donnemartin/interactive-coding-challenges · error · TypeError

s or t cannot be None

Error message

s or t cannot be None

What it means

find_diff (find the difference between two strings where t is a permutation of s plus one extra char) raises TypeError when either s or t is None. The counting-dict logic iterates both strings, so None would fail with a less clear error; the guard fails fast with an explicit message. The algorithm itself assumes both inputs are strings of the same alphabet.

Source

Thrown at online_judges/str_diff/str_diff_solution.ipynb:90

  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Solution(object):\n",
    "\n",
    "    def find_diff(self, s, t):\n",
    "        if s is None or t is None:\n",
    "            raise TypeError('s or t cannot be None')\n",
    "        seen = {}\n",
    "        for char in s:\n",
    "            if char in seen:\n",
    "                seen[char] += 1\n",
    "            else:\n",
    "                seen[char] = 1\n",
    "        for char in t:\n",
    "            try:\n",
    "                seen[char] -= 1\n",
    "            except KeyError:\n",
    "                return char\n",
    "            if seen[char] < 0:\n",
    "                return char\n",
    "        return None"
   ]
  },
  {
   "cell_type": "markdown",

View on GitHub (pinned to 358f2cc604)

Solutions

1. Pass both strings, using '' for the legitimately empty case
2. Default missing inputs to empty strings at the boundary: s = s or ''
3. Add tests covering the None contract so callers know it is intentionally rejected

Example fix

// before
sol.find_diff(s, t)  # t is None
// after
sol.find_diff(s if s is not None else '', t if t is not None else '')

s = s or ''
t = t or ''
sol.find_diff(s, t)

def is_str(x):
    return isinstance(x, str)

try:
    sol.find_diff(s, t)
except TypeError as e:
    if 's or t cannot be None' in str(e):
        raise ValueError('both strings are required') from e
    raise

When it happens

Trigger: Calling Solution().find_diff(None, 'abc') or find_diff('abc', None) or both None; passing t loaded from a file/stream that hit EOF and returned None.

Common situations: String pairs read from paired files or API fields where one side is missing; notebook variables not yet assigned before the cell runs; optional request parameters forwarded without defaults.

Related errors

• license_key must be a str
• license_key must not be empty
• file_system cannot be None
• string cannot be None
• array cannot be None

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/f73f3b45f8a3e047. Report an issue: GitHub.