Add read-only access to the children of a TreeNode

See #1398.

tests/tree/test_tree_node_children.py

@@ -0,0 +1,32 @@
+import pytest
+from textual.widgets import Tree, TreeNode
+
+def label_of(node: TreeNode[None]):
+    """Get the label of a node.
+
+    TODO: This is just a helper function to reduce the number of type
+    errors, which can and will be remove once this code is merged with a
+    version of main that also has the TreeNode.label PR merged.
+    """
+    return str(node._label)
+
+
+def test_tree_node_children() -> None:
+    """A node's children property should act like an immutable list."""
+    CHILDREN=23
+    tree = Tree[None]("Root")
+    for child in range(CHILDREN):
+        tree.root.add(str(child))
+    assert len(tree.root.children)==CHILDREN
+    for child in range(CHILDREN):
+        assert label_of(tree.root.children[child]) == str(child)
+    assert label_of(tree.root.children[0]) == "0"
+    assert label_of(tree.root.children[-1]) == str(CHILDREN-1)
+    assert [label_of(node) for node in tree.root.children] == [str(n) for n in range(CHILDREN)]
+    assert [label_of(node) for node in tree.root.children[:2]] == [str(n) for n in range(2)]
+    with pytest.raises(TypeError):
+        tree.root.children[0] = tree.root.children[1]
+    with pytest.raises(TypeError):
+        del tree.root.children[0]
+    with pytest.raises(TypeError):
+        del tree.root.children[0:2]