Default Argument Trap

Avoid mutable default arguments and make your function safe across calls.

default-arguments mutable immutability

In Python, using a mutable default argument like `def add_item(item, bag=[])` causes the same list to be shared across all calls, leading to unexpected behavior. Your task is to implement a function `add_item(item, bag=None)` that adds an item to a shopping bag.

If `bag` is not provided (i.e., `None`), the function should create a new empty list, add the item to it, and return the list. If `bag` is provided, add the item to that list and return it.

The function must not modify the default argument in any way. It should work correctly when called multiple times without a `bag` argument, each time returning a new list containing only the given item.

**Function signature:** `def add_item(item, bag=None):`

**Behavior:**
- When `bag` is `None`, a new list `[item]` is returned.
- When `bag` is a list, the item is appended and the same list is returned.
- The original list passed as `bag` should be modified (append) as expected.

Do not use any external libraries. Your solution should be a pure Python function.

Constraints

- `item` can be any hashable or unhashable Python object (numbers, strings, tuples, lists, etc.).
- `bag` is either `None` or a list.
- The function should return a list.
- Complexity: O(1) append.

Example

>>> add_item('apple')
['apple']
>>> add_item('banana')
['banana']
>>> my_bag = ['apple']
>>> add_item('banana', my_bag)
['apple', 'banana']
>>> my_bag
['apple', 'banana']

8 points ~10 min

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Hints

Check if bag is None inside the function and create a new list then.

Remember that if bag is not None, you should append to it and return it.

Avoid using a mutable default argument like `bag=[]`.

Test Results

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