code: def test_depth():
      """Test the tree_depth() method"""
      f = simple_forest()
      assert f.tree_depth() == 3
  
file_path: nipy/nipy/algorithms/graph/tests/test_forest.py
imports: import numpy
  from ..forest import Forest
label_desc: Use image node_id 12 for calling a global function with example usage: test_depth() without return types