class Queue(object):
    """实现一个队列"""

    def __init__(self, length = 20):
        self.list = []
        self.__length = length

    def enqueue(self, elem):
        """入队"""
        if self.size() > self.__length:
            self.dequeue()
            self.list.append(elem)
        else:
            self.list.append(elem)

    def dequeue(self):
        """出队"""
        return self.list.pop(0)

    def is_empty(self):
        return not self.list

    def size(self):
        """队列的大小"""
        return len(self.list)