      def __lt__(self, other):
          return self.key < other.key
  
      def add_neighbor(self, neighbor, weight=0):
          if neighbor is None:
              raise Exception('Invalid neighbor')
          neighbor.incoming_edges += 1
          self.adj_weights[neighbor.key] = weight
          self.adj_nodes[neighbor.key] = neighbor
  
      def remove_neighbor(self, neighbor):
 
</s> remove     "        if neighbor is None:\n",
    "            raise Exception('Invalid neighbor')\n",
 </s> add     "        if neighbor is None or weight is None:\n",
    "            raise TypeError('neighbor or weight cannot be None')\n", </s> remove         if neighbor is None or neighbor.key not in self.adj_nodes:
            raise Exception('Invalid neighbor')
 </s> add         if neighbor is None:
            raise TypeError('neighbor cannot be None')
        if neighbor.key not in self.adj_nodes:
            raise KeyError('neighbor not found') </s> remove     "        if neighbor is None or neighbor.key not in self.adj_nodes:\n",
    "            raise Exception('Invalid neighbor')\n",
 </s> add     "        if neighbor is None:\n",
    "            raise TypeError('neighbor cannot be None')\n",
    "        if neighbor.key not in self.adj_nodes:\n",
    "            raise KeyError('neighbor not found')\n", </s> remove     def add_undirected_edge(self, source_key, dest_key, weight=0):
        if source_key is None or dest_key is None:
            raise Exception('Invalid key')
        self.add_edge(source_key, dest_key, weight)
        self.nodes[dest_key].add_neighbor(self.nodes[source_key],
                                          weight) </s> add 
        if neighbor is None or weight is None:
            raise TypeError('neighbor or weight cannot be None')
      def __lt__(self, other):
          return self.key < other.key
  
      def add_neighbor(self, neighbor, weight=0):
        if neighbor is None or weight is None:
            raise TypeError('neighbor or weight cannot be None')
        if neighbor is None or weight is None:
            raise TypeError('neighbor or weight cannot be None')
          neighbor.incoming_edges += 1
          self.adj_weights[neighbor.key] = weight
          self.adj_nodes[neighbor.key] = neighbor
  
      def remove_neighbor(self, neighbor):
keep keep keep keep replace replace keep keep keep keep keep
 Update graph challenge (#117)

Use more specific exception types. Update constraints. Update add_undirected_edge to use add_edge. 
graphs_trees/graph/graph.py