Re-implement map_over_datasets

The main changes:

- It is implemented using zip_subtrees, which means it should properly
  handle DataTrees where the nodes are defined in a different order.
- For simplicity, I removed handling of `**kwargs`, in order to preserve
  some flexibility for adding keyword arugments.
- I removed automatic skipping of empty nodes, because there are almost
  assuredly cases where that would make sense. This could be restored
  with a option keyword arugment.

xarray/core/datatree.py

@@ -1387,7 +1387,6 @@ def map_over_datasets(
         self,
         func: Callable,
         *args: Iterable[Any],
-        **kwargs: Any,
     ) -> DataTree | tuple[DataTree, ...]:
         """
         Apply a function to every dataset in this subtree, returning a new tree which stores the results.
@@ -1406,8 +1405,6 @@ def map_over_datasets(
             Function will not be applied to any nodes without datasets.
         *args : tuple, optional
             Positional arguments passed on to `func`.
-        **kwargs : Any
-            Keyword arguments passed on to `func`.
 
         Returns
         -------
@@ -1417,7 +1414,7 @@ def map_over_datasets(
         # TODO this signature means that func has no way to know which node it is being called upon - change?
 
         # TODO fix this typing error
-        return map_over_datasets(func)(self, *args, **kwargs)
+        return map_over_datasets(func)(self, *args)
 
     def pipe(
         self, func: Callable | tuple[Callable, str], *args: Any, **kwargs: Any

xarray/core/datatree_mapping.py

@@ -2,14 +2,12 @@
 
 import functools
 import sys
-from collections.abc import Callable
-from itertools import repeat
-from typing import TYPE_CHECKING
+from collections.abc import Callable, Mapping
+from typing import TYPE_CHECKING, Any, cast
 
-from xarray.core.dataarray import DataArray
 from xarray.core.dataset import Dataset
 from xarray.core.formatting import diff_treestructure
-from xarray.core.treenode import NodePath, TreeNode
+from xarray.core.treenode import TreeNode, zip_subtrees
 
 if TYPE_CHECKING:
     from xarray.core.datatree import DataTree
@@ -125,110 +123,55 @@ def map_over_datasets(func: Callable) -> Callable:
     # TODO inspect function to work out immediately if the wrong number of arguments were passed for it?
 
     @functools.wraps(func)
-    def _map_over_datasets(*args, **kwargs) -> DataTree | tuple[DataTree, ...]:
+    def _map_over_datasets(*args) -> DataTree | tuple[DataTree, ...]:
         """Internal function which maps func over every node in tree, returning a tree of the results."""
         from xarray.core.datatree import DataTree
 
-        all_tree_inputs = [a for a in args if isinstance(a, DataTree)] + [
-            a for a in kwargs.values() if isinstance(a, DataTree)
-        ]
-
-        if len(all_tree_inputs) > 0:
-            first_tree, *other_trees = all_tree_inputs
-        else:
-            raise TypeError("Must pass at least one tree object")
-
-        for other_tree in other_trees:
-            # isomorphism is transitive so this is enough to guarantee all trees are mutually isomorphic
-            check_isomorphic(
-                first_tree, other_tree, require_names_equal=False, check_from_root=False
-            )
-
         # Walk all trees simultaneously, applying func to all nodes that lie in same position in different trees
         # We don't know which arguments are DataTrees so we zip all arguments together as iterables
         # Store tuples of results in a dict because we don't yet know how many trees we need to rebuild to return
-        out_data_objects = {}
-        args_as_tree_length_iterables = [
-            a.subtree if isinstance(a, DataTree) else repeat(a) for a in args
-        ]
-        n_args = len(args_as_tree_length_iterables)
-        kwargs_as_tree_length_iterables = {
-            k: v.subtree if isinstance(v, DataTree) else repeat(v)
-            for k, v in kwargs.items()
-        }
-        for node_of_first_tree, *all_node_args in zip(
-            first_tree.subtree,
-            *args_as_tree_length_iterables,
-            *list(kwargs_as_tree_length_iterables.values()),
-            strict=False,
-        ):
-            node_args_as_datasetviews = [
-                a.dataset if isinstance(a, DataTree) else a
-                for a in all_node_args[:n_args]
-            ]
-            node_kwargs_as_datasetviews = dict(
-                zip(
-                    [k for k in kwargs_as_tree_length_iterables.keys()],
-                    [
-                        v.dataset if isinstance(v, DataTree) else v
-                        for v in all_node_args[n_args:]
-                    ],
-                    strict=True,
-                )
+        out_data_objects: dict[str, Dataset | None | tuple[Dataset | None, ...]] = {}
+
+        tree_args = [arg for arg in args if isinstance(arg, DataTree)]
+        subtrees = zip_subtrees(*tree_args)
+
+        for node_tree_args in subtrees:
+
+            node_dataset_args = [arg.dataset for arg in node_tree_args]
+            for i, arg in enumerate(args):
+                if not isinstance(arg, DataTree):
+                    node_dataset_args.insert(i, arg)
+
+            path = (
+                "/"
+                if node_tree_args[0] is tree_args[0]
+                else node_tree_args[0].relative_to(tree_args[0])
             )
-            func_with_error_context = _handle_errors_with_path_context(
-                node_of_first_tree.path
-            )(func)
-
-            if node_of_first_tree.has_data:
-                # call func on the data in this particular set of corresponding nodes
-                results = func_with_error_context(
-                    *node_args_as_datasetviews, **node_kwargs_as_datasetviews
-                )
-            elif node_of_first_tree.has_attrs:
-                # propagate attrs
-                results = node_of_first_tree.dataset
-            else:
-                # nothing to propagate so use fastpath to create empty node in new tree
-                results = None
+            func_with_error_context = _handle_errors_with_path_context(path)(func)
+            results = func_with_error_context(*node_dataset_args)
 
-            # TODO implement mapping over multiple trees in-place using if conditions from here on?
-            out_data_objects[node_of_first_tree.path] = results
+            out_data_objects[path] = results
 
-        # Find out how many return values we received
         num_return_values = _check_all_return_values(out_data_objects)
 
-        # Reconstruct 1+ subtrees from the dict of results, by filling in all nodes of all result trees
-        original_root_path = first_tree.path
-        result_trees = []
-        for i in range(num_return_values):
-            out_tree_contents = {}
-            for n in first_tree.subtree:
-                p = n.path
-                if p in out_data_objects.keys():
-                    if isinstance(out_data_objects[p], tuple):
-                        output_node_data = out_data_objects[p][i]
-                    else:
-                        output_node_data = out_data_objects[p]
-                else:
-                    output_node_data = None
-
-                # Discard parentage so that new trees don't include parents of input nodes
-                relative_path = str(NodePath(p).relative_to(original_root_path))
-                relative_path = "/" if relative_path == "." else relative_path
-                out_tree_contents[relative_path] = output_node_data
-
-            new_tree = DataTree.from_dict(
-                out_tree_contents,
-                name=first_tree.name,
-            )
-            result_trees.append(new_tree)
+        if num_return_values is None:
+            out_data = cast(Mapping[str, Dataset | None], out_data_objects)
+            return DataTree.from_dict(out_data, name=tree_args[0].name)
 
-        # If only one result then don't wrap it in a tuple
-        if len(result_trees) == 1:
-            return result_trees[0]
-        else:
-            return tuple(result_trees)
+        out_data_tuples = cast(
+            Mapping[str, tuple[Dataset | None, ...]], out_data_objects
+        )
+        output_dicts: list[dict[str, Dataset | None]] = [
+            {} for _ in range(num_return_values)
+        ]
+        for path, outputs in out_data_tuples.items():
+            for output_dict, output in zip(output_dicts, outputs, strict=False):
+                output_dict[path] = output
+
+        return tuple(
+            DataTree.from_dict(output_dict, name=tree_args[0].name)
+            for output_dict in output_dicts
+        )
 
     return _map_over_datasets
 
@@ -260,62 +203,54 @@ def add_note(err: BaseException, msg: str) -> None:
         err.add_note(msg)
 
 
-def _check_single_set_return_values(
-    path_to_node: str, obj: Dataset | DataArray | tuple[Dataset | DataArray]
-):
+def _check_single_set_return_values(path_to_node: str, obj: Any) -> int | None:
     """Check types returned from single evaluation of func, and return number of return values received from func."""
-    if isinstance(obj, Dataset | DataArray):
-        return 1
-    elif isinstance(obj, tuple):
-        for r in obj:
-            if not isinstance(r, Dataset | DataArray):
-                raise TypeError(
-                    f"One of the results of calling func on datasets on the nodes at position {path_to_node} is "
-                    f"of type {type(r)}, not Dataset or DataArray."
-                )
-        return len(obj)
-    else:
+    if isinstance(obj, None | Dataset):
+        return None  # no need to pack results
+
+    if not isinstance(obj, tuple) or not all(
+        isinstance(r, Dataset | None) for r in obj
+    ):
         raise TypeError(
-            f"The result of calling func on the node at position {path_to_node} is of type {type(obj)}, not "
-            f"Dataset or DataArray, nor a tuple of such types."
+            f"the result of calling func on the node at position is not a Dataset or None "
+            f"or a tuple of such types: {obj!r}"
         )
 
+    return len(obj)
 
-def _check_all_return_values(returned_objects):
-    """Walk through all values returned by mapping func over subtrees, raising on any invalid or inconsistent types."""
 
-    if all(r is None for r in returned_objects.values()):
-        raise TypeError(
-            "Called supplied function on all nodes but found a return value of None for"
-            "all of them."
-        )
+def _check_all_return_values(returned_objects) -> int | None:
+    """Walk through all values returned by mapping func over subtrees, raising on any invalid or inconsistent types."""
 
     result_data_objects = [
-        (path_to_node, r)
-        for path_to_node, r in returned_objects.items()
-        if r is not None
+        (path_to_node, r) for path_to_node, r in returned_objects.items()
     ]
 
-    if len(result_data_objects) == 1:
-        # Only one node in the tree: no need to check consistency of results between nodes
-        path_to_node, result = result_data_objects[0]
-        num_return_values = _check_single_set_return_values(path_to_node, result)
-    else:
-        prev_path, _ = result_data_objects[0]
-        prev_num_return_values, num_return_values = None, None
-        for path_to_node, obj in result_data_objects[1:]:
-            num_return_values = _check_single_set_return_values(path_to_node, obj)
-
-            if (
-                num_return_values != prev_num_return_values
-                and prev_num_return_values is not None
-            ):
+    first_path, result = result_data_objects[0]
+    return_values = _check_single_set_return_values(first_path, result)
+
+    for path_to_node, obj in result_data_objects[1:]:
+        cur_return_values = _check_single_set_return_values(path_to_node, obj)
+
+        if return_values != cur_return_values:
+            if return_values is None:
                 raise TypeError(
-                    f"Calling func on the nodes at position {path_to_node} returns {num_return_values} separate return "
-                    f"values, whereas calling func on the nodes at position {prev_path} instead returns "
-                    f"{prev_num_return_values} separate return values."
+                    f"Calling func on the nodes at position {path_to_node} returns "
+                    f"a tuple of {cur_return_values} datasets, whereas calling func on the "
+                    f"nodes at position {first_path} instead returns a single dataset."
+                )
+            elif cur_return_values is None:
+                raise TypeError(
+                    f"Calling func on the nodes at position {path_to_node} returns "
+                    f"a single dataset, whereas calling func on the nodes at position "
+                    f"{first_path} instead returns a tuple of {return_values} datasets."
+                )
+            else:
+                raise TypeError(
+                    f"Calling func on the nodes at position {path_to_node} returns "
+                    f"a tuple of {cur_return_values} datasets, whereas calling func on "
+                    f"the nodes at position {first_path} instead returns a tuple of "
+                    f"{return_values} datasets."
                 )
 
-            prev_path, prev_num_return_values = path_to_node, num_return_values
-
-    return num_return_values
+    return return_values

xarray/core/treenode.py

@@ -791,6 +791,9 @@ def zip_subtrees(*trees: AnyNamedNode) -> Iterator[tuple[AnyNamedNode, ...]]:
     ------
     Tuples of matching subtrees.
     """
+    if not trees:
+        raise TypeError("Must pass at least one tree object")
+
     # https://en.wikipedia.org/wiki/Breadth-first_search#Pseudocode
     queue = collections.deque([trees])