diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
index 4a13f0a6751..a8b2f4d3f43 100644
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -12,7 +12,6 @@ from .util import (strclass, safe_repr, sorted_list_difference,
                    unorderable_list_difference)
 
 __unittest = True
-TRUNCATED_DIFF = '\n[diff truncated...]'
 
 class SkipTest(Exception):
     """
@@ -169,6 +168,11 @@ class TestCase(object):
 
     longMessage = False
 
+    # This attribute sets the maximum length of a diff in failure messsages
+    # by assert methods using difflib. It is looked up as an instance attribute
+    # so can be configured by individual tests if required.
+    maxDiff = 80*8
+
     # Attribute used by TestSuite for classSetUp
 
     _classSetupFailed = False
@@ -599,8 +603,7 @@ class TestCase(object):
     failUnlessRaises = _deprecate(assertRaises)
     failIf = _deprecate(assertFalse)
 
-    def assertSequenceEqual(self, seq1, seq2, msg=None, seq_type=None,
-                            max_diff=80*8):
+    def assertSequenceEqual(self, seq1, seq2, msg=None, seq_type=None):
         """An equality assertion for ordered sequences (like lists and tuples).
 
         For the purposes of this function, a valid ordered sequence type is one
@@ -613,7 +616,6 @@ class TestCase(object):
                     datatype should be enforced.
             msg: Optional message to use on failure instead of a list of
                     differences.
-            max_diff: Maximum size off the diff, larger diffs are not shown
         """
         if seq_type != None:
             seq_type_name = seq_type.__name__
@@ -700,13 +702,17 @@ class TestCase(object):
         diffMsg = '\n' + '\n'.join(
             difflib.ndiff(pprint.pformat(seq1).splitlines(),
                           pprint.pformat(seq2).splitlines()))
-        if max_diff is None or len(diffMsg) <= max_diff:
-            standardMsg += diffMsg
-        else:
-            standardMsg += diffMsg[:max_diff] + TRUNCATED_DIFF
+
+        standardMsg = self._truncateMessage(standardMsg, diffMsg)
         msg = self._formatMessage(msg, standardMsg)
         self.fail(msg)
 
+    def _truncateMessage(self, message, diff):
+        max_diff = self.maxDiff
+        if max_diff is None or len(diff) <= max_diff:
+            return message + diff
+        return message
+
     def assertListEqual(self, list1, list2, msg=None):
         """A list-specific equality assertion.
 
@@ -805,9 +811,10 @@ class TestCase(object):
         self.assert_(isinstance(d2, dict), 'Second argument is not a dictionary')
 
         if d1 != d2:
-            standardMsg = ('\n' + '\n'.join(difflib.ndiff(
+            diff = ('\n' + '\n'.join(difflib.ndiff(
                            pprint.pformat(d1).splitlines(),
                            pprint.pformat(d2).splitlines())))
+            standardMsg = self._truncateMessage('', diff)
             self.fail(self._formatMessage(msg, standardMsg))
 
     def assertDictContainsSubset(self, expected, actual, msg=None):
@@ -924,8 +931,9 @@ class TestCase(object):
                 'Second argument is not a string'))
 
         if first != second:
-            standardMsg = '\n' + ''.join(difflib.ndiff(first.splitlines(True),
+            diff = '\n' + ''.join(difflib.ndiff(first.splitlines(True),
                                                        second.splitlines(True)))
+            standardMsg = self._truncateMessage('', diff)
             self.fail(self._formatMessage(msg, standardMsg))
 
     def assertLess(self, a, b, msg=None):
diff --git a/Lib/unittest/test/test_case.py b/Lib/unittest/test/test_case.py
index 14a386a6f39..7de79310ed4 100644
--- a/Lib/unittest/test/test_case.py
+++ b/Lib/unittest/test/test_case.py
@@ -592,20 +592,38 @@ class Test_TestCase(unittest.TestCase, TestEquality, TestHashing):
         self.assertRaises(self.failureException, self.assertDictEqual, 1, 1)
 
     def testAssertSequenceEqualMaxDiff(self):
+        self.assertEqual(self.maxDiff, 80*8)
         seq1 = 'a' + 'x' * 80**2
         seq2 = 'b' + 'x' * 80**2
         diff = '\n'.join(difflib.ndiff(pprint.pformat(seq1).splitlines(),
                                        pprint.pformat(seq2).splitlines()))
+
+        self.maxDiff = len(diff)//2
         try:
-            self.assertSequenceEqual(seq1, seq2, max_diff=len(diff)//2)
-        except AssertionError as e:
+
+            self.assertSequenceEqual(seq1, seq2)
+        except self.failureException as e:
             msg = e.args[0]
+        else:
+            self.fail('assertSequenceEqual did not fail.')
         self.assertTrue(len(msg) < len(diff))
 
+        self.maxDiff = len(diff) * 2
         try:
-            self.assertSequenceEqual(seq1, seq2, max_diff=len(diff)*2)
-        except AssertionError as e:
+            self.assertSequenceEqual(seq1, seq2)
+        except self.failureException as e:
             msg = e.args[0]
+        else:
+            self.fail('assertSequenceEqual did not fail.')
+        self.assertTrue(len(msg) > len(diff))
+
+        self.maxDiff = None
+        try:
+            self.assertSequenceEqual(seq1, seq2)
+        except self.failureException as e:
+            msg = e.args[0]
+        else:
+            self.fail('assertSequenceEqual did not fail.')
         self.assertTrue(len(msg) > len(diff))
 
     def testAssertItemsEqual(self):