class Queue:
    def __init__(self):
        self.__list = []

    def enqueue(self, item):
        '''往对列添加元素'''
        self.__list.append(item)

    def dequeue(self):
        '''从头部删除一个元素'''
        return self.__list.pop(0)

    def is_empty(self):
        return not self.__list

    def size(self):
        return len(self.__list)

if __name__ == '__main__':
    queue = Queue()
    queue.enqueue(1)
    queue.enqueue(2)
    queue.enqueue(3)
    queue.enqueue(4)

    print(queue.dequeue())
    print(queue.dequeue())
    print(queue.dequeue())
    print(queue.dequeue())```