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Python subprocess.Popen and asynchronous output

I have simple Python script to execute test suite both under Windows and Linux. Every test writes its output to separate file. I use subprocess.Popen class to execute shell command in a cycle.

Every shell command starts like that:

def system_execute(self, command, path, out_file):
    params_list = command.split(' ') 
    file_path = os.path.join(path, out_file)
    f = open(file_path, "w")
    subprocess.Popen(params_list, stdout=f)
    f.close()

It works fine, but script finishes its work before all output files have been written. Actually, I getting hundreds of zero-size files, and it takes some time to finish writing output and close handles. Could anyone explain the reason why it works so strange, and is there a synchronous way to do the same job?

Thanks

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Python subprocess.Popen and asynchronous output

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