reps = 500
stdout = open('add_numbers_outputs.log', 'w')
stderr = open('add_numbers_errors.log', 'w')
external_command = './add_numbers'
parameter = str(1000000)
call_arguments = (
    """[
        '%s',
        '%s'], 
        stdout=stdout,
        stderr=stderr
"""
     % (external_command, parameter))
get how much time python subprocess spends