[Python] python subprocess dd and stdout

I am using subprocess to create a random file from /dev/random using unix dd. Now, if i want the data output of dd to be written to a file instead of stdout. so here's the code am using,

import subprocess
out_fd = open('test_file','w')
def os_system_dd():
global out_fd
out_fd.write("executing the time dd command\n")
cmd_list = ['time','dd','if=/dev/random', 'of=/home/anand/sys_entropy_random', 'bs=1M' ,'count=5']
a = subprocess.Popen(cmd_list,stdout=out_fd)
a.wait()

if __name__ == '__main__':
os_system_dd()


This doesn't print the dd output to the file and instead prints it in the stdout. Is this a specific functionality of dd command? Or am i missing some thing about how subprocess works?

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