queue derived from two stacks

queue.py

class Stack(object):
  def __init__(self):
    self.arr = []

  def __len__(self):
    return len(self.arr)

  def push(self, obj):
    self.arr.append(obj)

  def pop(self):
    if len(self.arr) == 0:
      raise Exception('Cannot pop when there are no elements!')

    v = self.arr[-1]
    del self.arr[-1]
    return v

class Queue(object):
  def __init__(self):
    self.stacks = [Stack(), Stack()]

  def enqueue(self, obj):
    self.stacks[0].push(obj)

  def dequeue(self):
    if len(self.stacks[0]) == 0:
      raise Exception('Nothing left to dequeue!')

    # swap stack 1 to stack 2 except last element
    for _ in xrange(len(self.stacks[0]) - 1):
      self.stacks[1].push(self.stacks[0].pop())

    # pop from stack 2 => val
    val = self.stacks[0].pop()

    # swap back to stack 1
    for _ in range(len(self.stacks[1])):
      self.stacks[0].push(self.stacks[1].pop())

    # ret val
    return val


def main():
  q = Queue()
  for i in xrange(1, 3):
    q.enqueue(i)
  print q.dequeue()
  q.enqueue(5)
  print q.dequeue()
  print q.dequeue()

if __name__ == '__main__':
  main()