code: def test_add_node_string(self):
      self.graph.add_node("a")
      self.assertListEqual(list(self.graph.nodes()), ["a"])
  
imports: from pgmpy.base import UndirectedGraph
  from pgmpy.tests import help_functions
  import unittest
label_desc: Use image node_id 4 for calling the TestUndirectedGraphCreation obj's underlying member method code with example usage: obj.test_add_node_string() without return types