CODEWITHSUNDEEP
pythonstdoutstderr
szabgab
szabgab

Reputation: 6302

how to capture output of external process in python

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

Upvotes: 0

Views: 2100

Answers (2)

c-urchin
c-urchin

Reputation: 4504

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.

Upvotes: 0

mgilson
mgilson

Reputation: 309889

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).

Upvotes: 2

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