donnemartin/interactive-coding-challenges · error · TypeError

root cannot be None

Error message

root cannot be None

What it means

InverseBst.invert_tree raises TypeError('root cannot be None') when called on an empty BST. The class delegates to _invert_tree starting from self.root, and the guard clause treats an empty tree as an invalid argument rather than a no-op, so callers must ensure the tree has at least one node before inverting.

Source

Thrown at graphs_trees/invert_tree/invert_tree_solution.ipynb:114

   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "%run ../bst/bst.py"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [],
   "source": [
    "class InverseBst(Bst):\n",
    "\n",
    "    def invert_tree(self):\n",
    "        if self.root is None:\n",
    "            raise TypeError('root cannot be None')\n",
    "        return self._invert_tree(self.root)\n",
    "\n",
    "    def _invert_tree(self, root):\n",
    "        if root is None:\n",
    "            return\n",
    "        self._invert_tree(root.left)\n",
    "        self._invert_tree(root.right)\n",
    "        root.left, root.right = root.right, root.left\n",
    "        return root"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Unit Test"
   ]
  },

View on GitHub (pinned to 358f2cc604)

Solutions

1. Insert at least one node (e.g. tree.insert(5)) before calling invert_tree()
2. Guard the call: if tree.root is not None: tree.invert_tree()
3. Catch TypeError in test code when intentionally exercising the empty-tree case

Example fix

// before
tree = InverseBst()
tree.invert_tree()  # TypeError
// after
tree = InverseBst()
if tree.root is not None:
    tree.invert_tree()

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

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

try:
    tree.invert_tree()
except TypeError:
    pass  # empty tree, nothing to invert

When it happens

Trigger: Calling invert_tree() on a freshly constructed InverseBst (or any Bst subclass) without inserting any nodes first, e.g. tree = InverseBst(); tree.invert_tree().

Common situations: Running the solution on an empty test case, initializing a BST from an empty array/list, or a test harness that iterates over edge cases including [] before populated trees.

Related errors

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

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/70c62d4cd9435677. Report an issue: GitHub.