python ros 回充demo

#!/usr/bin/env python
#coding=utf-

import rospy
from std_msgs.msg import String
i=
def talker():
    global i
    pub = rospy.Publisher('nav_goal',String, queue_size=)
    rospy.init_node('talker',anonymous=True)
    #rate = rospy.Rate() # 10hz
    #rospy.signal_shutdown("closed!")
    while not rospy.is_shutdown():
        rospy.loginfo("pub")
        pub.publish(String(data="charging_port"))
        i=i+
        #rate.sleep()

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