Preserve insertion order for extra dict items in assertion output

changelog/13503.bugfix.rst

@@ -0,0 +1,2 @@
+Preserved insertion order for extra dictionary items in assertion failure output.
+Display dictionary differences in assertion failures using the original key insertion order instead of sorted order.

src/_pytest/assertion/util.py

@@ -520,7 +520,7 @@ def _compare_eq_dict(
                 + " != "
                 + highlighter(saferepr({k: right[k]}))
             ]
-    extra_left = set_left - set_right
+    extra_left = [k for k in left if k not in right]
     len_extra_left = len(extra_left)
     if len_extra_left:
         explanation.append(
@@ -529,7 +529,7 @@ def _compare_eq_dict(
         explanation.extend(
             highlighter(pprint.pformat({k: left[k] for k in extra_left})).splitlines()
         )
-    extra_right = set_right - set_left
+    extra_right = [k for k in right if k not in left]
     len_extra_right = len(extra_right)
     if len_extra_right:
         explanation.append(

testing/test_assertion.py

@@ -2,6 +2,7 @@
 from __future__ import annotations
 
 from collections.abc import MutableSequence
+import re
 import sys
 import textwrap
 from typing import Any
@@ -2198,3 +2199,35 @@ def test_vvv():
         ]
     )
     result.stdout.no_fnmatch_line(expected_non_vvv_arg_line)
+
+
+def test_dict_extra_items_preserve_insertion_order(pytester):
+    pytester.makepyfile(
+        test_order="""
+        def test_order():
+            a = {
+                "first": 1,
+                "second": 2,
+            }
+            assert a == {}
+        """
+    )
+
+    result = pytester.runpytest()
+
+    stdout = result.stdout.str()
+    stdout = re.sub(r"\x1b\[[0-9;]*m", "", stdout)
+
+    assert "Left contains 2 more items:" in stdout
+
+    dict_line = next(
+        line
+        for line in stdout.splitlines()
+        if "{" in line and "}" in line and "first" in line
+    )
+
+    first = dict_line.find("first")
+    second = dict_line.find("second")
+
+    assert first != -1 and second != -1
+    assert first < second