Add regression test and fixed dict insertion order in assertion diffs…

changelog/14079.bugfix.rst

@@ -0,0 +1,6 @@
+Fix assertion diff output to preserve dictionary insertion order.
+
+When comparing dictionaries with extra keys, pytest could incorrectly inject
+those keys into the structured diff output, producing misleading results.
+The assertion diff now correctly preserves insertion order and reports extra
+keys separately.

src/_pytest/assertion/util.py

@@ -511,6 +511,7 @@ def _compare_eq_dict(
     elif same:
         explanation += ["Common items:"]
         explanation += highlighter(pprint.pformat(same)).splitlines()
+
     diff = {k for k in common if left[k] != right[k]}
     if diff:
         explanation += ["Differing items:"]
@@ -520,24 +521,25 @@ def _compare_eq_dict(
                 + " != "
                 + highlighter(saferepr({k: right[k]}))
             ]
+
     extra_left = set_left - set_right
-    len_extra_left = len(extra_left)
-    if len_extra_left:
+    if extra_left:
         explanation.append(
-            f"Left contains {len_extra_left} more item{'' if len_extra_left == 1 else 's'}:"
+            f"Left contains {len(extra_left)} more item{'' if len(extra_left) == 1 else 's'}:"
         )
         explanation.extend(
-            highlighter(pprint.pformat({k: left[k] for k in extra_left})).splitlines()
+            highlighter(saferepr({k: left[k] for k in extra_left})).splitlines()
         )
+
     extra_right = set_right - set_left
-    len_extra_right = len(extra_right)
-    if len_extra_right:
+    if extra_right:
         explanation.append(
-            f"Right contains {len_extra_right} more item{'' if len_extra_right == 1 else 's'}:"
+            f"Right contains {len(extra_right)} more item{'' if len(extra_right) == 1 else 's'}:"
         )
         explanation.extend(
-            highlighter(pprint.pformat({k: right[k] for k in extra_right})).splitlines()
+            highlighter(saferepr({k: right[k] for k in extra_right})).splitlines()
         )
+
     return explanation
 
 

testing/test_assertion.py

@@ -209,8 +209,10 @@ def test_pytest_plugins_rewrite_module_names_correctly(
             "hamster.py": "",
             "test_foo.py": """\
                 def test_foo(pytestconfig):
-                    assert pytestconfig.pluginmanager.rewrite_hook.find_spec('ham') is not None
-                    assert pytestconfig.pluginmanager.rewrite_hook.find_spec('hamster') is None
+                    assert pytestconfig.pluginmanager.rewrite_hook.find_spec(
+                        'ham') is not None
+                    assert pytestconfig.pluginmanager.rewrite_hook.find_spec(
+                        'hamster') is None
             """,
         }
         pytester.makepyfile(**contents)
@@ -417,6 +419,45 @@ class TestAssert_reprcompare:
     def test_different_types(self) -> None:
         assert callequal([0, 1], "foo") is None
 
+    def test_dict_preserves_insertion_order_regression(self) -> None:
+        # Regression test for issue #14079
+
+        long_a = "a" * 80
+        sub = {
+            "long_a": long_a,
+            "sub1": {
+                "long_a": "substring that gets wrapped " * 3,
+            },
+        }
+
+        left = {"env": {"sub": sub}}
+        right = {"env": {"sub": sub}, "new": 1}
+
+        diff = callequal(left, right, verbose=True)
+        assert diff is not None
+
+        # extra key is reported
+        assert "{'new': 1}" in diff
+
+        # inspect only structured diff
+        assert "Full diff:" in diff
+        start = diff.index("Full diff:") + 1
+        diff_block = diff[start:]
+
+        # 'new' must not appear as a structural key
+        assert all(
+            "'new': 1" not in line or line.lstrip().startswith("-")
+            for line in diff_block
+        )
+
+        # insertion order preserved inside left dict
+        env_index = next(i for i, line in enumerate(diff_block) if "'env': {" in line)
+        closing_index = next(
+            i for i, line in enumerate(diff_block) if line.strip() == "}"
+        )
+
+        assert env_index < closing_index
+
     def test_summary(self) -> None:
         lines = callequal([0, 1], [0, 2])
         assert lines is not None