Try it! The empty list in the first one is only created at the time the 'def' is executed. All subsequent calls to 'bad_append' hold the reference to this lis

Try it! The empty list in the first one is only created at the time the 'def' is executed. All subsequent calls to 'bad_append' hold the reference to this list instead of a new empty one. The 'good' one uses a sentinel value to indicate that you want to start with an empty list but also gives you the ability to pass in an existing one if needed.