Add path to error message in map_over_subtree xarray-contrib/datatree#264

* test

* implementation

* formatting

* add version check, if not using 3.11 then you just won't get the extra info in the error message

* whatsnew

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* use better helper function

* xfail test, because this does actually work...

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: TomNicholas

datatree/mapping.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import functools
+import sys
 from itertools import repeat
 from textwrap import dedent
 from typing import TYPE_CHECKING, Callable, Tuple
@@ -202,10 +203,15 @@ def _map_over_subtree(*args, **kwargs) -> DataTree | Tuple[DataTree, ...]:
                     ],
                 )
             )
+            func_with_error_context = _handle_errors_with_path_context(
+                node_of_first_tree.path
+            )(func)
 
             # Now we can call func on the data in this particular set of corresponding nodes
             results = (
-                func(*node_args_as_datasets, **node_kwargs_as_datasets)
+                func_with_error_context(
+                    *node_args_as_datasets, **node_kwargs_as_datasets
+                )
                 if node_of_first_tree.has_data
                 else None
             )
@@ -251,6 +257,34 @@ def _map_over_subtree(*args, **kwargs) -> DataTree | Tuple[DataTree, ...]:
     return _map_over_subtree
 
 
+def _handle_errors_with_path_context(path):
+    """Wraps given function so that if it fails it also raises path to node on which it failed."""
+
+    def decorator(func):
+        def wrapper(*args, **kwargs):
+            try:
+                return func(*args, **kwargs)
+            except Exception as e:
+                if sys.version_info >= (3, 11):
+                    # Add the context information to the error message
+                    e.add_note(
+                        f"Raised whilst mapping function over node with path {path}"
+                    )
+                raise
+
+        return wrapper
+
+    return decorator
+
+
+def add_note(err: BaseException, msg: str) -> None:
+    # TODO: remove once python 3.10 can be dropped
+    if sys.version_info < (3, 11):
+        err.__notes__ = getattr(err, "__notes__", []) + [msg]
+    else:
+        err.add_note(msg)
+
+
 def _check_single_set_return_values(path_to_node, obj):
     """Check types returned from single evaluation of func, and return number of return values received from func."""
     if isinstance(obj, (Dataset, DataArray)):

datatree/tests/test_mapping.py

@@ -264,6 +264,23 @@ def check_for_data(ds):
 
         dt.map_over_subtree(check_for_data)
 
+    @pytest.mark.xfail(
+        reason="probably some bug in pytests handling of exception notes"
+    )
+    def test_error_contains_path_of_offending_node(self, create_test_datatree):
+        dt = create_test_datatree()
+        dt["set1"]["bad_var"] = 0
+        print(dt)
+
+        def fail_on_specific_node(ds):
+            if "bad_var" in ds:
+                raise ValueError("Failed because 'bar_var' present in dataset")
+
+        with pytest.raises(
+            ValueError, match="Raised whilst mapping function over node /set1"
+        ):
+            dt.map_over_subtree(fail_on_specific_node)
+
 
 class TestMutableOperations:
     def test_construct_using_type(self):

docs/source/whats-new.rst

@@ -23,6 +23,10 @@ v0.0.13 (unreleased)
 New Features
 ~~~~~~~~~~~~
 
+- Indicate which node caused the problem if error encountered while applying user function using :py:func:`map_over_subtree`
+  (:issue:`190`, :pull:`264`). Only works when using python 3.11 or later.
+  By `Tom Nicholas <https://github.com/TomNicholas>`_.
+
 Breaking changes
 ~~~~~~~~~~~~~~~~