For the arraylist's underlying data storage mechanism you will use the built-in python list, constrained so that only the following operations (as would be supported by a primitive array) are available: lst[i] for getting and setting a value at an existing, positive index i len(lst) to obtain the number of slots lst. append(none) to grow the list by one slot at a time del lst[len(lst)-1] to delete the last slot in a list constrained least to keep us honest, we've defined an api-constrained sub-class of the built-in list -- constrained set -- an instance of which is assigned to the data attribute of each arraylist. you should not change the definition of constrainedlist, and ensure that your arraylist implementation never assigns a regular python list to its data attribute. so long as you use constrainedlist in your implementation, you can be certain you're not performing any "illegal" operations (i. e., outside the constraints established above). if you invoke a disallowed operation, an appropriate exception will be raised.