donnemartin/interactive-coding-challenges · error · TypeError

key cannot be None

Error message

key cannot be None

What it means

Raised by Graph.add_node when key is None. Node keys index the graph's nodes dict and drive all lookups (BFS, DFS, shortest path), so None keys are rejected outright. This is a defensive TypeError against malformed graph definitions.

Source

Thrown at graphs_trees/graph/graph.py:50

    def remove_neighbor(self, neighbor):
        if neighbor is None:
            raise TypeError('neighbor cannot be None')
        if neighbor.key not in self.adj_nodes:
            raise KeyError('neighbor not found')
        neighbor.incoming_edges -= 1
        del self.adj_weights[neighbor.key]
        del self.adj_nodes[neighbor.key]


class Graph:

    def __init__(self):
        self.nodes = {}  # Key = key, val = Node

    def add_node(self, key):
        if key is None:
            raise TypeError('key cannot be None')
        if key not in self.nodes:
            self.nodes[key] = Node(key)
        return self.nodes[key]

    def add_edge(self, source_key, dest_key, weight=0):
        if source_key is None or dest_key is None:
            raise KeyError('Invalid key')
        if source_key not in self.nodes:
            self.add_node(source_key)
        if dest_key not in self.nodes:
            self.add_node(dest_key)
        self.nodes[source_key].add_neighbor(self.nodes[dest_key], weight)

    def add_undirected_edge(self, src_key, dst_key, weight=0):
        if src_key is None or dst_key is None:
            raise TypeError('key cannot be None')
        self.add_edge(src_key, dst_key, weight)
        self.add_edge(dst_key, src_key, weight)

View on GitHub (pinned to 358f2cc604)

Solutions

1. Validate/skip records with missing keys before graph construction
2. Default None keys to a generated unique key (e.g. uuid or counter)
3. Assert key is not None in a wrapper around graph building code

Example fix

# before
graph.add_node(record.get('id'))

# after
key = record.get('id')
if key is not None:
    graph.add_node(key)

if key is not None:
    graph.add_node(key)

def valid_key(key) -> bool:
    return key is not None

try:
    graph.add_node(key)
except TypeError:
    log.warning('skipping node with missing key: %r', key)

When it happens

Trigger: Calling graph.add_node(None); passing a None key derived from data into add_node, add_edge, or add_undirected_edge.

Common situations: Building graphs from records with missing ID fields; a key computed as data.get('id') returning None; refactor changing key type to Optional.

Related errors

• neighbor or weight cannot be None
• neighbor cannot be None
• key cannot be None
• data cannot be None
• key cannot be None

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/657b486cc409580d. Report an issue: GitHub.