I can't clean inside my bag file with python

edit

In my code i am trying to clean my bag file in every 10 seconds and then trying to write again, but it doesn't work. I couldn't find any rosbag function to clean inside my bag file. I tried with python functions but it didn't work too.

Here is my code.

#!/usr/bin/env python3
# -*- coding: UTF-8 -*-

import sys
import rospy 
import rosbag
from geometry_msgs.msg import Twist
import time

current_time = time.time()
last_time=time.time()
filename='test2.bag'
print(current_time)
bag = rosbag.Bag(filename, 'w')
def move(msg):
    global last_time
    current_time = time.time()
    if current_time - last_time >10: 
        last_time=time.time()
        with open(filename, 'r+') as f:
            #f.read()
            f.truncate()
            print(f.encoding)
            #f.close()
        print("outside")
    else:

        bag.write('/cmd_vel', msg)  
        print("inside")



def main(args):
    rospy.init_node('bag', anonymous=True)
    rospy.Subscriber("/cmd_vel",Twist,move)
    rospy.spin()
    bag.close()
    print("Closing...")

if __name__ == '__main__':
  main(sys.argv)