code: def test_add_single_node_raises_error(self):
      self.assertRaises(TypeError, self.graph.add_node, "a")
  
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 3 for calling the TestJunctionTreeCreation obj's underlying member method code with example usage: obj.test_add_single_node_raises_error() without return types