diff --git a/xarray/tests/datatree/test_treenode.py b/xarray/tests/datatree/test_treenode.py
index 245e8fab4fc..4789a9be6e3 100644
--- a/xarray/tests/datatree/test_treenode.py
+++ b/xarray/tests/datatree/test_treenode.py
@@ -1,5 +1,8 @@
 from __future__ import annotations
 
+from collections.abc import Iterator
+from typing import cast
+
 import pytest
 
 from xarray.core.treenode import InvalidTreeError, NamedNode, NodePath, TreeNode
@@ -8,20 +11,20 @@
 
 class TestFamilyTree:
     def test_lonely(self):
-        root = TreeNode()
+        root: TreeNode = TreeNode()
         assert root.parent is None
         assert root.children == {}
 
     def test_parenting(self):
-        john = TreeNode()
-        mary = TreeNode()
+        john: TreeNode = TreeNode()
+        mary: TreeNode = TreeNode()
         mary._set_parent(john, "Mary")
 
         assert mary.parent == john
         assert john.children["Mary"] is mary
 
     def test_no_time_traveller_loops(self):
-        john = TreeNode()
+        john: TreeNode = TreeNode()
 
         with pytest.raises(InvalidTreeError, match="cannot be a parent of itself"):
             john._set_parent(john, "John")
@@ -29,8 +32,8 @@ def test_no_time_traveller_loops(self):
         with pytest.raises(InvalidTreeError, match="cannot be a parent of itself"):
             john.children = {"John": john}
 
-        mary = TreeNode()
-        rose = TreeNode()
+        mary: TreeNode = TreeNode()
+        rose: TreeNode = TreeNode()
         mary._set_parent(john, "Mary")
         rose._set_parent(mary, "Rose")
 
@@ -41,11 +44,11 @@ def test_no_time_traveller_loops(self):
             rose.children = {"John": john}
 
     def test_parent_swap(self):
-        john = TreeNode()
-        mary = TreeNode()
+        john: TreeNode = TreeNode()
+        mary: TreeNode = TreeNode()
         mary._set_parent(john, "Mary")
 
-        steve = TreeNode()
+        steve: TreeNode = TreeNode()
         mary._set_parent(steve, "Mary")
 
         assert mary.parent == steve
@@ -53,24 +56,24 @@ def test_parent_swap(self):
         assert "Mary" not in john.children
 
     def test_multi_child_family(self):
-        mary = TreeNode()
-        kate = TreeNode()
-        john = TreeNode(children={"Mary": mary, "Kate": kate})
+        mary: TreeNode = TreeNode()
+        kate: TreeNode = TreeNode()
+        john: TreeNode = TreeNode(children={"Mary": mary, "Kate": kate})
         assert john.children["Mary"] is mary
         assert john.children["Kate"] is kate
         assert mary.parent is john
         assert kate.parent is john
 
     def test_disown_child(self):
-        mary = TreeNode()
-        john = TreeNode(children={"Mary": mary})
+        mary: TreeNode = TreeNode()
+        john: TreeNode = TreeNode(children={"Mary": mary})
         mary.orphan()
         assert mary.parent is None
         assert "Mary" not in john.children
 
     def test_doppelganger_child(self):
-        kate = TreeNode()
-        john = TreeNode()
+        kate: TreeNode = TreeNode()
+        john: TreeNode = TreeNode()
 
         with pytest.raises(TypeError):
             john.children = {"Kate": 666}
@@ -79,22 +82,22 @@ def test_doppelganger_child(self):
             john.children = {"Kate": kate, "Evil_Kate": kate}
 
         john = TreeNode(children={"Kate": kate})
-        evil_kate = TreeNode()
+        evil_kate: TreeNode = TreeNode()
         evil_kate._set_parent(john, "Kate")
         assert john.children["Kate"] is evil_kate
 
     def test_sibling_relationships(self):
-        mary = TreeNode()
-        kate = TreeNode()
-        ashley = TreeNode()
+        mary: TreeNode = TreeNode()
+        kate: TreeNode = TreeNode()
+        ashley: TreeNode = TreeNode()
         TreeNode(children={"Mary": mary, "Kate": kate, "Ashley": ashley})
         assert kate.siblings["Mary"] is mary
         assert kate.siblings["Ashley"] is ashley
         assert "Kate" not in kate.siblings
 
     def test_ancestors(self):
-        tony = TreeNode()
-        michael = TreeNode(children={"Tony": tony})
+        tony: TreeNode = TreeNode()
+        michael: TreeNode = TreeNode(children={"Tony": tony})
         vito = TreeNode(children={"Michael": michael})
         assert tony.root is vito
         assert tony.parents == (michael, vito)
@@ -103,7 +106,7 @@ def test_ancestors(self):
 
 class TestGetNodes:
     def test_get_child(self):
-        steven = TreeNode()
+        steven: TreeNode = TreeNode()
         sue = TreeNode(children={"Steven": steven})
         mary = TreeNode(children={"Sue": sue})
         john = TreeNode(children={"Mary": mary})
@@ -126,8 +129,8 @@ def test_get_child(self):
         assert mary._get_item("Sue/Steven") is steven
 
     def test_get_upwards(self):
-        sue = TreeNode()
-        kate = TreeNode()
+        sue: TreeNode = TreeNode()
+        kate: TreeNode = TreeNode()
         mary = TreeNode(children={"Sue": sue, "Kate": kate})
         john = TreeNode(children={"Mary": mary})
 
@@ -138,7 +141,7 @@ def test_get_upwards(self):
         assert sue._get_item("../Kate") is kate
 
     def test_get_from_root(self):
-        sue = TreeNode()
+        sue: TreeNode = TreeNode()
         mary = TreeNode(children={"Sue": sue})
         john = TreeNode(children={"Mary": mary})  # noqa
 
@@ -147,8 +150,8 @@ def test_get_from_root(self):
 
 class TestSetNodes:
     def test_set_child_node(self):
-        john = TreeNode()
-        mary = TreeNode()
+        john: TreeNode = TreeNode()
+        mary: TreeNode = TreeNode()
         john._set_item("Mary", mary)
 
         assert john.children["Mary"] is mary
@@ -157,16 +160,16 @@ def test_set_child_node(self):
         assert mary.parent is john
 
     def test_child_already_exists(self):
-        mary = TreeNode()
-        john = TreeNode(children={"Mary": mary})
-        mary_2 = TreeNode()
+        mary: TreeNode = TreeNode()
+        john: TreeNode = TreeNode(children={"Mary": mary})
+        mary_2: TreeNode = TreeNode()
         with pytest.raises(KeyError):
             john._set_item("Mary", mary_2, allow_overwrite=False)
 
     def test_set_grandchild(self):
-        rose = TreeNode()
-        mary = TreeNode()
-        john = TreeNode()
+        rose: TreeNode = TreeNode()
+        mary: TreeNode = TreeNode()
+        john: TreeNode = TreeNode()
 
         john._set_item("Mary", mary)
         john._set_item("Mary/Rose", rose)
@@ -177,8 +180,8 @@ def test_set_grandchild(self):
         assert rose.parent is mary
 
     def test_create_intermediate_child(self):
-        john = TreeNode()
-        rose = TreeNode()
+        john: TreeNode = TreeNode()
+        rose: TreeNode = TreeNode()
 
         # test intermediate children not allowed
         with pytest.raises(KeyError, match="Could not reach"):
@@ -194,12 +197,12 @@ def test_create_intermediate_child(self):
         assert rose.parent == mary
 
     def test_overwrite_child(self):
-        john = TreeNode()
-        mary = TreeNode()
+        john: TreeNode = TreeNode()
+        mary: TreeNode = TreeNode()
         john._set_item("Mary", mary)
 
         # test overwriting not allowed
-        marys_evil_twin = TreeNode()
+        marys_evil_twin: TreeNode = TreeNode()
         with pytest.raises(KeyError, match="Already a node object"):
             john._set_item("Mary", marys_evil_twin, allow_overwrite=False)
         assert john.children["Mary"] is mary
@@ -214,8 +217,8 @@ def test_overwrite_child(self):
 
 class TestPruning:
     def test_del_child(self):
-        john = TreeNode()
-        mary = TreeNode()
+        john: TreeNode = TreeNode()
+        mary: TreeNode = TreeNode()
         john._set_item("Mary", mary)
 
         del john["Mary"]
@@ -226,16 +229,16 @@ def test_del_child(self):
             del john["Mary"]
 
 
-def create_test_tree():
-    a = NamedNode(name="a")
-    b = NamedNode()
-    c = NamedNode()
-    d = NamedNode()
-    e = NamedNode()
-    f = NamedNode()
-    g = NamedNode()
-    h = NamedNode()
-    i = NamedNode()
+def create_test_tree() -> tuple[NamedNode, NamedNode]:
+    a: NamedNode = NamedNode(name="a")
+    b: NamedNode = NamedNode()
+    c: NamedNode = NamedNode()
+    d: NamedNode = NamedNode()
+    e: NamedNode = NamedNode()
+    f: NamedNode = NamedNode()
+    g: NamedNode = NamedNode()
+    h: NamedNode = NamedNode()
+    i: NamedNode = NamedNode()
 
     a.children = {"b": b, "c": c}
     b.children = {"d": d, "e": e}
@@ -249,7 +252,9 @@ def create_test_tree():
 class TestIterators:
     def test_preorderiter(self):
         root, _ = create_test_tree()
-        result = [node.name for node in PreOrderIter(root)]
+        result: list[str | None] = [
+            node.name for node in cast(Iterator[NamedNode], PreOrderIter(root))
+        ]
         expected = [
             "a",
             "b",
@@ -265,7 +270,9 @@ def test_preorderiter(self):
 
     def test_levelorderiter(self):
         root, _ = create_test_tree()
-        result = [node.name for node in LevelOrderIter(root)]
+        result: list[str | None] = [
+            node.name for node in cast(Iterator[NamedNode], LevelOrderIter(root))
+        ]
         expected = [
             "a",  # root Node is unnamed
             "b",
@@ -358,11 +365,11 @@ def test_levels(self):
 
 class TestRenderTree:
     def test_render_nodetree(self):
-        sam = NamedNode()
-        ben = NamedNode()
-        mary = NamedNode(children={"Sam": sam, "Ben": ben})
-        kate = NamedNode()
-        john = NamedNode(children={"Mary": mary, "Kate": kate})
+        sam: NamedNode = NamedNode()
+        ben: NamedNode = NamedNode()
+        mary: NamedNode = NamedNode(children={"Sam": sam, "Ben": ben})
+        kate: NamedNode = NamedNode()
+        john: NamedNode = NamedNode(children={"Mary": mary, "Kate": kate})
 
         printout = john.__str__()
         expected_nodes = [