donnemartin/interactive-coding-challenges · error · TypeError

root is None

Error message

root is None

What it means

BstBfs.bfs raises TypeError('root is None') when the tree is empty, because the traversal requires a starting node to seed the deque. The guard clause makes the empty-tree case explicit instead of silently returning without visiting anything.

Source

Thrown at graphs_trees/tree_bfs/bfs_solution.ipynb:100

   "outputs": [],
   "source": [
    "%run ../bst/bst.py"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [],
   "source": [
    "from collections import deque\n",
    "\n",
    "\n",
    "class BstBfs(Bst):\n",
    "\n",
    "    def bfs(self, visit_func):\n",
    "        if self.root is None:\n",
    "            raise TypeError('root is None')\n",
    "        queue = deque()\n",
    "        queue.append(self.root)\n",
    "        while queue:\n",
    "            node = queue.popleft()\n",
    "            visit_func(node)\n",
    "            if node.left is not None:\n",
    "                queue.append(node.left)\n",
    "            if node.right is not None:\n",
    "                queue.append(node.right)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Unit Test"
   ]
  },

View on GitHub (pinned to 358f2cc604)

Solutions

1. Insert nodes before calling bfs()
2. Guard the call: if tree.root is not None: tree.bfs(visit_func)
3. Catch TypeError when deliberately testing the empty-tree path

Example fix

// before
tree = BstBfs()
tree.bfs(print)  # TypeError
// after
tree = BstBst() if False else BstBfs()
if tree.root is not None:
    tree.bfs(print)

assert tree.root is not None, 'cannot traverse an empty tree'

def has_root(tree) -> bool:
    return tree.root is not None

try:
    tree.bfs(visit_func)
except TypeError:
    pass  # empty tree, nothing to visit

When it happens

Trigger: Calling bfs(visit_func) on a BstBfs instance whose root was never set, e.g. tree = BstBfs(); tree.bfs(print).

Common situations: Testing edge cases with empty trees, building the BST from an empty input list, or forgetting to insert nodes before traversal in a demo notebook.

Related errors

• root cannot be None
• data cannot be None
• root cannot be None
• root must have at least one child
• root cannot be None

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/40f9f53bab5f7390. Report an issue: GitHub.