text: import unittest
  from dijkstra import shortest_distance
  
  graph = {'a': {'b': 7, 'd': 9, 'c': 14},
       'b': {'d': 10, 'e': 15},
       'c': {'d': 2, 'f': 9},
       'd': {'e': 11},
       'e': {'f': 6}}
  
  class TestShortestDistance(unittest.TestCase):
  
      def test_shortest_distance(self):
          self.assertEqual(shortest_distance(graph, 'a', 'f'), 23)
          self.assertEqual(shortest_distance(graph, 'a', 'e'), 20)
          self.assertEqual(shortest_distance(graph, 'c', 'b'), None)
  
  if __name__ == '__main__':
      unittest.main()
  