Display output failure message in insertion order of dictionary

changelog/13503.improvement.rst

@@ -0,0 +1,4 @@
+Fixed the order of dictionary keys in assertion failure messages.
+Previously, dictionary diffs were shown in alphabetical order, regardless of how the keys appeared in the original dicts.
+Now, common keys are shown in the insertion order of the left-hand dictionary,
+and differing keys are shown in the insertion order of their respective dictionaries

src/_pytest/_io/pprint.py

@@ -62,6 +62,7 @@ def __init__(
         indent: int = 4,
         width: int = 80,
         depth: int | None = None,
+        sort_dicts: bool = False,
     ) -> None:
         """Handle pretty printing operations onto a stream using a set of
         configured parameters.
@@ -75,6 +76,9 @@ def __init__(
         depth
             The maximum depth to print out nested structures.
 
+        sort_dicts
+            If true, dict keys are sorted.
+
         """
         if indent < 0:
             raise ValueError("indent must be >= 0")
@@ -85,6 +89,7 @@ def __init__(
         self._depth = depth
         self._indent_per_level = indent
         self._width = width
+        self._sort_dicts = sort_dicts
 
     def pformat(self, object: Any) -> str:
         sio = _StringIO()
@@ -162,7 +167,10 @@ def _pprint_dict(
     ) -> None:
         write = stream.write
         write("{")
-        items = sorted(object.items(), key=_safe_tuple)
+        if self._sort_dicts:
+            items = sorted(object.items(), key=_safe_tuple)
+        else:
+            items = object.items()
         self._format_dict_items(items, stream, indent, allowance, context, level)
         write("}")
 
@@ -608,7 +616,11 @@ def _safe_repr(
             components: list[str] = []
             append = components.append
             level += 1
-            for k, v in sorted(object.items(), key=_safe_tuple):
+            if self._sort_dicts:
+                items = sorted(object.items(), key=_safe_tuple)
+            else:
+                items = object.items()
+            for k, v in items:
                 krepr = self._safe_repr(k, context, maxlevels, level)
                 vrepr = self._safe_repr(v, context, maxlevels, level)
                 append(f"{krepr}: {vrepr}")

src/_pytest/assertion/util.py

@@ -505,29 +505,36 @@ def _compare_eq_dict(
     set_left = set(left)
     set_right = set(right)
     common = set_left.intersection(set_right)
-    same = {k: left[k] for k in common if left[k] == right[k]}
+    same = {k: left[k] for k in left if k in right and left[k] == right[k]}
     if same and verbose < 2:
         explanation += [f"Omitting {len(same)} identical items, use -vv to show"]
     elif same:
         explanation += ["Common items:"]
-        explanation += highlighter(pprint.pformat(same)).splitlines()
+        # Common items are displayed in the order of the left dict
+        explanation += highlighter(pprint.pformat(same, sort_dicts=False)).splitlines()
     diff = {k for k in common if left[k] != right[k]}
     if diff:
         explanation += ["Differing items:"]
-        for k in diff:
-            explanation += [
-                highlighter(saferepr({k: left[k]}))
-                + " != "
-                + highlighter(saferepr({k: right[k]}))
-            ]
+        # Differing items are displayed in the order of the left dict
+        for k in left:
+            if k in diff:
+                explanation += [
+                    highlighter(saferepr({k: left[k]}))
+                    + " != "
+                    + highlighter(saferepr({k: right[k]}))
+                ]
     extra_left = set_left - set_right
     len_extra_left = len(extra_left)
     if len_extra_left:
         explanation.append(
             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(
+                pprint.pformat(
+                    {k: left[k] for k in left if k in extra_left}, sort_dicts=False
+                )
+            ).splitlines()
         )
     extra_right = set_right - set_left
     len_extra_right = len(extra_right)
@@ -536,7 +543,11 @@ def _compare_eq_dict(
             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(
+                pprint.pformat(
+                    {k: right[k] for k in right if k in extra_right}, sort_dicts=False
+                )
+            ).splitlines()
         )
     return explanation
 

testing/io/test_pprint.py

@@ -406,3 +406,38 @@ class DataclassWithTwoItems:
 )
 def test_consistent_pretty_printer(data: Any, expected: str) -> None:
     assert PrettyPrinter().pformat(data) == textwrap.dedent(expected).strip()
+
+
+@pytest.mark.parametrize(
+    ("sort_dicts"),
+    (
+        pytest.param(True, id="sort_dicts-True"),
+        pytest.param(False, id="sort_dicts-False"),
+    ),
+)
+def test_pretty_printer_sort_dicts(sort_dicts: bool) -> None:
+    data = {
+        "b": 2,
+        "a": 1,
+        "c": 3,
+    }
+
+    if sort_dicts:
+        expected = textwrap.dedent("""
+            {
+                'a': 1,
+                'b': 2,
+                'c': 3,
+            }
+        """).strip()
+    else:
+        expected = textwrap.dedent("""
+            {
+                'b': 2,
+                'a': 1,
+                'c': 3,
+            }
+        """).strip()
+
+    actual = PrettyPrinter(sort_dicts=sort_dicts).pformat(data)
+    assert actual == expected

testing/test_error_diffs.py

@@ -198,6 +198,85 @@ def test_this():
         """,
         id="Compare dicts with differing items",
     ),
+    pytest.param(
+        """
+        def test_this():
+            result = {'d': 4, 'c': 3, 'b': 2, 'a': 1}
+            expected = {'d': 4, 'c': 3, 'e': 5}
+            assert result == expected
+        """,
+        """
+        >       assert result == expected
+        E       AssertionError: assert {'d': 4, 'c': 3, 'b': 2, 'a': 1} == {'d': 4, 'c': 3, 'e': 5}
+        E         Common items:
+        E         {'d': 4, 'c': 3}
+        E         Left contains 2 more items:
+        E         {'b': 2, 'a': 1}
+        E         Right contains 1 more item:
+        E         {'e': 5}
+        E         Full diff:
+        E           {
+        E               'd': 4,
+        E               'c': 3,
+        E         -     'e': 5,
+        E         ?      ^   ^
+        E         +     'b': 2,
+        E         ?      ^   ^
+        E         +     'a': 1,
+        E           }
+        """,
+        id="Compare dicts and check order of diff",
+    ),
+    pytest.param(
+        """
+        def test_this():
+            result = {
+                "b": {"m": 3, "n": 4},
+                "a": {"x": 1, "y": 2},
+                "c": {"k": 5, "l": 6}
+            }
+            expected = {
+                "c": {"l": 6, "k": 5},
+                "e": {"v": 8},
+                "d": {"u": 7}
+            }
+            assert result == expected
+        """,
+        """
+        >       assert result == expected
+                E       AssertionError: assert {'b': {'m': 3, 'n': 4}, 'a': {'x': 1, 'y': 2}, 'c': {'k': 5, 'l': 6}} \
+== {'c': {'l': 6, 'k': 5}, 'e': {'v': 8}, 'd': {'u': 7}}
+        E         Common items:
+        E         {'c': {'k': 5, 'l': 6}}
+        E         Left contains 2 more items:
+        E         {'b': {'m': 3, 'n': 4}, 'a': {'x': 1, 'y': 2}}
+        E         Right contains 2 more items:
+        E         {'e': {'v': 8}, 'd': {'u': 7}}
+        E         Full diff:
+        E           {
+        E         +     'b': {
+        E         +         'm': 3,
+        E         +         'n': 4,
+        E         +     },
+        E         +     'a': {
+        E         +         'x': 1,
+        E         +         'y': 2,
+        E         +     },
+        E               'c': {
+        E         +         'k': 5,
+        E                   'l': 6,
+        E         -         'k': 5,
+        E         -     },
+        E         -     'e': {
+        E         -         'v': 8,
+        E         -     },
+        E         -     'd': {
+        E         -         'u': 7,
+        E               },
+        E           }
+                """,
+        id="Compare nested dicts and check order of diff",
+    ),
     pytest.param(
         """
         def test_this():