code: def test_preorder_traversal():
      """Test order retrieval of DepthFirstTraversal"""
      # a
      # |   \  \   \
      # b    c  d   e
      # | \         |
      # f  g        h
      root = TreeNode("a")
      b = TreeNode("b", root)
      TreeNode("c", root)
      TreeNode("d", root)
      e = TreeNode("e", root)
  
      TreeNode("f", b)
      TreeNode("g", b)
  
      TreeNode("h", e)
  
      rval = [node.item for node in PreOrderTraversal(root)]
      assert rval == ["a", "b", "f", "g", "c", "d", "e", "h"]
  
imports: from orion.core.utils.tree import DepthFirstTraversal, PreOrderTraversal, TreeNode, flattened
label_desc: Use image node_id 20 for calling a global function with example usage: test_preorder_traversal() without return types