new_contents: class MyQueue(object):
      def __init__(self):
          self.front_stack = Stack()
          self.back_stack = Stack()
  
      def eq(self, data):
          self.back_stack.push(data)
  
      def dq(self):
          if self.front_stack.size == 0:
              self.rebuild()
          return self.front_stack.pop()
  
      def peek_front(self):
          if self.front_stack.size == 0:
              self.rebuild()
          return self.front_stack.peek()
      
      def rebuild(self):
          """
          Lazily rebuilds the front stack when a value is needed by pop/peek.
          When the front_stack empties, rebuild it by reversing the values in
          the back_stack.
          """
          while self.back_stack.size > 0:
              self.front_stack.push(self.back_stack.pop())
    
  
subject: Create 3.5_que_2stacks.py