如何用rospy编写一套小海龟走方形的publisher和subscriber，请给出具体代码

以下是使用rospy编写小海龟走方形的publisher和subscriber的示例代码：

#!/usr/bin/env python
import rospy
from geometry_msgs.msg import Twist
from turtlesim.msg import Pose

def pose_callback(pose):
    global x, y, theta
    x = pose.x
    y = pose.y
    theta = pose.theta

def move():
    rospy.init_node('turtle_square', anonymous=True)
    velocity_publisher = rospy.Publisher('/turtle1/cmd_vel', Twist, queue_size=10)
    pose_subscriber = rospy.Subscriber('/turtle1/pose', Pose, pose_callback)
    rate = rospy.Rate(10)

    # Move forward
    vel_msg = Twist()
    vel_msg.linear.x = 1
    vel_msg.linear.y = 
    vel_msg.linear.z = 
    vel_msg.angular.x = 
    vel_msg.angular.y = 
    vel_msg.angular.z = 
    t = rospy.Time.now().to_sec()
    while not rospy.is_shutdown():
        velocity_publisher.publish(vel_msg)
        t1 = rospy.Time.now().to_sec()
        if t1 - t > 2:
            break
        rate.sleep()

    # Stop and turn
    vel_msg.linear.x = 
    vel_msg.angular.z = 1
    t = rospy.Time.now().to_sec()
    while not rospy.is_shutdown():
        velocity_publisher.publish(vel_msg)
        t1 = rospy.Time.now().to_sec()
        if t1 - t > 1.5:
            break
        rate.sleep()

    # Move forward again
    vel_msg.linear.x = 1
    vel_msg.angular.z = 
    t = rospy.Time.now().to_sec()
    while not rospy.is_shutdown():
        velocity_publisher.publish(vel_msg)
        t1 = rospy.Time.now().to_sec()
        if t1 - t > 2:
            break
        rate.sleep()

    # Stop and turn again
    vel_msg.linear.x = 
    vel_msg.angular.z = 1
    t = rospy.Time.now().to_sec()
    while not rospy.is_shutdown():
        velocity_publisher.publish(vel_msg)
        t1 = rospy.Time.now().to_sec()
        if t1 - t > 1.5:
            break
        rate.sleep()

    # Move forward one last time
    vel_msg.linear.x = 1
    vel_msg.angular.z = 
    t = rospy.Time.now().to_sec()
    while not rospy.is_shutdown():
        velocity_publisher.publish(vel_msg)
        t1 = rospy.Time.now().to_sec()
        if t1 - t > 2:
            break
        rate.sleep()

    # Stop the turtle
    vel_msg.linear.x = 
    vel_msg.angular.z = 
    velocity_publisher.publish(vel_msg)

if __name__ == '__main__':
    try:
        move()
    except rospy.ROSInterruptException:
        pass

这段代码使用了geometry_msgs/Twist和turtlesim/Pose消息类型，其中Twist消息类型用于控制小海龟的运动，Pose消息类型用于获取小海龟的位置信息。在move函数中，我们首先初始化了节点，然后创建了一个publisher和一个subscriber，分别用于控制小海龟的运动和获取小海龟的位置信息。接着，我们使用Twist消息类型控制小海龟走方形，具体实现方式是先向前走一段时间，然后停下来并向左转，再向前走一段时间，再停下来并向左转，最后再向前走一段时间。在每个阶段结束时，我们都会检查时间是否已经超过了规定的时间，如果超过了就停止运动。最后，我们停止小海龟的运动并退出程序。