# 队列异常
class QueueError(Exception):
    pass


# 完成队列操作
class ArrayQueue:
    def __init__(self):
        # 使用列表存储队列数据
        self._elems = []

    def is_empty(self):
        return self._elems == []

    def enqueue(self, elem):
        self._elems.append(elem)

    def dequeue(self):
        if not self._elems:
            raise QueueError('Queue is empty')
        return self._elems.pop(0)


if __name__ == "__main__":
    sq = ArrayQueue()
    print(sq.is_empty())
    sq.enqueue(10)
    sq.enqueue(20)
    sq.enqueue(30)
    while not sq.is_empty():
        print(sq.dequeue())