Fix pytest.approx to correctly take account Mapping keys order to compare them (#13815)

This fixes an issue with `pytest.approx` where the error message incorrectly reported all elements as mismatched when comparing mappings with different key orders, even when only some values differed.

The original code paired values by position rather than by key. This caused incorrect mismatch reporting when dictionary keys were in different orders.

Fixes #12444

Author: cmnemoi

changelog/12444.bugfix.rst

@@ -0,0 +1 @@
+Fixed :func:`pytest.approx` which now correctly takes account Mapping keys order to compare them.

src/_pytest/python_api.py

@@ -242,7 +242,7 @@ def _repr_compare(self, other_side: Mapping[object, float]) -> list[str]:
                 f"Lengths: {len(self.expected)} and {len(other_side)}",
             ]
 
-        if set(self.expected.keys()) != set(other_side.keys()):
+        if self.expected.keys() != other_side.keys():
             return [
                 "comparison failed.",
                 f"Mappings has different keys: expected {self.expected.keys()} but got {other_side.keys()}",
@@ -256,9 +256,8 @@ def _repr_compare(self, other_side: Mapping[object, float]) -> list[str]:
         max_abs_diff = -math.inf
         max_rel_diff = -math.inf
         different_ids = []
-        for (approx_key, approx_value), other_value in zip(
-            approx_side_as_map.items(), other_side.values(), strict=True
-        ):
+        for approx_key, approx_value in approx_side_as_map.items():
+            other_value = other_side[approx_key]
             if approx_value != other_value:
                 if approx_value.expected is not None and other_value is not None:
                     try:

testing/python/approx.py

@@ -1062,6 +1062,46 @@ def test_approx_dicts_with_mismatch_on_keys(self) -> None:
         ):
             assert actual == approx(expected)
 
+    def test_approx_on_unordered_mapping_with_mismatch(
+        self, pytester: Pytester
+    ) -> None:
+        """https://github.com/pytest-dev/pytest/issues/12444"""
+        pytester.makepyfile(
+            """
+            import pytest
+
+            def test_approx_on_unordered_mapping_with_mismatch():
+                expected = {"a": 1, "b": 2, "c": 3, "d": 4}
+                actual = {"d": 4, "c": 5, "a": 8, "b": 2}
+                assert actual == pytest.approx(expected)
+            """
+        )
+        result = pytester.runpytest()
+        result.assert_outcomes(failed=1)
+        result.stdout.fnmatch_lines(
+            [
+                "*comparison failed.**Mismatched elements: 2 / 4:*",
+                "*Max absolute difference: 7*",
+                "*Index | Obtained | Expected *",
+                "* a * | 8 * | 1 *",
+                "* c * | 5 * | 3 *",
+            ]
+        )
+
+    def test_approx_on_unordered_mapping_matching(self, pytester: Pytester) -> None:
+        """https://github.com/pytest-dev/pytest/issues/12444"""
+        pytester.makepyfile(
+            """
+            import pytest
+            def test_approx_on_unordered_mapping_matching():
+                expected = {"a": 1, "b": 2, "c": 3, "d": 4}
+                actual = {"d": 4, "c": 3, "a": 1, "b": 2}
+                assert actual == pytest.approx(expected)
+            """
+        )
+        result = pytester.runpytest()
+        result.assert_outcomes(passed=1)
+
 
 class MyVec3:  # incomplete
     """sequence like"""