如何用两个栈模拟队列操作

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

    def empty(self):
        return len(self.items) == 0

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

    def peek(self):
        if not self.empty():
            return self.items[len(self.items) - 1]
        else:
            return None

    def pop(self):
        if not self.empty():
            return self.items.pop()
        else:
            print('栈已经为空')

    def push(self, x):
        self.items.append(x)


class MyStack:
    def __init__(self):
        self.A = Stack()  # 用来存储栈中元素
        self.B = Stack()  # 用来存储当前栈中最小的元素

    def push(self, data):
        self.A.push(data)

    def pop(self):
        if self.B.empty():
            while not self.A.empty():
                self.B.push(self.A.peek())
                self.A.pop()

        first = self.B.peek()
        self.B.pop()
        return first


if __name__ == '__main__':
    stack = MyStack()
    stack.push(1)
    stack.push(2)
    stack.push(3)
    print('队列首元素为：' + str(stack.pop()))
    print('队列首元素为：' + str(stack.pop()))
    stack.push(4)
    print('队列首元素为：' + str(stack.pop()))
    print('队列首元素为：' + str(stack.pop()))

运行结果如下：
队列首元素为：1
队列首元素为：2
队列首元素为：3
队列首元素为：4