gh-68968: Correcting message display issue with assertEqual (#103937)

1 files changed, 23 insertions, 8 deletions

diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
index 018f22e..001b640 100644
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -1217,19 +1217,34 @@ class TestCase(object):
 
     def assertMultiLineEqual(self, first, second, msg=None):
         """Assert that two multi-line strings are equal."""
-        self.assertIsInstance(first, str, 'First argument is not a string')
-        self.assertIsInstance(second, str, 'Second argument is not a string')
+        self.assertIsInstance(first, str, "First argument is not a string")
+        self.assertIsInstance(second, str, "Second argument is not a string")
 
         if first != second:
-            # don't use difflib if the strings are too long
+            # 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)
-            firstlines = first.splitlines(keepends=True)
-            secondlines = second.splitlines(keepends=True)
-            if len(firstlines) == 1 and first.strip('\r\n') == first:
-                firstlines = [first + '\n']
-                secondlines = [second + '\n']
+
+            # Append \n to both strings if either is missing the \n.
+            # This allows the final ndiff to show the \n difference. The
+            # exception here is if the string is empty, in which case no
+            # \n should be added
+            first_presplit = first
+            second_presplit = second
+            if first and second:
+                if first[-1] != '\n' or second[-1] != '\n':
+                    first_presplit += '\n'
+                    second_presplit += '\n'
+            elif second and second[-1] != '\n':
+                second_presplit += '\n'
+            elif first and first[-1] != '\n':
+                first_presplit += '\n'
+
+            firstlines = first_presplit.splitlines(keepends=True)
+            secondlines = second_presplit.splitlines(keepends=True)
+
+            # Generate the message and diff, then raise the exception
             standardMsg = '%s != %s' % _common_shorten_repr(first, second)
             diff = '\n' + ''.join(difflib.ndiff(firstlines, secondlines))
             standardMsg = self._truncateMessage(standardMsg, diff)