Record .bag File via Python

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Hey guys,

I'm facing a tricky problem. I want to write incoming data (live) in a .bag file. I'm doing that this way (not via command-rosbag record), because I want to change these data a bit. The problem is: If I am writing 2 different topics in the .bag file, there are errors while replaying, e.g. parsing header error, chunk error etc. Thats just the case when writing 2 topics in the bag. I have implemented it the same way as mentioned in the API. Heres my code:

bag = rosbag.Bag('/home/bryan/Schreibtisch/ros_workspace/src/bag_recorder/scripts/test.bag','w')

def recorder():

    rospy.init_node('bag_recorder')    
    sub = rospy.Subscriber('/imu/data', Imu, sub_imu)
    sub1 = rospy.Subscriber('/gps_ublox/fix', NavSatFix, sub_gps)

    rospy.spin()

    bag.close()

def sub_imu(data):  
    bag.write('/imu/data',data)

def sub_gps(data):
    bag.write('/gps_ublox/fix',data)

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

Do anybody know how to solve the problem? Recording both Sensors (around 50Hz and 10 Hz) with the command rosbag record works fine, but this way I need to postprocess the data via a python-script. The Errors occur at different time each time I want to record the signals. I also tried to kill this node properly via roskill or in python signal_shutdown but that won't effect the problem.

Maybe someone has an Idea.

Thank you.