From dd6001f1aa3b28a20a20ed1e3e3a352063517418 Mon Sep 17 00:00:00 2001
From: Charles-Meldhine Madi Mnemoi <63333367+cmnemoi@users.noreply.github.com>
Date: Sun, 25 Jan 2026 14:01:35 +0100
Subject: [PATCH] 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

(cherry picked from commit e95a843a5dbf8af618d0714705bbf97dfdb04044)
---
 changelog/12444.bugfix.rst |  1 +
 src/_pytest/python_api.py  |  7 +++----
 testing/python/approx.py   | 40 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 44 insertions(+), 4 deletions(-)
 create mode 100644 changelog/12444.bugfix.rst

diff --git a/changelog/12444.bugfix.rst b/changelog/12444.bugfix.rst
new file mode 100644
index 00000000000..146cfc7ab24
--- /dev/null
+++ b/changelog/12444.bugfix.rst
@@ -0,0 +1 @@
+Fixed :func:`pytest.approx` which now correctly takes account Mapping keys order to compare them.
diff --git a/src/_pytest/python_api.py b/src/_pytest/python_api.py
index 1e389eb0663..bab70aa4a8c 100644
--- a/src/_pytest/python_api.py
+++ b/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:
diff --git a/testing/python/approx.py b/testing/python/approx.py
index f870b9bd4d8..481df80565c 100644
--- a/testing/python/approx.py
+++ b/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"""