Show correct extra item in sequence comparison diffs

changelog/13652.bugfix.rst

@@ -0,0 +1 @@
+``contains one more item`` now reports the actual extra item in sequence diffs.

src/_pytest/assertion/_compare_sequence.py

@@ -52,6 +52,8 @@ def _compare_eq_sequence(
     comparing_bytes = isinstance(left, bytes) and isinstance(right, bytes)
     len_left = len(left)
     len_right = len(right)
+    found_diff = False
+    i = -1
     for i in range(min(len_left, len_right)):
         if left[i] != right[i]:
             if comparing_bytes:
@@ -73,6 +75,7 @@ def _compare_eq_sequence(
                 f"At index {i} diff:"
                 f" {highlighter(repr(left_value))} != {highlighter(repr(right_value))}"
             )
+            found_diff = True
             break
 
     if comparing_bytes:
@@ -84,11 +87,20 @@ def _compare_eq_sequence(
     if len_diff:
         if len_diff > 0:
             dir_with_more = "Left"
-            extra = saferepr(left[len_right])
+            # If the longer side has exactly one extra item and the tails after
+            # the first differing index align (offset by one), that item is the
+            # insertion.
+            if found_diff and len_diff == 1 and left[i + 1 :] == right[i:]:
+                extra = saferepr(left[i])
+            else:
+                extra = saferepr(left[len_right])
         else:
             len_diff = 0 - len_diff
             dir_with_more = "Right"
-            extra = saferepr(right[len_left])
+            if found_diff and len_diff == 1 and right[i + 1 :] == left[i:]:
+                extra = saferepr(right[i])
+            else:
+                extra = saferepr(right[len_left])
 
         if len_diff == 1:
             yield f"{dir_with_more} contains one more item: {highlighter(extra)}"

testing/test_assertion.py

@@ -1105,6 +1105,34 @@ def test_sequence_different_items(self) -> None:
             "  )",
         ]
 
+    @pytest.mark.parametrize(
+        ("left", "right", "expected"),
+        [
+            ([], [1], "Right contains one more item: 1"),
+            ([1], [], "Left contains one more item: 1"),
+            ([1, 2, 3], [0, 1, 2, 3], "Right contains one more item: 0"),
+            ([1, 2, 3], [1, 2, 3, 4], "Right contains one more item: 4"),
+            ([1, 2, 3], [1, 2, 0, 3], "Right contains one more item: 0"),
+            ([1, 2, 0, 3], [1, 2, 3], "Left contains one more item: 0"),
+            ([1, 1], [1, 0, 1], "Right contains one more item: 0"),
+            ([3, 4, 5], [1, 2], "Left contains one more item: 5"),
+            # Fallback: single extra item but tails don't align
+            ([1, 2, 3], [1, 9, 8, 3], "Right contains one more item: 3"),
+            ([1, 9, 8, 3], [1, 2, 3], "Left contains one more item: 3"),
+            (
+                [1, 2, 3],
+                [0, 1, 2, 3, 4],
+                "Right contains 2 more items, first extra item: 3",
+            ),
+        ],
+    )
+    def test_sequence_extra_item_message(
+        self, left: list[object], right: list[object], expected: str
+    ) -> None:
+        lines = callequal(left, right, verbose=1)
+        assert lines is not None
+        assert expected in lines
+
     def test_set(self) -> None:
         expl = callequal({0, 1}, {0, 2})
         assert expl is not None

testing/test_error_diffs.py

@@ -66,7 +66,7 @@ def test_this():
         >       assert result == expected
         E       assert [1, 3] == [1, 2, 3]
         E         At index 1 diff: 3 != 2
-        E         Right contains one more item: 3
+        E         Right contains one more item: 2
         E         Full diff: (-: missing in left side, +: extra in left side)
         E           [
         E               1,
@@ -76,6 +76,50 @@ def test_this():
         """,
         id="Compare lists, one item missing",
     ),
+    pytest.param(
+        """
+        def test_this():
+            result =   [1, 2, 3]
+            expected = [1, 2, 0, 3]
+            assert result == expected
+        """,
+        """
+        >       assert result == expected
+        E       assert [1, 2, 3] == [1, 2, 0, 3]
+        E         At index 2 diff: 3 != 0
+        E         Right contains one more item: 0
+        E         Full diff: (-: missing in left side, +: extra in left side)
+        E           [
+        E               1,
+        E               2,
+        E         -     0,
+        E               3,
+        E           ]
+        """,
+        id="Compare lists, one extra item inserted mid-list",
+    ),
+    pytest.param(
+        """
+        def test_this():
+            result =   [1, 2, 0, 3]
+            expected = [1, 2, 3]
+            assert result == expected
+        """,
+        """
+        >       assert result == expected
+        E       assert [1, 2, 0, 3] == [1, 2, 3]
+        E         At index 2 diff: 0 != 3
+        E         Left contains one more item: 0
+        E         Full diff: (-: missing in left side, +: extra in left side)
+        E           [
+        E               1,
+        E               2,
+        E         +     0,
+        E               3,
+        E           ]
+        """,
+        id="Compare lists, one extra item inserted mid-list on left",
+    ),
     pytest.param(
         """
         def test_this():