[7.4.x] Fix error assertion handling in approx when None in dict comparison (#11180)

Co-authored-by: Zac Hatfield-Dodds <zac.hatfield.dodds@gmail.com>

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

@@ -266,6 +266,7 @@ class ApproxMapping(ApproxBase):
             approx_side_as_map.items(), other_side.values()
         ):
             if approx_value != other_value:
+                if approx_value.expected is not None and other_value is not None:
                     max_abs_diff = max(
                         max_abs_diff, abs(approx_value.expected - other_value)
                     )

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],