fix(testing): pytest.approx returns a clearer error mesage when comparing mappings with different keys

Author: cmnemoi

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()
         }

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"""