diff -r f961e9179998 Lib/unittest/case.py
--- a/Lib/unittest/case.py	Mon Apr 04 01:47:52 2011 +0200
+++ b/Lib/unittest/case.py	Tue Apr 05 02:28:32 2011 +0300
@@ -169,6 +169,10 @@
 
     maxDiff = 80*8
 
+    # If a string is longer than _diffThreshold, use normal comparison instead
+    # of difflib.
+    _diffThreshold = 2**16
+
     # Attribute used by TestSuite for classSetUp
 
     _classSetupFailed = False
@@ -899,7 +903,11 @@
         self.assertIsInstance(second, basestring,
                 'Second argument is not a string')
 
-        if first != second:
+        # don't use difflib if the strings are too long
+        if (len(first) > self._diffThreshold or
+            len(second) > self._diffThreshold):
+            self._baseAssertEqual(first, second, msg)
+        elif first != second:
             firstlines = first.splitlines(True)
             secondlines = second.splitlines(True)
             if len(firstlines) == 1 and first.strip('\r\n') == first:
diff -r f961e9179998 Lib/unittest/test/test_case.py
--- a/Lib/unittest/test/test_case.py	Mon Apr 04 01:47:52 2011 +0200
+++ b/Lib/unittest/test/test_case.py	Tue Apr 05 02:28:32 2011 +0300
@@ -667,6 +667,42 @@
         else:
             self.fail('assertMultiLineEqual did not fail')
 
+    def testAssertEqual_diffThreshold(self):
+        # check threshold value
+        self.assertEqual(self._diffThreshold, 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**8
+        self.addCleanup(lambda: setattr(self, '_diffThreshold', old_threshold))
+
+        # under the threshold: diff marker (^) in error message
+        s = u'x' * (2**7)
+        with self.assertRaises(self.failureException) as cm:
+            self.assertEqual(s + 'a', s + 'b')
+        self.assertIn('^', str(cm.exception))
+        self.assertEqual(s + 'a', s + 'a')
+
+        # over the threshold: diff not used and marker (^) not in error message
+        s = u'x' * (2**9)
+        # 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.assertEqual(s1, s2)
+        self.assertNotIn('^', str(cm.exception))
+        self.assertEqual(str(cm.exception), '%r != %r' % (s1, s2))
+        self.assertEqual(s + 'a', s + 'a')
+
     def testAssertItemsEqual(self):
         a = object()
         self.assertItemsEqual([1, 2, 3], [3, 2, 1])