JuniorTheoryCommon

What does the program print?

Python

def add_item(item, box=[]):
    box.append(item)
    return box


add_item(1)
print(add_item(2))

[1, 2]

Expecting [2], as if each call got a fresh empty list: the default [] is created once, when def runs, so both calls append to the same list.
Expecting a TypeError, as if a list could not be a default: any object can be one, and the trap is that this one list is shared by every call.

How would you rewrite add_item with a None default so that each call starts from an empty list?
What does add_item(3, []) return after these two calls?

JuniorTheoryCommon

Python

def add_item(item, box=[]):
    box.append(item)
    return box


add_item(1)
print(add_item(2))

[1, 2]

Expecting [2], as if each call got a fresh empty list: the default [] is created once, when def runs, so both calls append to the same list.
Expecting a TypeError, as if a list could not be a default: any object can be one, and the trap is that this one list is shared by every call.

How would you rewrite add_item with a None default so that each call starts from an empty list?
What does add_item(3, []) return after these two calls?