class ArrayStack(object):

""" LIFO Stack implementation using a Python list as underlying storage. """

def __init__(self):

""" make an empty stack. """

self._data = [] # non-public list instance

def __len__(self):

"""Return the # of elements in the stack. """

return len(self._data)

def is_empty(self):

""" Return True is stack is empty. """

return len(self._data) == 0

def push(self, e):

""" Add element e to the top of the stack. """

self._data.append(e) # new item stored at end of list

def top(self):

""" Return (but do not remove) the element at the top of the stack.

Raise Empty exception if the stack is empty.

"""

if self.is_empty():

raise ('Stack is empty!')

return self._data[-1] # the last item in the list

def pop(self):

""" Remove and return the element from the top of the stack (i.e., LIFO)."""

if self.is_empty():

raise ('Stack is empty!')

return self._data.pop()