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