code: def test_iter_dfs():
      tree = BinaryBranch(
          BinaryBranch(Leaf(no=3), Leaf(no=4), no=2), Leaf(no=5), no=1
      )
  
      for i, node in enumerate(tree.iter_dfs(), start=1):
          assert i == node.no
  
file_path: river/river/tree/test_base.py
imports: from __future__ import annotations
  from river.tree.base import Branch, Leaf
label_desc: Use image node_id 3 for calling a global function with example usage: test_iter_dfs() without return types