mutation

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."""
   return a_list + [val, val]

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


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