how to capture output of external process in python

Question

I am trying to run an external program (in this case just python -V) and capture the standard error in the memory.

It works if I redirect to disk:

import sys, os
import subprocess
import tempfile
err = tempfile.mkstemp()
print err[1]
p = subprocess.call([sys.executable, '-V'], stderr=err[0] )

but that's not fun. Then I'd need to read that file into memory.

I thought I can create something in-memory that would act like a file using StringIO but this attempt failed:

import sys, os
import subprocess
import tempfile
import StringIO

err = StringIO.StringIO()
p = subprocess.call([sys.executable, '-V'], stderr=err )

I got:

AttributeError: StringIO instance has no attribute 'fileno'

ps. Once this works I'll want to capture stdout as well, but I guess that's the same. ps2. I tried the above on Windows and Python 2.7.3

mgilson · Accepted Answer · 2013-01-14 16:33:47Z

2

You need to set stderr = subprocess.PIPE

e.g.:

p = subprocess.Popen(...,stderr = subprocess.PIPE)
stdout,stderr = p.communicate()
#p.stderr.read() could work too.

note, for this to work, you need access to the Popen object, so you can't really use subprocess.call here (you really need subprocess.Popen).

answered Jan 14, 2013 at 16:33

mgilson

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5 Comments

szabgab Over a year ago

The documentation recommends against that: "Do not use stdout=PIPE or stderr=PIPE with this function. " or is that different than subprocess.PIPE that you suggest ?

mgilson Over a year ago

@szabgab -- Sorry, I must have been editing while you were commenting. You can't use PIPE with subprocess.call (or at least you shouldn't). You can use it with subprocess.Popen as I have demonstrated in my answer.

szabgab Over a year ago

Indeed, my previous comment was made when only the first line of the answer was seen. subprocess.Popen worked nicely. Thanks

mgilson Over a year ago

@szabgab -- Sorry. I suppose I should have taken my time a little more to write a bit more to the answer before hitting the original submit :)

szabgab Over a year ago

@mgilson no problem, I just wanted to clarify my comment in a comment in a comment ... :)

c-urchin · Accepted Answer · 2013-01-15 18:24:50Z

0

Use subprocess.check_output. From the docs

subprocess.check_output(args, *, stdin=None, stderr=None, shell=False, universal_newlines=False)
Run command with arguments and return its output as a byte string.

answered Jan 15, 2013 at 18:24

c-urchin

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