From d3193dab35e7861cd09728fc0c70d884d5b78e6a Mon Sep 17 00:00:00 2001
From: Charles-Meldhine Madi Mnemoi <icdk5b4zf@relay.firefox.com>
Date: Wed, 15 Oct 2025 14:51:36 +0200
Subject: [PATCH 1/4] fix(testing): `pytest.approx` returns a clearer error
 mesage when comparing mappings with different keys

---
 src/_pytest/python_api.py |  6 ++++++
 testing/python/approx.py  | 15 +++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/src/_pytest/python_api.py b/src/_pytest/python_api.py
index c64587bb219..1e389eb0663 100644
--- a/src/_pytest/python_api.py
+++ b/src/_pytest/python_api.py
@@ -242,6 +242,12 @@ 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()):
+            return [
+                "comparison failed.",
+                f"Mappings has different keys: expected {self.expected.keys()} but got {other_side.keys()}",
+            ]
+
         approx_side_as_map = {
             k: self._approx_scalar(v) for k, v in self.expected.items()
         }
diff --git a/testing/python/approx.py b/testing/python/approx.py
index 9032e6c0cb4..251df723e12 100644
--- a/testing/python/approx.py
+++ b/testing/python/approx.py
@@ -11,6 +11,7 @@
 import operator
 from operator import eq
 from operator import ne
+import re
 
 from _pytest.pytester import Pytester
 from _pytest.python_api import _recursive_sequence_map
@@ -1047,6 +1048,20 @@ def test_strange_sequence(self):
         assert b == pytest.approx(a, abs=2)
         assert b != pytest.approx(a, abs=0.5)
 
+    def test_approx_dicts_with_mismatch_on_keys(self) -> None:
+        """https://github.com/pytest-dev/pytest/issues/13816"""
+        expected = {"a": 1, "c": 3}
+        actual = {"a": 1, "b": 3}
+
+        with pytest.raises(
+            AssertionError,
+            match=re.escape(
+                "comparison failed.\n  Mappings has different keys: "
+                "expected dict_keys(['a', 'b']) but got dict_keys(['a', 'c'])"
+            ),
+        ):
+            assert expected == approx(actual)
+
 
 class MyVec3:  # incomplete
     """sequence like"""

From 4ac631ff678212ab34606aac1c99a578f622026c Mon Sep 17 00:00:00 2001
From: Charles-Meldhine Madi Mnemoi <icdk5b4zf@relay.firefox.com>
Date: Wed, 15 Oct 2025 14:54:55 +0200
Subject: [PATCH 2/4] docs: add changelog entry

---
 changelog/13816.bugfix.rst | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog/13816.bugfix.rst

diff --git a/changelog/13816.bugfix.rst b/changelog/13816.bugfix.rst
new file mode 100644
index 00000000000..432fc519ed5
--- /dev/null
+++ b/changelog/13816.bugfix.rst
@@ -0,0 +1 @@
+Fixed :func:`pytest.approx` which now returns a clearer error message when comparing mappings with different keys.

From 8af256646f5901cd6608b611f7e4f2d66bb14003 Mon Sep 17 00:00:00 2001
From: Charles-Meldhine Madi Mnemoi <icdk5b4zf@relay.firefox.com>
Date: Wed, 15 Oct 2025 14:55:22 +0200
Subject: [PATCH 3/4] docs: add Charles-Meldhine Madi Mnemoi (cmnemoi) to
 AUTHORS

---
 AUTHORS | 1 +
 1 file changed, 1 insertion(+)

diff --git a/AUTHORS b/AUTHORS
index b28f49776d6..5849228200b 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -83,6 +83,7 @@ Carlos Jenkins
 Ceridwen
 Charles Cloud
 Charles Machalow
+Charles-Meldhine Madi Mnemoi (cmnemoi)
 Charnjit SiNGH (CCSJ)
 Cheuk Ting Ho
 Chris Mahoney

From 10c83c54dc4818a8d47878d7db3db1397e702ea4 Mon Sep 17 00:00:00 2001
From: Charles-Meldhine Madi Mnemoi <icdk5b4zf@relay.firefox.com>
Date: Wed, 15 Oct 2025 15:05:53 +0200
Subject: [PATCH 4/4] fix(testing): correct expected and actual dictionary keys

---
 testing/python/approx.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/testing/python/approx.py b/testing/python/approx.py
index 251df723e12..f870b9bd4d8 100644
--- a/testing/python/approx.py
+++ b/testing/python/approx.py
@@ -1050,8 +1050,8 @@ def test_strange_sequence(self):
 
     def test_approx_dicts_with_mismatch_on_keys(self) -> None:
         """https://github.com/pytest-dev/pytest/issues/13816"""
-        expected = {"a": 1, "c": 3}
-        actual = {"a": 1, "b": 3}
+        expected = {"a": 1, "b": 3}
+        actual = {"a": 1, "c": 3}
 
         with pytest.raises(
             AssertionError,
@@ -1060,7 +1060,7 @@ def test_approx_dicts_with_mismatch_on_keys(self) -> None:
                 "expected dict_keys(['a', 'b']) but got dict_keys(['a', 'c'])"
             ),
         ):
-            assert expected == approx(actual)
+            assert actual == approx(expected)
 
 
 class MyVec3:  # incomplete