class arraystack """lifo stack implementation using python list as underlying storage "" def init (self) """create an empty stack ""nonpublic list instance self data def len (self) """return the number of elements in the stack "" return len(self data def is empty(self) """return true if the stack is empty "" return len(self data= def push(selfe) """add element to the top of the stack ""new item stored at end of list self data append( def top(self) """return (but do not removethe element at the top of the stack raise empty exception if the stack is empty "" if self is empty) raise emptystack is empty the last item in the list return self data[- def pop(self) """remove and return the element from the top of the stack ( lifo raise empty exception if the stack is empty "" if self is empty) raise emptystack is empty remove last item from list return self data popcode fragment implementing stack using python list as storage