Default list creation
Input
append_item(1)
Output
[1]
No list provided; a new list [1] is created and returned.
Full lesson preview
Implement a function with a default argument
Practice using default arguments safely by creating a function that appends an item to a list, avoiding the common mutable-default pitfall.
Python practice10 minFunctions & ScopeBeginnerLast updated March 17, 2026
Problem statement
Implement a function append_item(item, lst=None) that appends item to the list lst and returns the resulting list. If lst is omitted or None, the function should create and return a new list containing only item. Do not use a mutable object as a default argument (e.g., avoid lst=[] in the signature). The function must ensure separate calls with the default create distinct lists.
Task
Write append_item(item, lst=None) that appends item to lst. If lst is not provided, create a fresh list each call.
Examples
Input format
First argument: item to append. Second optional argument: list to append to (or None to use a new list).
Output format
The list after appending the item.
Constraints
Ensure that a new list is created when lst is None. Do not mutate a shared default list across calls.
Samples
Sample 1
Input
append_item('a', ['x'])
Output
['x', 'a']
Appends 'a' to the provided list ['x'] and returns it.