code: def test_zero_hops(self, tree_graph):
      bfw = DirectedBreadthFirstNeighbours(tree_graph)
      # By consensus, a zero-length walk raises an error.
      with pytest.raises(ValueError):
          subgraph = bfw.run(
              nodes=["root"], n=1, in_size=[], out_size=[]
          )
  
file_path: stellargraph/tests/data/test_directed_breadth_first_sampler.py
imports: import random
  import pytest
  import pandas
  import numpy
  from stellargraph.data.explorer import DirectedBreadthFirstNeighbours
  from stellargraph.core.graph import StellarDiGraph
  from ..test_utils.graphs import create_test_graph, tree_graph, example_graph_random, weighted_tree
label_desc: Use image node_id 2 for calling the TestDirectedBreadthFirstNeighbours obj's underlying member method code with example usage: obj.test_zero_hops(tree_graph) without return types