# Implementation of the Queue ADT using a Python list.
class Queue :
   # Creates an empty queue.
  def __init__( self ):
    self._qList = list()
    
   # Returns True if the queue is empty.
  def is_empty( self ):
    return len( self ) == 0
    
   # Returns the number of items in the queue.
  def __len__( self ):
    return len( self._qList )
    
   # Adds the given item to the queue.
  def enqueue( self, item ):
    self._qList.append( item )
    
   # Removes and returns the first item in the queue.
  def dequeue( self ):
    assert not self.is_empty(), "Cannot dequeue from an empty queue."
    return self._qList.pop( 0 )

  # Examine the first item without removing it
  def peek( self ):
    assert not self.is_empty(), "Cannot peek from an empty queue."
    item = self._qList[ 0 ]
    return item