Python: running subprocess in parallel [duplicate]

Answer 1

1. Yes, these md5sum processes will be started in parallel.
2. Yes, the order of md5sums writes will be unpredictable. And generally it is considered a bad practice to share a single resource like file from many processes this way.

Also your way of making p.wait() after the for loop will wait just for the last of md5sum processes to finish and the rest of them might still be running.

But you can modify this code slightly to still have benefits of parallel processing and predictability of synchronized output if you collect the md5sum output into temporary files and collect it back into one file once all processes are done.

import subprocess
import os

processes = []
for file in files_output:
    f = os.tmpfile()
    p = subprocess.Popen(['md5sum',file],stdout=f)
    processes.append((p, f))

for p, f in processes:
    p.wait()
    f.seek(0)
    logfile.write(f.read())
    f.close()