Add inherit='all_coords' option to DataTree.to_dataset()

doc/whats-new.rst

@@ -14,6 +14,9 @@ v2026.03.0 (unreleased)
 New Features
 ~~~~~~~~~~~~
 
+- Added ``inherit='all_coords'`` option to :py:meth:`DataTree.to_dataset` to inherit
+  all parent coordinates, not just indexed ones (:issue:`10812`, :pull:`11230`).
+  By `Alfonso Ladino <https://github.com/aladinor>`_.
 
 Breaking Changes
 ~~~~~~~~~~~~~~~~

xarray/core/datatree.py

@@ -588,6 +588,28 @@ def _coord_variables(self) -> ChainMap[Hashable, Variable]:
             *(p._node_coord_variables_with_index for p in self.parents),  # type: ignore[arg-type]
         )
 
+    @property
+    def _coord_variables_all(self) -> ChainMap[Hashable, Variable]:
+        return ChainMap(
+            self._node_coord_variables,
+            *(p._node_coord_variables for p in self.parents),
+        )
+
+    def _resolve_inherit(
+        self, inherit: bool | Literal["all_coords", "indexes"]
+    ) -> tuple[Mapping[Hashable, Variable], dict[Hashable, Index]]:
+        """Resolve the inherit parameter to (coord_vars, indexes)."""
+        if inherit is False:
+            return self._node_coord_variables, dict(self._node_indexes)
+        if inherit is True or inherit == "indexes":
+            return self._coord_variables, dict(self._indexes)
+        if inherit == "all_coords":
+            return self._coord_variables_all, dict(self._indexes)
+        raise ValueError(
+            f"Invalid value for inherit: {inherit!r}. "
+            "Expected True, False, 'indexes', or 'all'."
+        )
+
     @property
     def _dims(self) -> ChainMap[Hashable, int]:
         return ChainMap(self._node_dims, *(p._node_dims for p in self.parents))
@@ -596,8 +618,12 @@ def _dims(self) -> ChainMap[Hashable, int]:
     def _indexes(self) -> ChainMap[Hashable, Index]:
         return ChainMap(self._node_indexes, *(p._node_indexes for p in self.parents))
 
-    def _to_dataset_view(self, rebuild_dims: bool, inherit: bool) -> DatasetView:
-        coord_vars = self._coord_variables if inherit else self._node_coord_variables
+    def _to_dataset_view(
+        self,
+        rebuild_dims: bool,
+        inherit: bool | Literal["all_coords", "indexes"] = True,
+    ) -> DatasetView:
+        coord_vars, indexes = self._resolve_inherit(inherit)
         variables = dict(self._data_variables)
         variables |= coord_vars
         if rebuild_dims:
@@ -636,10 +662,10 @@ def _to_dataset_view(self, rebuild_dims: bool, inherit: bool) -> DatasetView:
             dims = dict(self._node_dims)
         return DatasetView._constructor(
             variables=variables,
-            coord_names=set(self._coord_variables),
+            coord_names=set(coord_vars),
             dims=dims,
             attrs=self._attrs,
-            indexes=dict(self._indexes if inherit else self._node_indexes),
+            indexes=indexes,
             encoding=self._encoding,
             close=None,
         )
@@ -669,30 +695,39 @@ def dataset(self, data: Dataset | None = None) -> None:
     # xarray-contrib/datatree
     ds = dataset
 
-    def to_dataset(self, inherit: bool = True) -> Dataset:
+    def to_dataset(
+        self, inherit: bool | Literal["all_coords", "indexes"] = True
+    ) -> Dataset:
         """
         Return the data in this node as a new xarray.Dataset object.
 
         Parameters
         ----------
-        inherit : bool, optional
-            If False, only include coordinates and indexes defined at the level
-            of this DataTree node, excluding any inherited coordinates and indexes.
+        inherit : bool or {"all_coords", "indexes"}, default True
+            Controls which coordinates are inherited from parent nodes.
+
+            - True or "indexes": inherit only indexed coordinates (default).
+            - "all_coords": inherit all coordinates, including non-index coordinates.
+            - False: only include coordinates defined at this node.
 
         See Also
         --------
         DataTree.dataset
         """
-        coord_vars = self._coord_variables if inherit else self._node_coord_variables
+        coord_vars, indexes = self._resolve_inherit(inherit)
         variables = dict(self._data_variables)
         variables |= coord_vars
-        dims = calculate_dimensions(variables) if inherit else dict(self._node_dims)
+        dims = (
+            dict(self._node_dims)
+            if inherit is False
+            else calculate_dimensions(variables)
+        )
         return Dataset._construct_direct(
             variables,
             set(coord_vars),
             dims,
             None if self._attrs is None else dict(self._attrs),
-            dict(self._indexes if inherit else self._node_indexes),
+            indexes,
             None if self._encoding is None else dict(self._encoding),
             None,
         )

xarray/tests/test_datatree.py

@@ -243,6 +243,28 @@ def test_to_dataset_inherited(self) -> None:
         assert_identical(tree.to_dataset(inherit=True), base)
         assert_identical(subtree.to_dataset(inherit=True), sub_and_base)
 
+    def test_to_dataset_inherit_all(self) -> None:
+        base = xr.Dataset(coords={"a": [1], "b": 2})
+        sub = xr.Dataset(coords={"c": [3]})
+        tree = DataTree.from_dict({"/": base, "/sub": sub})
+        subtree = typing.cast(DataTree, tree["sub"])
+
+        expected = xr.Dataset(coords={"a": [1], "b": 2, "c": [3]})
+        assert_identical(subtree.to_dataset(inherit="all_coords"), expected)
+        assert_identical(tree.to_dataset(inherit="all_coords"), base)
+
+        mid = xr.Dataset(coords={"c": 3.0})
+        leaf = xr.Dataset(coords={"d": [4]})
+        deep = DataTree.from_dict({"/": base, "/mid": mid, "/mid/leaf": leaf})
+        leaf_node = typing.cast(DataTree, deep["/mid/leaf"])
+        result = leaf_node.to_dataset(inherit="all_coords")
+        assert set(result.coords) == {"a", "b", "c", "d"}
+
+    def test_to_dataset_inherit_invalid(self) -> None:
+        tree = DataTree()
+        with pytest.raises(ValueError, match="Invalid value for inherit"):
+            tree.to_dataset(inherit="invalid")  # type: ignore[arg-type]
+
 
 class TestVariablesChildrenNameCollisions:
     def test_parent_already_has_variable_with_childs_name(self) -> None: