Removed the new max_diff argument to assertSequenceEqual. All unittest.TestCase assert methods that use difflib to produce failure messages now truncate overly long messages. New class attribute unittest.TestCase.maxDiff to configure this if necessary. Issue 8351.

Lib/unittest/case.py

@@ -13,7 +13,6 @@
 )
 
 __unittest = True
-TRUNCATED_DIFF = '\n[diff truncated...]'
 
 class SkipTest(Exception):
     """
@@ -157,6 +156,11 @@ 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
@@ -589,8 +593,7 @@ def deprecated_func(*args, **kwargs):
     failUnlessRaises = _deprecate(assertRaises)
     failIf = _deprecate(assertFalse)
 
-    def assertSequenceEqual(self, seq1, seq2, msg=None, seq_type=None,
+    def assertSequenceEqual(self, seq1, seq2, msg=None, seq_type=None):
-                            max_diff=80*8):
         """An equality assertion for ordered sequences (like lists and tuples).
 
         For the purposes of this function, a valid ordered sequence type is one
@@ -603,7 +606,6 @@ def assertSequenceEqual(self, seq1, seq2, msg=None, seq_type=None,
                     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 is not None:
             seq_type_name = seq_type.__name__
@@ -690,14 +692,15 @@ def assertSequenceEqual(self, seq1, seq2, msg=None, seq_type=None,
         diffMsg = '\n' + '\n'.join(
             difflib.ndiff(pprint.pformat(seq1).splitlines(),
                           pprint.pformat(seq2).splitlines()))
-        standardMsg = self._truncateMessage(standardMsg, diffMsg, max_diff)
+        standardMsg = self._truncateMessage(standardMsg, diffMsg)
         msg = self._formatMessage(msg, standardMsg)
         self.fail(msg)
 
-    def _truncateMessage(self, message, diff, max_diff):
+    def _truncateMessage(self, message, diff):
+        max_diff = self.maxDiff
         if max_diff is None or len(diff) <= max_diff:
             return message + diff
-        return message + diff[:max_diff] + TRUNCATED_DIFF
+        return message
 
     def assertListEqual(self, list1, list2, msg=None):
         """A list-specific equality assertion.
@@ -797,9 +800,10 @@ def assertDictEqual(self, d1, d2, msg=None):
         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):
@@ -882,8 +886,9 @@ def assertMultiLineEqual(self, first, second, msg=None):
                 '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):

Lib/unittest/test/test_case.py

@@ -591,20 +591,37 @@ def testEquality(self):
         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)
+            self.assertSequenceEqual(seq1, seq2)
-        except AssertionError as e:
+        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)
+            self.assertSequenceEqual(seq1, seq2)
-        except AssertionError as e:
+        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):