disable=redefined-builtin
 from sonnet.python.modules import base
 from sonnet.python.modules import basic
 import tensorflow as tf
@@ -419,7 +420,7 @@ def _affine_grid_warper_inverse(inputs):
       index = iter(range(6))
       def get_variable(constraint):
         if constraint is None:
-          i = index.next()
+          i = next(index)
           return inputs[:, i:i+1]
         else:
           return tf.fill(constant_shape, tf.constant(constraint,
diff --git a/sonnet/python/modules/util.py b/sonnet/python/modules/util.py
index 20cc01bd..997b4e38 100644
--- a/sonnet/python/modules/util.py
+++ b/sonnet/python/modules/util.py
@@ -21,6 +21,7 @@
 import re
 
 # Dependency imports
+import six
 import tensorflow as tf
 
 
@@ -82,7 +83,7 @@ def _check_nested_callables(dictionary, object_name):
     TypeError: If the dictionary contains something that is not either a
       dictionary or a callable.
   """
-  for key, entry in dictionary.iteritems():
+  for key, entry in six.iteritems(dictionary):
     if isinstance(entry, dict):
       _check_nested_callables(entry, object_name)
     elif not callable(entry):
@@ -311,7 +312,7 @@ def get_saver(scope, collections=(tf.GraphKeys.GLOBAL_VARIABLES,),
 
 def has_variable_scope(obj):
   """Determines whether the given object has a variable scope."""
-  return hasattr(obj, "variable_scope") or "variable_scope" in dir(obj)
+  return "variable_scope" in dir(obj)
 
 
 def _format_table(rows):
diff --git a/sonnet/python/ops/nest.py b/sonnet/python/ops/nest.py
index 4516895a..21afaa8c 100644
--- a/sonnet/python/ops/nest.py
+++ b/sonnet/python/ops/nest.py
@@ -290,7 +290,7 @@ def map(fn_or_op, *inputs):  # pylint: disable=redefined-builtin
 def _sorted(dict_):
   """Returns a sorted list from the dict, with error if keys not sortable."""
   try:
-    return sorted(dict_.iterkeys())
+    return sorted(six.iterkeys(dict_))
   except TypeError:
     raise TypeError("nest only supports dicts with sortable keys.")
 
@@ -307,7 +307,7 @@ def _iterable_like(instance, args):
     `args` with the type of `instance`.
   """
   if isinstance(instance, collections.OrderedDict):
-    return collections.OrderedDict(zip(instance.iterkeys(), args))
+    return collections.OrderedDict(zip(six.iterkeys(instance), args))
   elif isinstance(instance, dict):
     return dict(zip(_sorted(instance), args))
   elif (isinstance(instance, tuple) and
diff --git a/sonnet/python/ops/nest_test.py b/sonnet/python/ops/nest_test.py
index 32319a88..546378ab 100644
--- a/sonnet/python/ops/nest_test.py
+++ b/sonnet/python/ops/nest_test.py
@@ -24,9 +24,12 @@
 # Dependency imports
 
 import numpy as np
+import six
 from sonnet.python.ops import nest
 import tensorflow as tf
 
+typekw = "class" if six.PY3 else "type"
+
 
 class NestTest(tf.test.TestCase):
 
@@ -35,7 +38,7 @@ def testAssertShallowStructure(self):
     inp_abc = ["a", "b", "c"]
     with self.assertRaises(ValueError) as cm:
       nest.assert_shallow_structure(inp_abc, inp_ab)
-    self.assertEqual(cm.exception.message,
+    self.assertEqual(str(cm.exception),
                      "The two structures don't have the same sequence length. "
                      "Input structure has length 2, while shallow structure "
                      "has length 3.")
@@ -44,10 +47,10 @@ def testAssertShallowStructure(self):
     inp_ab2 = [[1, 1], [2, 2]]
     with self.assertRaises(TypeError) as cm:
       nest.assert_shallow_structure(inp_ab2, inp_ab1)
-    self.assertEqual(cm.exception.message,
+    self.assertEqual(str(cm.exception),
                      "The two structures don't have the same sequence type. "
-                     "Input structure has type <type 'tuple'>, while shallow "
-                     "structure has type <type 'list'>.")
+                     "Input structure has type <{0} 'tuple'>, while shallow "
+                     "structure has type <{0} 'list'>.".format(typekw))
 
   def testFlattenUpTo(self):
     # Normal application (Example 1).
@@ -123,9 +126,9 @@ def testFlattenUpTo(self):
     with self.assertRaises(TypeError) as cm:
       flattened_input_tree = nest.flatten_up_to(shallow_tree, input_tree)
     flattened_shallow_tree = nest.flatten_up_to(shallow_tree, shallow_tree)
-    self.assertEqual(cm.exception.message,
-                     "If shallow structure is a sequence, input must also "
-                     "be a sequence. Input has type: <type 'str'>.")
+    self.assertEqual(str(cm.exception),
+                     "If shallow structure is a sequence, input must also be "
+                     "a sequence. Input has type: <{} 'str'>.".format(typekw))
     self.assertEqual(flattened_shallow_tree, shallow_tree)
 
     input_tree = "input_tree"
@@ -133,9 +136,9 @@ def testFlattenUpTo(self):
     with self.assertRaises(TypeError) as cm:
       flattened_input_tree = nest.flatten_up_to(shallow_tree, input_tree)
     flattened_shallow_tree = nest.flatten_up_to(shallow_tree, shallow_tree)
-    self.assertEqual(cm.exception.message,
-                     "If shallow structure is a sequence, input must also "
-                     "be a sequence. Input has type: <type 'str'>.")
+    self.assertEqual(str(cm.exception),
+                     "If shallow structure is a sequence, input must also be "
+                     "a sequence. Input has type: <{} 'str'>.".format(typekw))
     self.assertEqual(flattened_shallow_tree, shallow_tree)
 
     # Using non-iterable elements.
@@ -144,9 +147,9 @@ def testFlattenUpTo(self):
     with self.assertRaises(TypeError) as cm:
       flattened_input_tree = nest.flatten_up_to(shallow_tree, input_tree)
     flattened_shallow_tree = nest.flatten_up_to(shallow_tree, shallow_tree)
-    self.assertEqual(cm.exception.message,
-                     "If shallow structure is a sequence, input must also "
-                     "be a sequence. Input has type: <type 'int'>.")
+    self.assertEqual(str(cm.exception),
+                     "If shallow structure is a sequence, input must also be "
+                     "a sequence. Input has type: <{} 'int'>.".format(typekw))
     self.assertEqual(flattened_shallow_tree, shallow_tree)
 
     input_tree = 0
@@ -154,9 +157,9 @@ def testFlattenUpTo(self):
     with self.assertRaises(TypeError) as cm:
       flattened_input_tree = nest.flatten_up_to(shallow_tree, input_tree)
     flattened_shallow_tree = nest.flatten_up_to(shallow_tree, shallow_tree)
-    self.assertEqual(cm.exception.message,
-                     "If shallow structure is a sequence, input must also "
-                     "be a sequence. Input has type: <type 'int'>.")
+    self.assertEqual(str(cm.exception),
+                     "If shallow structure is a sequence, input must also be "
+                     "a sequence. Input has type: <{} 'int'>.".format(typekw))
     self.assertEqual(flattened_shallow_tree, shallow_tree)
 
   def testMapUpTo(self):
diff --git a/sonnet/python/ops/resampler_test.py b/sonnet/python/ops/resampler_test.py
index 5a298a6c..186f0160 100644
--- a/sonnet/python/ops/resampler_test.py
+++ b/sonnet/python/ops/resampler_test.py
@@ -22,6 +22,7 @@
 # Dependency imports
 
 import numpy as np
+from six.moves import xrange  # pylint: disable=redefined-builtin
 import sonnet as snt
 from sonnet.testing import parameterized
 
diff --git a/sonnet/testing/parameterized/parameterized.py b/sonnet/testing/parameterized/parameterized.py
index d2e432e7..037b8877 100644
--- a/sonnet/testing/parameterized/parameterized.py
+++ b/sonnet/testing/parameterized/parameterized.py
@@ -147,6 +147,7 @@ def testSumIsZero(self, arg):
 import unittest
 import uuid
 
+import six
 from tensorflow.python.platform import googletest
 
 ADDR_RE = re.compile(r'\<([a-zA-Z0-9_\-\.]+) object at 0x[a-fA-F0-9]+\>')
@@ -167,13 +168,13 @@ def _StrClass(cls):
 
 def _NonStringIterable(obj):
   return (isinstance(obj, collections.Iterable) and not
-          isinstance(obj, basestring))
+          isinstance(obj, six.string_types))
 
 
 def _FormatParameterList(testcase_params):
   if isinstance(testcase_params, collections.Mapping):
     return ', '.join('%s=%s' % (argname, _CleanRepr(value))
-                     for argname, value in testcase_params.iteritems())
+                     for argname, value in six.iteritems(testcase_params))
   elif _NonStringIterable(testcase_params):
     return ', '.join(map(_CleanRepr, testcase_params))
   else:
@@ -265,7 +266,7 @@ def _ModifyClass(class_object, testcases, naming_type):
       'Cannot add parameters to %s,'
       ' which already has parameterized methods.' % (class_object,))
   class_object._id_suffix = id_suffix = {}
-  for name, obj in class_object.__dict__.items():
+  for name, obj in list(six.iteritems(class_object.__dict__)):
     if (name.startswith(unittest.TestLoader.testMethodPrefix)
         and isinstance(obj, types.FunctionType)):
       delattr(class_object, name)
@@ -273,7 +274,7 @@ def _ModifyClass(class_object, testcases, naming_type):
       _UpdateClassDictForParamTestCase(
           methods, id_suffix, name,
           _ParameterizedTestIter(obj, testcases, naming_type))
-      for name, meth in methods.iteritems():
+      for name, meth in six.iteritems(methods):
         setattr(class_object, name, meth)
 
 
@@ -353,7 +354,7 @@ class TestGeneratorMetaclass(type):
 
   def __new__(mcs, class_name, bases, dct):
     dct['_id_suffix'] = id_suffix = {}
-    for name, obj in dct.items():
+    for name, obj in list(six.iteritems(dct)):
       if (name.startswith(unittest.TestLoader.testMethodPrefix) and
           _NonStringIterable(obj)):
         iterator = iter(obj)
@@ -385,9 +386,9 @@ def _UpdateClassDictForParamTestCase(dct, id_suffix, name, iterator):
     id_suffix[new_name] = getattr(func, '__x_extra_id__', '')
 
 
-class ParameterizedTestCase(googletest.TestCase):
+class ParameterizedTestCase(
+    six.with_metaclass(TestGeneratorMetaclass, googletest.TestCase)):
   """Base class for test cases using the Parameters decorator."""
-  __metaclass__ = TestGeneratorMetaclass
 
   def _OriginalName(self):
     return self._testMethodName.split(_SEPARATOR)[0]
diff --git a/sonnet/testing/parameterized/parameterized_test.py b/sonnet/testing/parameterized/parameterized_test.py
index a35af9ad..98977480 100644
--- a/sonnet/testing/parameterized/parameterized_test.py
+++ b/sonnet/testing/parameterized/parameterized_test.py
@@ -19,6 +19,8 @@
 import unittest
 
 # Dependency imports
+import six
+from six.moves import xrange  # pylint: disable=redefined-builtin
 from sonnet.testing import parameterized
 
 from tensorflow.python.platform import googletest
@@ -355,7 +357,9 @@ def testSomething(unused_self, unused_obj):  # pylint: disable=invalid-name
 
     expected_testcases = [1, 2, 3, 4, 5, 6]
     self.assertTrue(hasattr(testSomething, 'testcases'))
-    self.assertItemsEqual(expected_testcases, testSomething.testcases)
+    assert_items_equal = (self.assertCountEqual if six.PY3
+                          else self.assertItemsEqual)
+    assert_items_equal(expected_testcases, testSomething.testcases)
 
   def testChainedDecorator(self):
     ts = unittest.makeSuite(self.ChainedTests)