donnemartin/interactive-coding-challenges · error · TypeError

matrices cannot be None

Error message

matrices cannot be None

What it means

Raised by MatrixMultiplicationCost.find_min_cost when matrices is None. The method builds a square DP table of size len(matrices) and reads matrix dimensions from each entry, so a None list is rejected explicitly before T is allocated.

Source

Thrown at recursion_dynamic/matrix_mult/find_min_cost_solution.ipynb:200

    "    def __init__(self, first, second):\n",
    "        self.first = first\n",
    "        self.second = second"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [],
   "source": [
    "import sys\n",
    "\n",
    "\n",
    "class MatrixMultiplicationCost(object):\n",
    "\n",
    "    def find_min_cost(self, matrices):\n",
    "        if matrices is None:\n",
    "            raise TypeError('matrices cannot be None')\n",
    "        if not matrices:\n",
    "            return 0\n",
    "        size = len(matrices)\n",
    "        T = [[0] * size for _ in range(size)]\n",
    "        for offset in range(1, size):\n",
    "            for i in range(size-offset):\n",
    "                j = i + offset\n",
    "                min_cost = sys.maxsize\n",
    "                for k in range(i, j):\n",
    "                    cost = (T[i][k] + T[k+1][j] +\n",
    "                            matrices[i].first *\n",
    "                            matrices[k].second *\n",
    "                            matrices[j].second)\n",
    "                    if cost < min_cost:\n",
    "                        min_cost = cost\n",
    "                T[i][j] = min_cost\n",
    "        return T[0][size-1]"
   ]

View on GitHub (pinned to 358f2cc604)

Solutions

1. Pass a list of matrix dimension tuples (e.g. [(2,3),(3,6)]) and default to [] when absent
2. Fix the parser to return [] on failure instead of None
3. Guard the call site with 'if matrices is not None'

Example fix

// before
cost = mmc.find_min_cost(parse_dims(text))
// after
dims = parse_dims(text) or []
cost = mmc.find_min_cost(dims)

matrices = matrices or []
mmc.find_min_cost(matrices)

def is_dim_list(x):
    return isinstance(x, list) and all(isinstance(m, tuple) and len(m) == 2 for m in x)

try:
    mmc.find_min_cost(matrices)
except TypeError:
    cost = 0

When it happens

Trigger: Calling find_min_cost(None). An empty list is valid and returns 0; only None raises the TypeError.

Common situations: Matrix dimension chains parsed from input that may be absent; a chain-building helper returning None on parse failure; passing an uninitialized variable.

Related errors

• coins or total cannot be None
• input_items or total_weight cannot be None
• items or total_weight cannot be None
• str input cannot be None
• seq cannot be None

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/6ee6092e5aecf5ce. Report an issue: GitHub.