Fix error assertion handling in approx when None in dict comparison

Dict comparsion in the ApproxMapping class did not check if values were None before attempting to subtract for max_abs_diff stat, which was throwing an TypeError instead of being handled by pytest error assertion. Check for None has been added before these calculations, so that None will properly show as Obtained/Expected in pytest assert message

changelog/10702.bugfix.rst

@@ -0,0 +1 @@
+Fixed error assertion handling in :func:`pytest.approx` when ``None`` is an expected or received value when comparing dictionaries.

src/_pytest/python_api.py

@@ -265,19 +265,20 @@ class ApproxMapping(ApproxBase):
             approx_side_as_map.items(), other_side.values()
         ):
             if approx_value != other_value:
-                max_abs_diff = max(
-                    max_abs_diff, abs(approx_value.expected - other_value)
-                )
-                if approx_value.expected == 0.0:
-                    max_rel_diff = math.inf
-                else:
-                    max_rel_diff = max(
-                        max_rel_diff,
-                        abs(
-                            (approx_value.expected - other_value)
-                            / approx_value.expected
-                        ),
+                if not any((approx_value.expected is None, other_value is None)):
+                    max_abs_diff = max(
+                        max_abs_diff, abs(approx_value.expected - other_value)
                     )
+                    if approx_value.expected == 0.0:
+                        max_rel_diff = math.inf
+                    else:
+                        max_rel_diff = max(
+                            max_rel_diff,
+                            abs(
+                                (approx_value.expected - other_value)
+                                / approx_value.expected
+                            ),
+                        )
                 different_ids.append(approx_key)
 
         message_data = [

testing/python/approx.py

@@ -122,6 +122,23 @@ class TestApprox:
             ],
         )
 
+        assert_approx_raises_regex(
+            {"a": 1.0, "b": None, "c": None},
+            {
+                "a": None,
+                "b": 1000.0,
+                "c": None,
+            },
+            [
+                r"  comparison failed. Mismatched elements: 2 / 3:",
+                r"  Max absolute difference: -inf",
+                r"  Max relative difference: -inf",
+                r"  Index \| Obtained\s+\| Expected\s+",
+                rf"  a     \| {SOME_FLOAT} \| None",
+                rf"  b     \| None\s+\| {SOME_FLOAT} ± {SOME_FLOAT}",
+            ],
+        )
+
         assert_approx_raises_regex(
             [1.0, 2.0, 3.0, 4.0],
             [1.0, 3.0, 3.0, 5.0],