diff -r 768234f5c246 Lib/unittest/case.py
--- a/Lib/unittest/case.py	Sat Jun 25 15:03:52 2011 +0200
+++ b/Lib/unittest/case.py	Sun Jun 26 17:12:16 2011 +0200
@@ -1034,9 +1034,20 @@
             if len(firstlines) == 1 and first.strip('\r\n') == first:
                 firstlines = [first + '\n']
                 secondlines = [second + '\n']
-            standardMsg = '%s != %s' % (safe_repr(first, True),
-                                        safe_repr(second, True))
+
             diff = '\n' + ''.join(difflib.ndiff(firstlines, secondlines))
+            # Get the maxsize to truncate to, which is the index when
+            # the two strings first, second start to differ.
+            seq = difflib.SequenceMatcher(a=repr(first), b=repr(second))
+            truncate_size = seq.find_longest_match(
+                                0, len(repr(first)), 0, len(repr(second))).size
+            # If a diff was found, the trancate should include the next char.
+            if truncate_size != 0:
+                truncate_size += 1
+            standardMsg = '%s != %s' % (
+                            safe_repr(first, True, max_length=truncate_size),
+                            safe_repr(second, True, max_length=truncate_size))
+
             standardMsg = self._truncateMessage(standardMsg, diff)
             self.fail(self._formatMessage(msg, standardMsg))
 
diff -r 768234f5c246 Lib/unittest/test/test_case.py
--- a/Lib/unittest/test/test_case.py	Sat Jun 25 15:03:52 2011 +0200
+++ b/Lib/unittest/test/test_case.py	Sun Jun 26 17:12:16 2011 +0200
@@ -756,6 +756,16 @@
         self.assertEqual(str(cm.exception), '%r != %r' % (s1, s2))
         self.assertEqual(s + 'a', s + 'a')
 
+    def testAssertEqualHumanReadableOutput(self):
+        # Issue #12038: Make the assertion error message more human redable
+        # for long strings.
+        s1 = "aaaaaa"
+        s2 = "abbbbb"
+        with self.assertRaises(self.failureException) as cm:
+            self.assertEqual(s1, s2)
+        self.assertIn("aa [truncated]... != 'ab [truncated]...",
+                      str(cm.exception))
+
     def testAssertCountEqual(self):
         a = object()
         self.assertCountEqual([1, 2, 3], [3, 2, 1])
diff -r 768234f5c246 Lib/unittest/util.py
--- a/Lib/unittest/util.py	Sat Jun 25 15:03:52 2011 +0200
+++ b/Lib/unittest/util.py	Sun Jun 26 17:12:16 2011 +0200
@@ -5,14 +5,31 @@
 __unittest = True
 
 _MAX_LENGTH = 80
-def safe_repr(obj, short=False):
+def safe_repr(obj, short=False, max_length=None):
     try:
         result = repr(obj)
     except Exception:
         result = object.__repr__(obj)
-    if not short or len(result) < _MAX_LENGTH:
+
+    len_result = len(result)
+    truncate_size = max_length or _MAX_LENGTH
+    if not short or len_result <= truncate_size:
         return result
-    return result[:_MAX_LENGTH] + ' [truncated]...'
+
+    if truncate_size <= _MAX_LENGTH:
+        result = result[:truncate_size]
+    else:
+        #  0         _MAX_LENGTH     truncate_size    len(result)
+        #  begin                        trunc             end
+        #  <----------------------------------------------->
+        #  begin   ...   trunc
+        #  <-------------->
+        result = (result[:_MAX_LENGTH // 2] + '...' +
+                  result[-_MAX_LENGTH // 2:truncate_size])
+
+    if truncate_size < len_result:
+        return result + ' [truncated]...'
+    return result
 
 def strclass(cls):
     return "%s.%s" % (cls.__module__, cls.__name__)