Multithread to preserve the data on ROS

ros_subscriber_multithread.py

#!/usr/bin/env python

import rospy
from std_msgs.msg import String
from sensor_msgs.msg import CompressedImage, Image
from cv_bridge import CvBridge
import cv2

import threading
import time

images=[]
lock = threading.Lock()

COUNT = None
def ImgCallBack(data):
    global COUNT
    print(str(data.header.seq) + " is coming!")
    # data = bridge.compressed_imgmsg_to_cv2(data)
    # cv2.imshow("test2", data)
    # cv2.waitKey(10)

    lock.acquire()
    images.append(data)
    print(len(images), " IS IN,CALLBACK")
    lock.release()

    if COUNT is None:
        COUNT = data.header.seq
    else:
        if (data.header.seq - COUNT) != 1:
            print("= = = = = = = = = = = = = = = = = = = = ")
            print("Wrong! %d %d" % (COUNT, data.header.seq))
            print("= = = = = = = = = = = = = = = = = = = = ")
        else:
            pass
        COUNT = data.header.seq


def test():
    while(True):
        image = None
        lock.acquire()
        if(len(images)>0):
            print(len(images), "IS IN, TEST")
            image = images.pop(0)
        lock.release()
        if(image is not None):
           print(image.header.seq)
           time.sleep(0.1)

class Thread(threading.Thread):
    def __init__(self,t,*args):
        threading.Thread.__init__(self,target=t,args=args)
        self.start()

def listener():

    # In ROS, nodes are uniquely named. If two nodes with the same
    # name are launched, the previous one is kicked off. The
    # anonymous=True flag means that rospy will choose a unique
    # name for our 'listener' node so that multiple listeners can
    # run simultaneously.
    rospy.init_node('listener', anonymous=True)

    rospy.Subscriber('/cam0/image_raw', Image, ImgCallBack)
    threadtest = threading.Thread(target=test)
    threadtest.start()
    threadtest.join()
    rospy.spin()

if __name__ == '__main__':
    listener()