The explanation of the behavior presented on the previous page is:

• the 'a_list' object is a compound object;
• we’ve run a shallow copy that constructs a new compound object, b_list in our example, and then populated it with references to the objects found in the original;
• as you can see, a shallow copy is only one level deep. The copying process does not recurse and therefore does not create copies of the child objects, but instead populates b_list with references to the already existing objects.