donnemartin/interactive-coding-challenges · error · TypeError

grid cannot be None

Error message

grid cannot be None

What it means

Raised by Solution.island_perimeter when grid is None. The method immediately does len(grid) and len(grid[0]), which would raise TypeError on None; the guard states the contract: a 2D list of 0/1 cells is required.

Source

Thrown at online_judges/island_perimeter/island_perimeter_solution.ipynb:105

  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Solution(object):\n",
    "\n",
    "    def island_perimeter(self, grid):\n",
    "        if grid is None:\n",
    "            raise TypeError('grid cannot be None')\n",
    "        sides = 0\n",
    "        num_rows = len(grid)\n",
    "        num_cols = len(grid[0])\n",
    "        for i in range(num_rows):\n",
    "            for j in range(num_cols):\n",
    "                if grid[i][j] == 1:\n",
    "                    # Check left\n",
    "                    if j == 0 or grid[i][j - 1] == 0:\n",
    "                        sides += 1\n",
    "                    # Check right\n",
    "                    if j == num_cols - 1 or grid[i][j + 1] == 0:\n",
    "                        sides += 1\n",
    "                    # Check up\n",
    "                    if i == 0 or grid[i - 1][j] == 0:\n",
    "                        sides += 1\n",
    "                    # Check down\n",
    "                    if i == num_rows - 1 or grid[i + 1][j] == 0:\n",
    "                        sides += 1\n",

View on GitHub (pinned to 358f2cc604)

Solutions

1. Pass a well-formed 2D list, e.g. [[0,1,0,0],[1,1,1,0],...]
2. Default the loader to [] or fail loudly when the grid source is missing
3. Ensure the notebook cell building the grid has executed before the call

Example fix

# before
solution.island_perimeter(grid)  # grid never assigned / loader returned None

# after
grid = load_grid(path)
if grid is None:
    raise FileNotFoundError(path)
solution.island_perimeter(grid)

if grid is None: raise ValueError('grid is required')
solution.island_perimeter(grid)

def is_grid(g): return isinstance(g, list) and all(isinstance(r, list) for r in g)

try:
    p = solution.island_perimeter(grid)
except TypeError as e:
    raise ValueError('missing grid input') from e

When it happens

Trigger: Calling island_perimeter(None) — e.g. a matrix loader returned None, or grid came from a failed parse or an optional parameter.

Common situations: Reading grids from files/APIs that may be absent; tests exercising the None guard; notebooks where the cell defining the grid wasn't run.

Related errors

• num cannot be None
• max_num cannot be None
• val cannot be None
• n cannot be None
• a or b cannot be None

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/a3346f8084414aa1. Report an issue: GitHub.