donnemartin/interactive-coding-challenges · error · TypeError

a or b cannot be None

Error message

a or b cannot be None

What it means

sum_two implements addition with bitwise operators (XOR plus shifted carry, recursing until carry is 0) and raises TypeError when a or b is None. The guard protects the public API since the internal recursion always passes ints. Without it, a ^ b on None would raise a confusing TypeError about unsupported operand types.

Source

Thrown at online_judges/sum_two/sum_two_solution.ipynb:126

  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Solution(object):\n",
    "\n",
    "    def sum_two(self, a, b):\n",
    "        if a is None or b is None:\n",
    "            raise TypeError('a or b cannot be None')\n",
    "        result = a ^ b;\n",
    "        carry = (a&b) << 1\n",
    "        if carry != 0:\n",
    "            return self.sum_two(result, carry)\n",
    "        return result;"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Unit Test"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},

View on GitHub (pinned to 358f2cc604)

Solutions

1. Pass non-negative ints, converting explicitly beforehand
2. Resolve None-defaults at the call site: b = b if b is not None else 0
3. Replace with a + b for production use; the bitwise version is an interview exercise

Example fix

// before
def add(x, y=None):
    return sol.sum_two(x, y)
// after
def add(x, y=0):
    return sol.sum_two(x, y)

a = a if a is not None else 0
b = b if b is not None else 0
sol.sum_two(a, b)

def is_nonneg_int(x):
    return isinstance(x, int) and not isinstance(x, bool) and x >= 0

try:
    sol.sum_two(a, b)
except TypeError as e:
    if 'a or b cannot be None' in str(e):
        raise ValueError('operands must be provided') from e
    raise

When it happens

Trigger: Calling Solution().sum_two(None, 3) or sum_two(3, None); forwarding optional numeric params (e.g. defaults of None) directly into the method.

Common situations: Optional fields in deserialized data left as None and passed through; misreading the API as accepting nullable numbers; notebook cells referencing unassigned variables. Beware also that with negative ints this carry loop can recurse very long in Python (no fixed-width wrap).

Related errors

• a or b cannot be None
• a or b cannot be None
• a or b cannot be None
• Cannot have a None input
• array cannot be None or empty

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/8420128da34542f4. Report an issue: GitHub.