os.popen on windows: loosing stdout of child process


G

Greg Ercolano

When I use os.popen(cmd,'w'), I find that under windows, the stdout
of the child process disappears, instead of appearing in the DOS window
the script is invoked from. eg:

C:\> type foo.py
import os
import sys
file = os.popen("nslookup", 'w')
file.write("google.com\n")
file.close()

C:\> python foo.py
<-- nothing is printed
C:\>

This just seems wrong. The following DOS equivalent works fine:

C:\> echo google.com | nslookup
Default Server: dns.erco.x
Address: 192.168.1.14
[..expected output..]

When I run the same python program on a unix box, the output
from 'nslookup' appears in the terminal, as I'd expect.

Shouldn't popen() be consistent in its handling of the child's
stdout and stderr across platforms?

Maybe I'm missing something, being somewhat new to python, but
an old hand at unix and win32 and functions like popen(). Didn't
see anything in the docs for popen(), and I googled around quite
a bit on the web and groups for eg. 'python windows popen stdout lost'
and found nothing useful.

FWIW, I'm using the windows version of python 2.5 from activestate.
 
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G

Gabriel Genellina

When I use os.popen(cmd,'w'), I find that under windows, the stdout
of the child process disappears, instead of appearing in the DOS window
the script is invoked from. eg: [...]
When I run the same python program on a unix box, the output
from 'nslookup' appears in the terminal, as I'd expect.

Shouldn't popen() be consistent in its handling of the child's
stdout and stderr across platforms?

Maybe I'm missing something, being somewhat new to python, but
an old hand at unix and win32 and functions like popen(). Didn't
see anything in the docs for popen(), and I googled around quite
a bit on the web and groups for eg. 'python windows popen stdout lost'
and found nothing useful.

Using the subprocess module is the recommended approach (as you can see on
the os.popen documentation) and does what you want:

C:\TEMP>type foo2.py
import subprocess
p = subprocess.Popen("nslookup", stdin=subprocess.PIPE)
p.stdin.write("google.com\n")
p.stdin.close()

C:\TEMP>python foo2.py

C:\TEMP>Servidor predeterminado: coyote.softlabbsas.com.ar
Address: 192.168.0.116
Servidor: coyote.softlabbsas.com.ar
Address: 192.168.0.116

Respuesta no autoritativa:
Nombre: google.com
Addresses: 64.233.187.99, 64.233.167.99, 72.14.207.99
C:\TEMP>

For more info about subprocess usage, see
http://docs.python.org/lib/module-subprocess.html
 
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H

half.italian

When I use os.popen(cmd,'w'), I find that under windows, the stdout
of the child process disappears, instead of appearing in the DOS window
the script is invoked from. eg:

C:\> type foo.py
import os
import sys
file = os.popen("nslookup", 'w')
file.write("google.com\n")
file.close()

C:\> python foo.py
<-- nothing is printed
C:\>

This just seems wrong. The following DOS equivalent works fine:

C:\> echo google.com | nslookup
Default Server: dns.erco.x
Address: 192.168.1.14
[..expected output..]

When I run the same python program on a unix box, the output
from 'nslookup' appears in the terminal, as I'd expect.

Shouldn't popen() be consistent in its handling of the child's
stdout and stderr across platforms?

Maybe I'm missing something, being somewhat new to python, but
an old hand at unix and win32 and functions like popen(). Didn't
see anything in the docs for popen(), and I googled around quite
a bit on the web and groups for eg. 'python windows popen stdout lost'
and found nothing useful.

FWIW, I'm using the windows version of python 2.5 from activestate.

Glad to see you're finally coming into the light Greg! I've used Rush
in a few different studios over the past couple of years. We even had
sushi once. :)

I'm no expert like you, but I think I can point you in the right
direction. You need os.popen2 which returns a tuple of file-like
objects. The first pointing to stdin, and the second pointing to
stdout. Write to stdin, and read from stdout.

import os
import sys
stdin, stdout = os.popen2("nslookup")
stdin.write("google.com\n")
stdin.close()

print stdout.read()
stdout.close()

I don't use windows much, but I believe the os.popen functionality is
being replaced by subprocess.Popen:

from subprocess import *
import sys

p = Popen("nslookup", shell=True, bufsize=1024, stdin=PIPE,
stdout=PIPE, close_fds=True)
p.stdin.write("google.com\n")
p.stdin.close()

print p.stdout.read()
p.stdout.close()

I found these:
http://pydoc.org/2.4.1/subprocess.html
http://docs.python.org/lib/module-subprocess.html

~Sean DiZazzo
Curious George
 

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