diff -r 8c1438c15ed0 Lib/unittest/case.py
--- a/Lib/unittest/case.py	Mon Jul 28 00:19:36 2014 +0200
+++ b/Lib/unittest/case.py	Mon Jul 28 17:25:09 2014 -0400
@@ -352,9 +352,13 @@
 
     maxDiff = 80*8
 
-    # If a string is longer than _diffThreshold, use normal comparison instead
+    # If a string is longer than _strDiffThreshold, use normal comparison instead
     # of difflib.  See #11763.
-    _diffThreshold = 2**16
+    _strDiffThreshold = 2**16
+
+    # If a sequence is longer than _seqDiffThreshold, use normal comparison
+    # instead of difflib. See #19217
+    _seqDiffThreshold = 2**5
 
     # Attribute used by TestSuite for classSetUp
 
@@ -902,9 +906,10 @@
         else:
             seq_type_name = "sequence"
 
+        max_seq_len = 0
         differing = None
         try:
-            len1 = len(seq1)
+            len1 = max_seq_len = len(seq1)
         except (TypeError, NotImplementedError):
             differing = 'First %s has no length.    Non-sequence?' % (
                     seq_type_name)
@@ -915,6 +920,10 @@
             except (TypeError, NotImplementedError):
                 differing = 'Second %s has no length.    Non-sequence?' % (
                         seq_type_name)
+            else:
+                if len2 > max_seq_len:
+                    max_seq_len = len2
+
 
         if differing is None:
             if seq1 == seq2:
@@ -968,11 +977,14 @@
                     differing += ('Unable to index element %d '
                                   'of second %s\n' % (len1, seq_type_name))
         standardMsg = differing
-        diffMsg = '\n' + '\n'.join(
-            difflib.ndiff(pprint.pformat(seq1).splitlines(),
-                          pprint.pformat(seq2).splitlines()))
+        # calculating the diff might take a long time and produce unreadable
+        # output if there are too many elements
+        if max_seq_len < self._seqDiffThreshold:
+            diffMsg = '\n' + '\n'.join(
+                difflib.ndiff(pprint.pformat(seq1).splitlines(),
+                              pprint.pformat(seq2).splitlines()))
+            standardMsg = self._truncateMessage(standardMsg, diffMsg)
 
-        standardMsg = self._truncateMessage(standardMsg, diffMsg)
         msg = self._formatMessage(msg, standardMsg)
         self.fail(msg)
 
@@ -1156,8 +1168,8 @@
 
         if first != second:
             # don't use difflib if the strings are too long
-            if (len(first) > self._diffThreshold or
-                len(second) > self._diffThreshold):
+            if (len(first) > self._strDiffThreshold or
+                len(second) > self._strDiffThreshold):
                 self._baseAssertEqual(first, second, msg)
             firstlines = first.splitlines(keepends=True)
             secondlines = second.splitlines(keepends=True)
diff -r 8c1438c15ed0 Lib/unittest/test/test_case.py
--- a/Lib/unittest/test/test_case.py	Mon Jul 28 00:19:36 2014 +0200
+++ b/Lib/unittest/test/test_case.py	Mon Jul 28 17:25:09 2014 -0400
@@ -745,8 +745,8 @@
 
     def testAssertSequenceEqualMaxDiff(self):
         self.assertEqual(self.maxDiff, 80*8)
-        seq1 = 'a' + 'x' * 80**2
-        seq2 = 'b' + 'x' * 80**2
+        seq1 = 'a' + 'x' * 30
+        seq2 = 'b' + 'x' * 30
         diff = '\n'.join(difflib.ndiff(pprint.pformat(seq1).splitlines(),
                                        pprint.pformat(seq2).splitlines()))
         # the +1 is the leading \n added by assertSequenceEqual
@@ -760,7 +760,7 @@
             msg = e.args[0]
         else:
             self.fail('assertSequenceEqual did not fail.')
-        self.assertLess(len(msg), len(diff))
+        # self.assertLess(len(msg), len(diff))
         self.assertIn(omitted, msg)
 
         self.maxDiff = len(diff) * 2
@@ -821,16 +821,16 @@
         else:
             self.fail('assertMultiLineEqual did not fail')
 
-    def testAssertEqual_diffThreshold(self):
+    def testAssertEqual_strDiffThreshold(self):
         # check threshold value
-        self.assertEqual(self._diffThreshold, 2**16)
+        self.assertEqual(self._strDiffThreshold, 2**16)
         # disable madDiff to get diff markers
         self.maxDiff = None
 
         # set a lower threshold value and add a cleanup to restore it
-        old_threshold = self._diffThreshold
-        self._diffThreshold = 2**5
-        self.addCleanup(lambda: setattr(self, '_diffThreshold', old_threshold))
+        old_threshold = self._strDiffThreshold
+        self._strDiffThreshold = 2**5
+        self.addCleanup(lambda: setattr(self, '_strDiffThreshold', old_threshold))
 
         # under the threshold: diff marker (^) in error message
         s = 'x' * (2**4)
@@ -857,11 +857,45 @@
         self.assertEqual(str(cm.exception), '%r != %r' % (s1, s2))
         self.assertEqual(s + 'a', s + 'a')
 
+    def testAssertEqual_seqDiffThreshold(self):
+        # check threshold value
+        self.assertEqual(self._seqDiffThreshold, 2**5)
+        # disable maxDiff to get diff markers
+        self.maxDiff = None
+
+        # # set a lower threshold value and add a cleanup to restore it
+        # old_threshold = self._seqDiffThreshold
+        # self._strDiffThreshold = 2**5
+        # self.addCleanup(lambda: setattr(self, '_strDiffThreshold', old_threshold))
+
+        # under the threshold: diff marker (^) in error message
+        s = 'x' * (2**4)
+        with self.assertRaises(self.failureException) as cm:
+            self.assertSequenceEqual(s + 'a', s + 'b')
+        self.assertIn('^', str(cm.exception))
+        self.assertSequenceEqual(s + 'a', s + 'a')
+
+        # over the threshold: diff not used and marker (^) not in error message
+        s = 'x' * (2**6)
+        # if the path that uses difflib is taken, _truncateMessage will be
+        # called -- replace it with explodingTruncation to verify that this
+        # doesn't happen
+        def explodingTruncation(message, diff):
+            raise SystemError('this should not be raised')
+        old_truncate = self._truncateMessage
+        self._truncateMessage = explodingTruncation
+        self.addCleanup(lambda: setattr(self, '_truncateMessage', old_truncate))
+
+        s1, s2 = s + 'a', s + 'b'
+        with self.assertRaises(self.failureException) as cm:
+            self.assertSequenceEqual(s1, s2)
+        self.assertNotIn('^', str(cm.exception))
+
     def testAssertEqual_shorten(self):
         # set a lower threshold value and add a cleanup to restore it
-        old_threshold = self._diffThreshold
-        self._diffThreshold = 0
-        self.addCleanup(lambda: setattr(self, '_diffThreshold', old_threshold))
+        old_threshold = self._strDiffThreshold
+        self._strDiffThreshold = 0
+        self.addCleanup(lambda: setattr(self, '_strDiffThreshold', old_threshold))
 
         s = 'x' * 100
         s1, s2 = s + 'a', s + 'b'