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

Answer 1

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).