donnemartin/interactive-coding-challenges · error · TypeError

data cannot be None

Error message

data cannot be None

What it means

Bst.insert raises TypeError('data cannot be None') when inserting a None value. A BST needs comparable keys to route left/right during insertion, and None cannot be compared with <, so it is rejected before tree traversal.

Source

Thrown at graphs_trees/bst/bst_solution.ipynb:133

    "\n",
    "    def __init__(self, data):\n",
    "        self.data = data\n",
    "        self.left = None\n",
    "        self.right = None\n",
    "        self.parent = None\n",
    "\n",
    "    def __repr__(self):\n",
    "        return str(self.data)\n",
    "\n",
    "\n",
    "class Bst(object):\n",
    "\n",
    "    def __init__(self, root=None):\n",
    "        self.root = root\n",
    "\n",
    "    def insert(self, data):\n",
    "        if data is None:\n",
    "            raise TypeError('data cannot be None')\n",
    "        if self.root is None:\n",
    "            self.root = Node(data)\n",
    "            return self.root\n",
    "        else:\n",
    "            return self._insert(self.root, data)\n",
    "\n",
    "    def _insert(self, node, data):\n",
    "        if node is None:\n",
    "            return Node(data)\n",
    "        if data <= node.data:\n",
    "            if node.left is None:\n",
    "                node.left = self._insert(node.left, data)\n",
    "                node.left.parent = node\n",
    "                return node.left\n",
    "            else:\n",
    "                return self._insert(node.left, data)\n",
    "        else:\n",
    "            if node.right is None:\n",

View on GitHub (pinned to 358f2cc604)

Solutions

1. Filter out None values before inserting: for x in items: if x is not None: bst.insert(x)
2. Fix the data source to supply a default (e.g. coalesce to a sentinel value)
3. Validate records upstream and log/reject rows missing the key field

Example fix

# before
for value in values:
    bst.insert(value)  # values may contain None

# after
for value in values:
    if value is None:
        continue
    bst.insert(value)

for value in values:
    if value is None:
        continue  # or raise, per policy
    bst.insert(value)

def is_insertable(v) -> bool:
    return v is not None

try:
    bst.insert(data)
except TypeError as e:
    if 'cannot be None' in str(e):
        logger.warning('skipping null record')
    else:
        raise

When it happens

Trigger: bst.insert(None), inserting values read from a stream/file where a record is missing a field, or a loop over a list that contains None entries.

Common situations: ETL/ingest pipelines inserting rows with nullable columns; lists containing None from map/filter chains; API payloads with absent optional fields.

Related errors

• root cannot be None
• number cannot be None
• a or b cannot be None
• Invalid argument: None
• num cannot be None

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/db8fa0dd51edfc3c. Report an issue: GitHub.