code: def test_no_labels(self):
      with pytest.raises(nx.NetworkXError):
          G = nx.path_graph(4)
          node_classification.harmonic_function(G)
  
imports: import pytest
  import networkx
  from networkx.algorithms import node_classification
label_desc: Use image node_id 2 for calling the TestHarmonicFunction obj's underlying member method code with example usage: obj.test_no_labels() without return types