donnemartin/interactive-coding-challenges · error · TypeError

neighbor cannot be None

Error message

neighbor cannot be None

What it means

Raised by Node.remove_neighbor(neighbor) in graph_solution.ipynb when neighbor is None. The method dereferences neighbor.key to look up and delete entries in adj_weights and adj_nodes, so a None neighbor would crash with AttributeError; the guard converts that into an explicit TypeError.

Source

Thrown at graphs_trees/graph/graph_solution.ipynb:203

    "        self.adj_nodes = {}  # Key = key, val = Node\n",
    "        self.adj_weights = {}  # Key = key, val = weight\n",
    "\n",
    "    def __repr__(self):\n",
    "        return str(self.key)\n",
    "\n",
    "    def __lt__(self, other):\n",
    "        return self.key < other.key\n",
    "\n",
    "    def add_neighbor(self, neighbor, weight=0):\n",
    "        if neighbor is None or weight is None:\n",
    "            raise TypeError('neighbor or weight cannot be None')\n",
    "        neighbor.incoming_edges += 1\n",
    "        self.adj_weights[neighbor.key] = weight\n",
    "        self.adj_nodes[neighbor.key] = neighbor\n",
    "\n",
    "    def remove_neighbor(self, neighbor):\n",
    "        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",
    "        neighbor.incoming_edges -= 1\n",
    "        del self.adj_weights[neighbor.key]\n",
    "        del self.adj_nodes[neighbor.key]\n",
    "\n",
    "\n",
    "class Graph:\n",
    "\n",
    "    def __init__(self):\n",
    "        self.nodes = {}  # Key = key, val = Node\n",
    "\n",
    "    def add_node(self, key):\n",
    "        if key is None:\n",
    "            raise TypeError('key cannot be None')\n",
    "        if key not in self.nodes:\n",
    "            self.nodes[key] = Node(key)\n",
    "        return self.nodes[key]\n",

View on GitHub (pinned to 358f2cc604)

Solutions

1. Check the neighbor object is not None before calling remove_neighbor
2. Verify the node exists via graph.nodes.get(key) and skip when the lookup returns None
3. Sanitize adjacency lists at graph-build time so None nodes never enter the structure

Example fix

# before
node.remove_neighbor(maybe_none)  # TypeError

# after
if maybe_none is not None:
    node.remove_neighbor(maybe_none)

if neighbor is not None:
    node.remove_neighbor(neighbor)

def is_removable(node, neighbor):
    return neighbor is not None and neighbor.key in node.adj_nodes

try:
    node.remove_neighbor(neighbor)
except TypeError:
    pass  # nothing to remove
except KeyError as e:
    if 'neighbor not found' in str(e):
        pass  # already removed
    else:
        raise

When it happens

Trigger: Calling remove_neighbor(None) directly, or removing an edge where the destination node was never created (variable still None). Also hit when iterating a list of neighbors that contains None entries from malformed graph construction.

Common situations: Tearing down edges from untrusted datasets; test code that removes a neighbor it never added; refactor where the node variable name holds None after a failed lookup in self.nodes.

Related errors

• neighbor or weight cannot be None
• neighbor not found
• key cannot be None
• graph cannot be None
• Input node keys cannot be None

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/ce824aae052bd24c. Report an issue: GitHub.