code_debug_env/tasks/medium/task_003_mutable_default.json

{
  "task_id": "task_003_mutable_default",
  "difficulty": "medium",
  "description": "Fix the bug in the add_item and build_shopping_lists functions. build_shopping_lists should return independent lists for each group of items. Currently all lists share the same underlying list due to a mutable default argument.",
  "buggy_code": "def add_item(item, lst=[]):\n    lst.append(item)\n    return lst\n\n\ndef build_shopping_lists(items_per_list):\n    result = []\n    for items in items_per_list:\n        current_list = add_item(items[0])\n        for item in items[1:]:\n            current_list = add_item(item)\n        result.append(current_list)\n    return result\n",
  "solution": "def add_item(item, lst=None):\n    if lst is None:\n        lst = []\n    lst.append(item)\n    return lst\n\n\ndef build_shopping_lists(items_per_list):\n    result = []\n    for items in items_per_list:\n        current_list = add_item(items[0])\n        for item in items[1:]:\n            current_list = add_item(item, current_list)\n        result.append(current_list)\n    return result\n",
  "test_descriptions": [
    "build_shopping_lists([['a', 'b']]) should return [['a', 'b']]",
    "build_shopping_lists([['a', 'b'], ['c', 'd']]) should return [['a', 'b'], ['c', 'd']] (independent lists)",
    "build_shopping_lists([['x']]) should return [['x']]",
    "Calling build_shopping_lists twice should not carry state between calls"
  ],
  "test_code": "try:\n    r1 = build_shopping_lists([['a', 'b']])\n    results.append({\"test_name\": \"single_list\", \"passed\": r1 == [['a', 'b']], \"expected\": \"[['a', 'b']]\", \"actual\": str(r1), \"error\": \"\"})\nexcept Exception as e:\n    results.append({\"test_name\": \"single_list\", \"passed\": False, \"expected\": \"[['a', 'b']]\", \"actual\": \"\", \"error\": str(e)})\n\ntry:\n    r2 = build_shopping_lists([['a', 'b'], ['c', 'd']])\n    results.append({\"test_name\": \"independent_lists\", \"passed\": r2 == [['a', 'b'], ['c', 'd']], \"expected\": \"[['a', 'b'], ['c', 'd']]\", \"actual\": str(r2), \"error\": \"\"})\nexcept Exception as e:\n    results.append({\"test_name\": \"independent_lists\", \"passed\": False, \"expected\": \"[['a', 'b'], ['c', 'd']]\", \"actual\": \"\", \"error\": str(e)})\n\ntry:\n    r3 = build_shopping_lists([['x']])\n    results.append({\"test_name\": \"single_item\", \"passed\": r3 == [['x']], \"expected\": \"[['x']]\", \"actual\": str(r3), \"error\": \"\"})\nexcept Exception as e:\n    results.append({\"test_name\": \"single_item\", \"passed\": False, \"expected\": \"[['x']]\", \"actual\": \"\", \"error\": str(e)})\n\ntry:\n    call1 = build_shopping_lists([['a']])\n    call2 = build_shopping_lists([['b']])\n    results.append({\"test_name\": \"no_state_leak\", \"passed\": call1 == [['a']] and call2 == [['b']], \"expected\": \"[['a']] and [['b']]\", \"actual\": f\"{call1} and {call2}\", \"error\": \"\"})\nexcept Exception as e:\n    results.append({\"test_name\": \"no_state_leak\", \"passed\": False, \"expected\": \"[['a']] and [['b']]\", \"actual\": \"\", \"error\": str(e)})",
  "max_steps": 7
}