using rosbag in callback

edit

For my project I am interested in many frames of reference contained in a list called frame_list, each with Pose messages published to their own topic e.g. 'frame_transform'. I am trying to subscribe to all of these topics and use rosbag to save the Poses as they are published. The code below creates the .bag file, but does not write any data into it even though messages are being published to the topics. Is this a good way to achieve what I want? Can anybody spot where I am going wrong? Thanks.

#!/usr/bin/env python  
import roslib
import rospy
import rosbag
from geometry_msgs.msg import Pose

def callback(data, args):

    bag = rosbag.Bag('data.bag', 'w')
    bag.write(str(args), data)

if __name__ == '__main__':

    if rospy.has_param('init_complete'): #check if initialisation is complete
        rospy.init_node('rosbag_writer') #initiate node
        frame_list = rospy.get_param('frame_list') #import list of frames from rosparam
        for frame in frame_list:
            rospy.Subscriber(str(frame) + '_transform', Pose, callback, frame) #check for frame transforms
        rospy.spin()