better error messages for DataTree

xarray/core/datatree.py

@@ -191,7 +191,7 @@ def __init__(
         coords: Mapping[Any, Any] | None = None,
         attrs: Mapping[Any, Any] | None = None,
     ):
-        raise AttributeError("DatasetView objects are not to be initialized directly")
+        raise TypeError("DatasetView objects are not to be initialized directly")
 
     @classmethod
     def _constructor(

xarray/core/treenode.py

@@ -456,14 +456,14 @@ def _get_item(self: Tree, path: str | NodePath) -> Tree | T_DataArray:
         for part in parts:
             if part == "..":
                 if current_node.parent is None:
-                    raise KeyError(f"Could not find node at {path}")
+                    raise KeyError(path)
                 else:
                     current_node = current_node.parent
             elif part in ("", "."):
                 pass
             else:
                 if current_node.get(part) is None:
-                    raise KeyError(f"Could not find node at {path}")
+                    raise KeyError(path)
                 else:
                     current_node = current_node.get(part)
         return current_node
@@ -511,7 +511,7 @@ def _set_item(
             path = NodePath(path)
 
         if not path.name:
-            raise ValueError("Can't set an item under a path which has no name")
+            raise ValueError("cannot set an item under a path which has no name")
 
         if path.root:
             # absolute path
@@ -528,7 +528,7 @@ def _set_item(
                 if part == "..":
                     if current_node.parent is None:
                         # We can't create a parent if `new_nodes_along_path=True` as we wouldn't know what to name it
-                        raise KeyError(f"Could not reach node at path {path}")
+                        raise ValueError(f"could not reach node at path {path}")
                     else:
                         current_node = current_node.parent
                 elif part in ("", "."):
@@ -542,14 +542,14 @@ def _set_item(
                         current_node._set(part, new_node)
                         current_node = current_node.children[part]
                     else:
-                        raise KeyError(f"Could not reach node at path {path}")
+                        raise ValueError(f"could not reach node at path {path}")
 
         if name in current_node.children:
             # Deal with anything already existing at this location
             if allow_overwrite:
                 current_node._set(name, item)
             else:
-                raise KeyError(f"Already a node object at path {path}")
+                raise ValueError(f"already a node object at path {path}")
         else:
             current_node._set(name, item)
 
@@ -560,7 +560,10 @@ def __delitem__(self: Tree, key: str):
             del self._children[key]
             child.orphan()
         else:
-            raise KeyError("Cannot delete")
+            try:
+                raise ValueError("cannot delete key not found in child nodes")
+            except ValueError as e:
+                raise KeyError(key) from e
 
     def same_tree(self, other: Tree) -> bool:
         """True if other node is in the same tree as this node."""

xarray/tests/test_treenode.py

@@ -163,7 +163,7 @@ def test_child_already_exists(self):
         mary: TreeNode = TreeNode()
         john: TreeNode = TreeNode(children={"Mary": mary})
         mary_2: TreeNode = TreeNode()
-        with pytest.raises(KeyError):
+        with pytest.raises(ValueError):
             john._set_item("Mary", mary_2, allow_overwrite=False)
 
     def test_set_grandchild(self):
@@ -184,7 +184,7 @@ def test_create_intermediate_child(self):
         rose: TreeNode = TreeNode()
 
         # test intermediate children not allowed
-        with pytest.raises(KeyError, match="Could not reach"):
+        with pytest.raises(ValueError, match="could not reach"):
             john._set_item(path="Mary/Rose", item=rose, new_nodes_along_path=False)
 
         # test intermediate children allowed
@@ -203,7 +203,7 @@ def test_overwrite_child(self):
 
         # test overwriting not allowed
         marys_evil_twin: TreeNode = TreeNode()
-        with pytest.raises(KeyError, match="Already a node object"):
+        with pytest.raises(ValueError, match="already a node object"):
             john._set_item("Mary", marys_evil_twin, allow_overwrite=False)
         assert john.children["Mary"] is mary
         assert marys_evil_twin.parent is None