Fix error message for comparing single line strings in unittest.TestCase.assertEqual.

Issue 9174.

Lib/unittest/case.py

@@ -940,9 +940,14 @@ def assertMultiLineEqual(self, first, second, msg=None):
                 'Second argument is not a string'))
 
         if first != second:
-            standardMsg = '%s != %s' % (safe_repr(first, True), safe_repr(second, True))
-            diff = '\n' + ''.join(difflib.ndiff(first.splitlines(True),
-                                                       second.splitlines(True)))
+            firstlines = first.splitlines(True)
+            secondlines = second.splitlines(True)
+            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))
             standardMsg = self._truncateMessage(standardMsg, diff)
             self.fail(self._formatMessage(msg, standardMsg))
 

Lib/unittest/test/test_case.py

@@ -841,6 +841,21 @@ def testAssertMultiLineEqual(self):
             # so can't use assertEqual either. Just use assertTrue.
             self.assertTrue(sample_text_error == error)
 
+    def testAsertEqualSingleLine(self):
+        sample_text = "laden swallows fly slowly"
+        revised_sample_text = "unladen swallows fly quickly"
+        sample_text_error = """\
+- laden swallows fly slowly
+?                    ^^^^
++ unladen swallows fly quickly
+? ++                   ^^^^^
+"""
+        try:
+            self.assertEqual(sample_text, revised_sample_text)
+        except self.failureException as e:
+            error = str(e).split('\n', 1)[1]
+            self.assertTrue(sample_text_error == error)
+
     def testAssertIsNone(self):
         self.assertIsNone(None)
         self.assertRaises(self.failureException, self.assertIsNone, False)