Why subprocess stdout to a file is written out of order?

Question

I have a python script that calls an executable. The executable's output is redirected to a log file along with some info about the time it was called. For example, using python -V as the executable to illustrate:

import time, subprocess
with open('./LOGFILE.txt', 'a') as F:
    F.write('******\n')
    F.write('Events on %s :\n'%time.strftime("%Y-%m-%d %H:%M:%S", time.localtime()))
    EXE_output = subprocess.call(['python', '-V'], stdout=F, stderr=F)

The output of the file LOGFILE.txt is:

Python 2.7.3
******
Events on 2013-04-10 19:27:25 :

Where I was expecting it as follows:

******
Events on 2013-04-10 19:27:25 :
Python 2.7.3

I wrote the ****** and time info in the opened log file before running the subprocess and redirecting its output and error into the file. Why is ordering like that? and how can I reorder?

Answer 1

You should call F.flush() before running the subprocess. The reason for this is that the subprocess will flush the buffers when it finishes, whereas you are not.