Expand on the implementation of set_dims to make the trivial case easier

Author: hmaarrfk

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