Python standard output and subprocesses

Question

I've got two files:

run.py

import subprocess
import time

while True:
  time.sleep(1)
  print 'hello'
  proc = subprocess.call(['./writer.sh'])

writer.sh (chmod 777'd)

#!/bin/sh
echo 'write something here'

and I'm confused by the following outputs:

$ python run.py
hello
write something here
hello
write something here
hello
write something here
....

$ python run.py | tee out.log
write something here
write something here
(hello disappears)

....

$ python run.py > out.log
# Nothing, but out.log has the following:
write something here
write something here
write something here
write something here
hello
hello
hello
hello
... # and the two basically "expand" the longer I run this (instead of appending)

What is happening, and how can I get everything to output like the first command?

zhangyangyu · Accepted Answer · 2013-08-06 02:26:02Z

2

The output of your main script is buffered. Call sys.stdout.flush() right before running the subprocess.

edited Aug 6, 2013 at 2:26

zhangyangyu

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answered Aug 6, 2013 at 2:23

tdelaney

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1 Comment

theicfire Over a year ago

Ah, and both tee and redirecting to a file make the buffer not line-by-line, whereas just running to a tty flushes on every line. Thanks!

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