Do not rely on np.broadcast_to to perform trivial dimension insertion

doc/whats-new.rst

@@ -97,6 +97,11 @@ Internal Changes
 ~~~~~~~~~~~~~~~~
 - Avoid stacking when grouping by a chunked array. This can be a large performance improvement.
   By `Deepak Cherian <https://github.com/dcherian>`_.
+- The implementation of ``Variable.set_dims`` has changed to use array indexing syntax
+  instead of ``np.broadcast_to`` to perform dimension expansions where
+  all new dimensions have a size of 1. This should improve compatibility with
+  duck arrays that do not support broadcasting (:issue:`9462`, :pull:`10277`).
+  By `Mark Harfouche <https://github.com/hmaarrfk>`_.
 
 .. _whats-new.2025.03.1:
 

xarray/core/variable.py

@@ -1355,7 +1355,7 @@ def set_dims(self, dim, shape=None):
             dim = [dim]
 
         if shape is None and is_dict_like(dim):
-            shape = dim.values()
+            shape = tuple(dim.values())
 
         missing_dims = set(self.dims) - set(dim)
         if missing_dims:
@@ -1371,13 +1371,18 @@ def set_dims(self, dim, shape=None):
             # don't use broadcast_to unless necessary so the result remains
             # writeable if possible
             expanded_data = self.data
-        elif shape is not None:
+        elif shape is None or all(
+            s == 1 for s, e in zip(shape, dim, strict=True) if e not in self_dims
+        ):
+            # "Trivial" broadcasting, i.e. simply inserting a new dimension
+            # This is typically easier for duck arrays to implement
+            # than the full "broadcast_to" semantics
+            indexer = (None,) * (len(expanded_dims) - self.ndim) + (...,)
+            expanded_data = self.data[indexer]
+        else:  # elif shape is not None:
             dims_map = dict(zip(dim, shape, strict=True))
             tmp_shape = tuple(dims_map[d] for d in expanded_dims)
             expanded_data = duck_array_ops.broadcast_to(self._data, tmp_shape)
-        else:
-            indexer = (None,) * (len(expanded_dims) - self.ndim) + (...,)
-            expanded_data = self.data[indexer]
 
         expanded_var = Variable(
             expanded_dims, expanded_data, self._attrs, self._encoding, fastpath=True

xarray/tests/test_variable.py

@@ -1653,6 +1653,74 @@ def test_set_dims_object_dtype(self):
         expected = Variable(["x"], exp_values)
         assert_identical(actual, expected)
 
+    def test_set_dims_without_broadcast(self):
+        class ArrayWithoutBroadcastTo(NDArrayMixin, indexing.ExplicitlyIndexed):
+            def __init__(self, array):
+                self.array = array
+
+            # Broadcasting with __getitem__ is "easier" to implement
+            # especially for dims of 1
+            def __getitem__(self, key):
+                return self.array[key]
+
+            def __array_function__(self, *args, **kwargs):
+                raise NotImplementedError(
+                    "Not we don't want to use broadcast_to here "
+                    "https://github.com/pydata/xarray/issues/9462"
+                )
+
+        arr = ArrayWithoutBroadcastTo(np.zeros((3, 4)))
+        # We should be able to add a new axis without broadcasting
+        assert arr[np.newaxis, :, :].shape == (1, 3, 4)
+        with pytest.raises(NotImplementedError):
+            np.broadcast_to(arr, (1, 3, 4))
+
+        v = Variable(["x", "y"], arr)
+        v_expanded = v.set_dims(["z", "x", "y"])
+        assert v_expanded.dims == ("z", "x", "y")
+        assert v_expanded.shape == (1, 3, 4)
+
+        v_expanded = v.set_dims(["x", "z", "y"])
+        assert v_expanded.dims == ("x", "z", "y")
+        assert v_expanded.shape == (3, 1, 4)
+
+        v_expanded = v.set_dims(["x", "y", "z"])
+        assert v_expanded.dims == ("x", "y", "z")
+        assert v_expanded.shape == (3, 4, 1)
+
+        # Explicitly asking for a shape of 1 triggers a different
+        # codepath in set_dims
+        # https://github.com/pydata/xarray/issues/9462
+        v_expanded = v.set_dims(["z", "x", "y"], shape=(1, 3, 4))
+        assert v_expanded.dims == ("z", "x", "y")
+        assert v_expanded.shape == (1, 3, 4)
+
+        v_expanded = v.set_dims(["x", "z", "y"], shape=(3, 1, 4))
+        assert v_expanded.dims == ("x", "z", "y")
+        assert v_expanded.shape == (3, 1, 4)
+
+        v_expanded = v.set_dims(["x", "y", "z"], shape=(3, 4, 1))
+        assert v_expanded.dims == ("x", "y", "z")
+        assert v_expanded.shape == (3, 4, 1)
+
+        v_expanded = v.set_dims({"z": 1, "x": 3, "y": 4})
+        assert v_expanded.dims == ("z", "x", "y")
+        assert v_expanded.shape == (1, 3, 4)
+
+        v_expanded = v.set_dims({"x": 3, "z": 1, "y": 4})
+        assert v_expanded.dims == ("x", "z", "y")
+        assert v_expanded.shape == (3, 1, 4)
+
+        v_expanded = v.set_dims({"x": 3, "y": 4, "z": 1})
+        assert v_expanded.dims == ("x", "y", "z")
+        assert v_expanded.shape == (3, 4, 1)
+
+        with pytest.raises(NotImplementedError):
+            v.set_dims({"z": 2, "x": 3, "y": 4})
+
+        with pytest.raises(NotImplementedError):
+            v.set_dims(["z", "x", "y"], shape=(2, 3, 4))
+
     def test_stack(self):
         v = Variable(["x", "y"], [[0, 1], [2, 3]], {"foo": "bar"})
         actual = v.stack(z=("x", "y"))