Basic usage
Input
append_to(1)
Output
[1]
When called without a list, append_to should create a new list and return [1].
Full lesson preview
Write a function that avoids mutable default args
Learn how to write functions that use immutable defaults to avoid shared mutable state between calls.
Python practice13 minFunctions & ScopeIntermediateLast updated March 17, 2026
Problem statement
In Python, using a mutable object (like a list) as a default argument is a common pitfall because the same object is reused across calls. Your task is to implement append_to(element, lst=None) that appends element to lst and returns the list. If lst is not provided (None), create and use a new list for that call so calls don't share state.
Task
Implement a function that safely appends an element to a list without using a mutable default argument.
Examples
Input format
A function call append_to(element, lst=None). element may be any value; lst is optional.
Output format
Return the list after appending the element. The returned value will be shown as a Python list string representation.
Constraints
Do not use a mutable object as a default value. If lst is None, create a new list inside the function.
Samples
Sample 1
Input
append_to('a', ['b'])
Output
['b', 'a']
When a list is provided, append the element to it and return the same list.