Is rospython subscriber thread safe?

edit

Hey
I'm trying to write a python node, which has one class and two member functions as callback functions.
so a TimeSynchronizer and Subscriber in a node.
I saw in this link and tested that python subscribers are event-driven model and are in separate threads.
https://robotics.stackexchange.com/qu...

My question here is , are they thread safe when they are working on the same class member? e.g. self.a below. Do I need to write a lock? Where can I find more information about this?
Thank you in advanced!!!!

class mr_listener(object):

    def __init__(self):
        self.a =  [1,2,3,4,5]
        rospy.init_node('mr_listener', anonymous=True)
        scan_subscriber = message_filters.Subscriber('/scan',LaserScan)
        pose_subscriber = message_filters.Subscriber('/slam_out_pose', PoseStamped)
        ts = message_filters.ApproximateTimeSynchronizer([scan_subscriber,projected_scan_subscriber,pose_subscriber], 10, 0.1, allow_headerless=True)
        ts.registerCallback(self.callback1)
        rospy.Subscriber("chatter", String, self.callback2)
        rospy.spin()

    def callback1(self,scan,pose):
        somthing

    def callback2(self,chatt):
        somingthing
        rospy.sleep(1.)