code: def test_add_single_node(self):
      self.graph.add_node(("a", "b"))
      self.assertListEqual(list(self.graph.nodes()), [("a", "b")])
  
imports: import unittest
  import numpy
  from pgmpy.factors.discrete import DiscreteFactor
  from pgmpy.models import JunctionTree
  from pgmpy.tests import help_functions
label_desc: Use image node_id 2 for calling the TestJunctionTreeCreation obj's underlying member method code with example usage: obj.test_add_single_node() without return types