Fix off-by-one in trailing assertion diff skipping (#14637)

AUTHORS

@@ -10,6 +10,7 @@ Abhijeet Kasurde
 Adam Johnson
 Adam Stewart
 Adam Uhlir
+Aditya Tripuraneni
 Ahn Ki-Wook
 Akhilesh Ramakrishnan
 Akiomi Kamakura

changelog/14637.bugfix.rst

@@ -0,0 +1 @@
+Pytest now correctly skips identical trailing characters in assertion text diffs even when the compared strings differ at the beginning.

src/_pytest/assertion/compare_text.py

@@ -59,7 +59,7 @@ def _diff_text(
             left = left[i:]
             right = right[i:]
         if len(left) == len(right):
-            for i in range(len(left)):
+            for i in range(1, len(left) + 1):
                 if left[-i] != right[-i]:
                     break
             if i > 42:

testing/test_assertion.py

@@ -507,6 +507,14 @@ def test_text_skipping_trailing(self) -> None:
         for line in lines:
             assert "z" * 50 not in line
 
+    def test_text_skipping_trailing_when_prefix_differs(self) -> None:
+        # The trailing-skip branch must still work when the first characters differ.
+        lines = callequal("x" + "z" * 50, "y" + "z" * 50)
+        assert lines is not None
+        assert any("identical trailing" in line for line in lines)
+        for line in lines:
+            assert "z" * 50 not in line
+
     def test_text_skipping_verbose(self) -> None:
         lines = callequal("a" * 50 + "spam", "a" * 50 + "eggs", verbose=1)
         assert lines is not None