Mutation.py

def append_twice_okay(a_list, val):
    """mutates argument."""
    a_list.append(val)
    a_list.append(val)

def append_twice_good(a_list, val):
    """returns new list."""
    a_list = a_list + [val, val]
    return a_list

nums_okay = [1, 2, 3]
append_twice_okay(nums_okay, 7)
print(nums_okay)


nums_good = [1, 2, 3]
append_twice_good(nums_good, 7)
nums_good_new = print(nums_good)