Bugfix for approx on mismatched dictionaries

changelog/12444.bugfix.rst

@@ -0,0 +1 @@
+In :pr:`13046`, fixes a bug where the .approx method incorrectly reported all key-value pairs as mismatched when only one was incorrect in unordered dictionaries.

src/_pytest/python_api.py

@@ -254,9 +254,8 @@
         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()
-        ):
+        for approx_key, approx_value in approx_side_as_map.items():
+            other_value = other_side.get(approx_key, None)
             if approx_value != other_value:
                 if approx_value.expected is not None and other_value is not None:
                     try:

testing/python/approx.py

@@ -1022,6 +1022,17 @@ def test_strange_sequence(self):
         assert b == pytest.approx(a, abs=2)
         assert b != pytest.approx(a, abs=0.5)
 
+    def test_approx_unordered_dicts_with_mismatch(self):
+        """https://github.com/pytest-dev/pytest/pull/12445"""
+        dict1 = {"a": 1.0, "b": 1.0, "c": 3.1}
+        dict2 = {"b": 2.0, "a": 1.0, "c": 3.1}  # 'b' has a different value
+
+        with pytest.raises(
+            AssertionError,
+            match="Mismatched elements: 1 / 3:\n  Max absolute difference: 1.0\n",
+        ):
+            assert dict1 == pytest.approx(dict2)
+
 
 class MyVec3:  # incomplete
     """sequence like"""