properly diff objects with arrays as attributes on variables

doc/whats-new.rst

@@ -35,7 +35,8 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
-
+- Allow diffing objects with array attributes on variables (:issue:`9153`, :pull:`9169`).
+  By `Justus Magin <https://github.com/keewis>`_.
 
 Documentation
 ~~~~~~~~~~~~~

properties/test_properties.py

@@ -0,0 +1,13 @@
+from hypothesis import given
+
+import xarray as xr
+import xarray.testing.strategies as xrst
+
+
+@given(attrs=xrst.attrs())
+def test_assert_identical(attrs):
+    v = xr.Variable(dims=(), data=0, attrs=attrs)
+    xr.testing.assert_identical(v, v)
+
+    ds = xr.Dataset(attrs=attrs)
+    xr.testing.assert_identical(ds, ds)

xarray/core/formatting.py

@@ -765,6 +765,12 @@ def _diff_mapping_repr(
     a_indexes=None,
     b_indexes=None,
 ):
+    def compare_attr(a, b):
+        if is_duck_array(a) or is_duck_array(b):
+            return array_equiv(a, b)
+        else:
+            return a == b
+
     def extra_items_repr(extra_keys, mapping, ab_side, kwargs):
         extra_repr = [
             summarizer(k, mapping[k], col_width, **kwargs[k]) for k in extra_keys
@@ -801,11 +807,7 @@ def extra_items_repr(extra_keys, mapping, ab_side, kwargs):
             is_variable = True
         except AttributeError:
             # compare attribute value
-            if is_duck_array(a_mapping[k]) or is_duck_array(b_mapping[k]):
-                compatible = array_equiv(a_mapping[k], b_mapping[k])
-            else:
-                compatible = a_mapping[k] == b_mapping[k]
-
+            compatible = compare_attr(a_mapping[k], b_mapping[k])
             is_variable = False
 
         if not compatible:
@@ -821,7 +823,11 @@ def extra_items_repr(extra_keys, mapping, ab_side, kwargs):
 
                 attrs_to_print = set(a_attrs) ^ set(b_attrs)
                 attrs_to_print.update(
-                    {k for k in set(a_attrs) & set(b_attrs) if a_attrs[k] != b_attrs[k]}
+                    {
+                        k
+                        for k in set(a_attrs) & set(b_attrs)
+                        if not compare_attr(a_attrs[k], b_attrs[k])
+                    }
                 )
                 for m in (a_mapping, b_mapping):
                     attr_s = "\n".join(

xarray/tests/test_formatting.py

@@ -399,6 +399,36 @@ def test_diff_attrs_repr_with_array(self) -> None:
         actual = formatting.diff_attrs_repr(attrs_a, attrs_c, "equals")
         assert expected == actual
 
+    def test__diff_mapping_repr_array_attrs_on_variables(self) -> None:
+        a = {
+            "a": xr.DataArray(
+                dims="x",
+                data=np.array([1], dtype="int16"),
+                attrs={"b": np.array([1, 2], dtype="int8")},
+            )
+        }
+        b = {
+            "a": xr.DataArray(
+                dims="x",
+                data=np.array([1], dtype="int16"),
+                attrs={"b": np.array([2, 3], dtype="int8")},
+            )
+        }
+        actual = formatting.diff_data_vars_repr(a, b, compat="identical", col_width=8)
+        expected = dedent(
+            """\
+            Differing data variables:
+            L   a   (x) int16 2B 1
+                Differing variable attributes:
+                    b: [1 2]
+            R   a   (x) int16 2B 1
+                Differing variable attributes:
+                    b: [2 3]
+            """.rstrip()
+        )
+
+        assert actual == expected
+
     def test_diff_dataset_repr(self) -> None:
         ds_a = xr.Dataset(
             data_vars={