code: def test_single():
      """Joining just one tree yields a tree with one more node."""
      T = nx.empty_graph(1)
      trees = [(T, 0)]
      actual_with_label = nx.join_trees(
          trees, label_attribute="custom_label"
      )
      expected = nx.path_graph(2)
      assert nodes_equal(list(expected), list(actual_with_label))
      assert edges_equal(
          list(expected.edges()), list(actual_with_label.edges())
      )
  
file_path: networkx/networkx/algorithms/tree/tests/test_operations.py
imports: from itertools import chain
  import networkx
  from networkx.utils import edges_equal, nodes_equal
label_desc: Use image node_id 3 for calling a global function with example usage: test_single() without return types