Add an arithmetic_compat option to xr.set_options, which determines how non-index coordinates of the same name are compared for potential conflicts when performing binary operations.

The default of compat='minimal' matches the previous behaviour.

Author: mjwillson

doc/whats-new.rst

@@ -13,6 +13,15 @@ v2025.12.1 (unreleased)
 
 New Features
 ~~~~~~~~~~~~
+
+- Improved ``pydap`` backend behavior and performance when using :py:func:`open_dataset`, :py:func:`open_datatree`
+  when downloading dap4 (opendap) dimensions data (:issue:`10628`, :pull:`10629`). In addition ``checksums=True|False``
+  is added as optional argument to be passed to ``pydap`` backend.
+  By `Miguel Jimenez-Urias <https://github.com/Mikejmnez>`_.
+
+- :py:func:`combine_nested` now support :py:class:`DataTree` objects
+  (:pull:`10849`).
+  By `Stephan Hoyer <https://github.com/shoyer>`_.
 - :py:func:`set_options` now supports an ``arithmetic_compat`` option which determines how non-index coordinates
   of the same name are compared for potential conflicts when performing binary operations. The default for it is
   ``arithmetic_compat='minimal'`` which matches the existing behaviour.

xarray/tests/test_datatree.py

@@ -2423,6 +2423,43 @@ def test_arithmetic_inherited_coords(self) -> None:
         expected["/foo/bar"].data = np.array([8, 10, 12])
         assert_identical(actual, expected)
 
+    def test_binary_op_compat_setting(self) -> None:
+        # Setting up a clash of non-index coordinate 'foo':
+        a = DataTree(xr.Dataset(
+            data_vars={"var": (["x"], [0, 0, 0])},
+            coords={
+                "x": [1, 2, 3],
+                "foo": (["x"], [1.0, 2.0, np.nan]),
+            },
+        ))
+        b = DataTree(xr.Dataset(
+            data_vars={"var": (["x"], [0, 0, 0])},
+            coords={
+                "x": [1, 2, 3],
+                "foo": (["x"], [np.nan, 2.0, 3.0]),
+            },
+        ))
+
+        with xr.set_options(arithmetic_compat="minimal"):
+            expected = DataTree(a.dataset.drop_vars('foo'))
+            assert_equal(a + b, expected)
+
+        with xr.set_options(arithmetic_compat="override"):
+            assert_equal(a + b, a)
+            assert_equal(b + a, b)
+
+        with xr.set_options(arithmetic_compat="no_conflicts"):
+            expected = DataTree(a.dataset.assign_coords(
+                foo=(["x"], [1.0, 2.0, 3.0])))
+            assert_equal(a + b, expected)
+            assert_equal(b + a, expected)
+
+        with xr.set_options(arithmetic_compat="equals"):
+            with pytest.raises(xr.MergeError):
+                a + b
+            with pytest.raises(xr.MergeError):
+                b + a
+
     def test_binary_op_commutativity_with_dataset(self) -> None:
         # regression test for #9365