handling exit status popen python

Question

I am tring to handle status exit with popen but it gives a error, the code is:

import os
try:
    res = os.popen("ping -c 4 www.google.com")
except IOError:
    print "ISPerror: popen"
try:
    #wait = [0,0]
    wait = os.wait()
except IOError:
    print "ISPerror:os.wait"

if wait[1] != 0:
    print("  os.wait:exit status != 0\n")
else:
    print ("os.wait:"+str(wait))
print("before read")
result = res.read()

print ("after read:")

print ("exiting")

But it if giving the following error:

close failed in file object destructor: IOError: [Errno 10] No child processes

Use `subprocess.Popen()` instead: https://docs.python.org/2/library/subprocess.html#replacing-os-popen-os-popen2-os-popen3 — Jonathon Reinhart, Jul 31 '16 at 22:56

score 3 · Answer 1 · edited May 23 '17 at 12:14

Error Explanation

It looks like this error is occurring because upon exiting, the program tries to destroy res, which involves calling the res.close() method. But somehow invoking os.wait() has already closed the object. So it's trying to close res twice, resulting in the error. If the call to os.wait() is removed, the error no longer occurs.

import os
try:
    res = os.popen("ping -c 4 www.google.com")
except IOError:
    print "ISPerror: popen"

print("before read")
result = res.read()
res.close() # explicitly close the object
print ("after read: {}".format(result)

print ("exiting")

But this leaves you with the problem of knowing when the process has finished. And since res just has type file, your options are limited. I would instead move to using subprocess.Popen

Using subprocess.Popen

To use subprocess.Popen, you pass your command in as a list of strings. To be able to access the output of the process, you set the stdout argument to subprocess.PIPE, which allows you to access stdout later on using file operations. Then, instead of using the regular os.wait() method, subprocess.Popen objects have their own wait methods you call directly on the object, this also sets the returncode value which represents the exit status.

import os
import subprocess

# list of strings representing the command
args = ['ping', '-c', '4', 'www.google.com']

try:
    # stdout = subprocess.PIPE lets you redirect the output
    res = subprocess.Popen(args, stdout=subprocess.PIPE)
except OSError:
    print "error: popen"
    exit(-1) # if the subprocess call failed, there's not much point in continuing

res.wait() # wait for process to finish; this also sets the returncode variable inside 'res'
if res.returncode != 0:
    print("  os.wait:exit status != 0\n")
else:
    print ("os.wait:({},{})".format(res.pid, res.returncode)

# access the output from stdout
result = res.stdout.read()
print ("after read: {}".format(result))

print ("exiting")

handling exit status popen python

1 Answers1

Error Explanation

Using subprocess.Popen

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