class Node:    """Single node in a data structure"""      def __init__(self, data):       """Node constructor"""              self._data = data       self._nextNode = None         def __str__(self):       """Node data representation"""         return str(self._data)       class List:    """Linked list"""      def __init__(self):       """List constructor"""         self._firstNode = None       self._lastNode = None      def __str__(self):       """List string representation"""         if self.isEmpty():          return "empty"         currentNode = self._firstNode       output = []         while currentNode is not None:          output.append(str(currentNode._data))          currentNode = currentNode._nextNode         return " ".join(output)          def insertAtFront(self, value):       """Insert node at front of list"""         newNode = Node(value)         if self.isEmpty():  # List is empty          self._firstNode = self._lastNode = newNode       else:   # List is not empty          newNode._nextNode = self._firstNode          self._firstNode = newNode             def insertAtBack(self, value):       """Insert node at back of list"""         newNode = Node(value)         if self.isEmpty():  # List is empty          self._firstNode = self._lastNode = newNode       else:  # List is not empty          self._lastNode._nextNode = newNode          self._lastNode = newNode      def removeFromFront(self):       """Delete node from front of list"""         if self.isEmpty():  # raise exception on empty list          raise IndexError, "remove from empty list"         tempNode = self._firstNode         if self._firstNode is self._lastNode:  # one node in list          self._firstNode = self._lastNode = None       else:          self._firstNode = self._firstNode._nextNode         return tempNode      def removeFromBack(self):       """Delete node from back of list"""         if self.isEmpty():  # raise exception on empty list          raise IndexError, "remove from empty list"             tempNode = self._lastNode         if self._firstNode is self._lastNode:  # one node in list          self._firstNode = self._lastNode = None       else:          currentNode = self._firstNode            # locate second-to-last node          while currentNode._nextNode is not self._lastNode:                currentNode = currentNode._nextNode                          currentNode._nextNode = None          self._lastNode = currentNode         return tempNode         def isEmpty(self):       """Returns true if List is empty"""         return self._firstNode is None