# Implementation of the Stack ADT using a Python list.
class Stack :
    """ A stack class implemented using Python list.
    The end of the list is the 'top' of the stack"""
    def __init__( self ):
        """ Create the stack""" 
        self._the_items = list()
  
    def is_empty( self ):
        """ Check if empty """
        return len( self ) == 0
  
    def __len__ ( self ):
        """ Return the length """
        return len( self._the_items ) 

    def peek( self ):
        """ Return the top item of the stack"""
        assert not self.is_empty(), "Cannot peek at an empty stack"
        return self._the_items[-1]
	   
    def pop( self ):
        """ Return and remove the top element of the stack"""
        assert not self.is_empty(), "Cannot pop from an empty stack"
        return self._the_items.pop()
  
    def push( self, item ):
        """ Add a new item to the top of the stack"""
        self._the_items.append( item )