fix assert_equal for DataTree

doc/whats-new.rst

@@ -30,7 +30,8 @@ Bug fixes
   By `Deepak Cherian <https://github.com/dcherian>`_.
 - Check and fix character array string dimension names, issue warnings as needed (:issue:`6352`, :pull:`10395`).
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
-
+- Fix the error message of :py:func:`testing.assert_equal` when two different :py:class:`DataTree` objects
+  are passed (:pull:`10440`). By `Mathias Hauser <https://github.com/mathause>`_.
 
 
 Documentation
@@ -1007,7 +1008,7 @@ New Features
   for example, will retain the object.  However, one cannot do operations that are not possible on the ``ExtensionArray``
   then, such as broadcasting. (:issue:`5287`, :issue:`8463`, :pull:`8723`)
   By `Ilan Gold <https://github.com/ilan-gold>`_.
-- :py:func:`testing.assert_allclose`/:py:func:`testing.assert_equal` now accept a new argument ``check_dims="transpose"``, controlling whether a transposed array is considered equal. (:issue:`5733`, :pull:`8991`)
+- :py:func:`testing.assert_allclose` / :py:func:`testing.assert_equal` now accept a new argument ``check_dims="transpose"``, controlling whether a transposed array is considered equal. (:issue:`5733`, :pull:`8991`)
   By `Ignacio Martinez Vazquez <https://github.com/ignamv>`_.
 - Added the option to avoid automatically creating 1D pandas indexes in :py:meth:`Dataset.expand_dims()`, by passing the new kwarg
   ``create_index_for_new_dim=False``. (:pull:`8960`)

xarray/core/formatting.py

@@ -1039,15 +1039,13 @@ def diff_dataset_repr(a, b, compat):
 def diff_nodewise_summary(a: DataTree, b: DataTree, compat):
     """Iterates over all corresponding nodes, recording differences between data at each location."""
 
-    compat_str = _compat_to_str(compat)
-
     summary = []
     for path, (node_a, node_b) in group_subtrees(a, b):
         a_ds, b_ds = node_a.dataset, node_b.dataset
 
         if not a_ds._all_compat(b_ds, compat):
             path_str = "root node" if path == "." else f"node {path!r}"
-            dataset_diff = diff_dataset_repr(a_ds, b_ds, compat_str)
+            dataset_diff = diff_dataset_repr(a_ds, b_ds, compat)
             data_diff = indent(
                 "\n".join(dataset_diff.split("\n", 1)[1:]), prefix="    "
             )

xarray/tests/test_formatting.py

@@ -721,6 +721,27 @@ def test_diff_datatree_repr_node_data(self):
         actual = formatting.diff_datatree_repr(dt_1, dt_2, "identical")
         assert actual == expected
 
+    def test_diff_datatree_repr_equals(self) -> None:
+        ds1 = xr.Dataset(data_vars={"data": ("y", [5, 2])})
+        ds2 = xr.Dataset(data_vars={"data": (("x", "y"), [[5, 2]])})
+        dt1 = xr.DataTree.from_dict({"node": ds1})
+        dt2 = xr.DataTree.from_dict({"node": ds2})
+
+        expected = dedent(
+            """\
+            Left and right DataTree objects are not equal
+
+            Data at node 'node' does not match:
+                Differing dimensions:
+                    (y: 2) != (x: 1, y: 2)
+                Differing data variables:
+                L   data     (y) int64 16B 5 2
+                R   data     (x, y) int64 16B 5 2"""
+        )
+
+        actual = formatting.diff_datatree_repr(dt1, dt2, "equals")
+        assert actual == expected
+
 
 def test_inline_variable_array_repr_custom_repr() -> None:
     class CustomArray: