code: def edge_cost(self, node_a, node_b):
          
          cost = float('inf')
          node_object_a = self.get_node(node_a)
          for edge_id in node_object_a['edges']:
              edge = self.get_edge(edge_id)
              tpl = (node_a, node_b)
              if edge['vertices'] == tpl:
                  cost = edge['cost']
                  break
          return cost
docs: Returns the cost of moving between the edge that connects node_a to node_b.
          Returns +inf if no such edge exists.