old_contents: #!/usr/bin/env python
  
  '''Implementation of a simple queue data structure.
  The queue has `enqueue`, `dequeue`, and `peek` methods.
  Items in the queue have `value` and `behind` attributes.
  The queue has a `front` attribute.
  '''
  
  
  class Item(object):
      def __init__(self, value, behind=None):
          self.value = value
          self.behind = behind
  
      def __str__(self):
          return self.value
  
  
  class Queue(object):
      def __init__(self, front=None):
          self.front = front
  
      def enqueue(self, value):
          pass
  
      def dequeue(self):
          pass
  
      def peek(self):
          pass
  
new_contents: #!/usr/bin/env python
  
  '''Implementation of a simple queue data structure.
  The queue has `enqueue`, `dequeue`, and `peek` methods.
  Items in the queue have `value` and `behind` attributes.
  The queue has a `front` attribute.
  '''
  
  
  class Item(object):
      def __init__(self, value, behind=None):
          self.value = value
          self.behind = behind
  
      def __str__(self):
          return self.value
  
  
  class Queue(object):
      def __init__(self, front=None):
          self.front = front
  
      def enqueue(self, value):
          pass
  
      def dequeue(self):
          pass
  
      def peek(self):
          try:
              return self.front.value
          except AttributeError:
              raise ValueError('No items in queue')
  
subject: Add peek function to Queue class