Adding timeout in subprocess.check_output

Question

I am developing a small tool in Python 2.7 and using subprocess module. I am using this module to run commands on remote devices using its check_output function. There might be a situation in which the remote device is not functioning and therefore I am getting the following response: Timeout: No Response from 10.xxx.xxx.xxx Following is my code:

try:
    x=subprocess.check_output(command, shell=True)
except Exception:
    print ("Some issues in fetching details")
    exit()
else:
    print (x)

I want to put timeout into this function so that if after a certain amount of time, no response is recieved, my code goes in the Exception part and prints the given message. I tried using timeout argument in the check_output command but after running my script with timeout argument, it is immediately printing the message given in the Exception part. What I tried:

try:
    x=subprocess.check_output(command, shell=True, timeout=5)
except Exception:
    print ("Some issues in fetching details")
    exit()
else:
    print (x)

Answer 1

My guess is that you are running your code in Python 2.

If that is the case, subprocess.check_output() does not accept a timeout parameter, and the function will fail immediately with:

TypeError: __init__() got an unexpected keyword argument 'timeout'

But, because you are catching all exceptions and printing a generic message, you don't see the actual exception, and you assume that the command is timing out immediately.

One way to fix this problem is to run your code in Python 3.

Whether you are running Python 2 or 3, I recommend that you do not catch all exceptions, or that you at least print the value of the exception so that you can see the actual cause, e.g.

try:
    x=subprocess.check_output(command, shell=True, timeout=5)
except subprocess.TimeoutExpired as exc:
    print("Command timed out: {}".format(exc))
    exit()
else:
    print (x)

which explicitly checks for a timeout exception. All other exceptions are propagated as usual and so are not masked by your "catch all" code. Or,

try:
    x=subprocess.check_output(command, shell=True, timeout=5)
except Exception as exc:
    print("Command failed: {}".format(exc))
    exit()
else:
    print (x)

but the former is preferred.

Edit

OP can't use Python 3. If you are using Linux then you could use the timeout command, e.g.

x = subprocess.check_output('timeout 5 {}'.format(command), shell=True)

On timeout this will raise an exception with a particular exit status value of 124:

subprocess.CalledProcessError: Command 'timeout 5 sleep 10' returned non-zero exit status 124

BTW you shouldn't use the shell=True option as there are security implications as mentioned in the documentation. Instead you should pass a list of strings to check_output() like this:

from shlex import shlex

command = shlex('timeout 5 {}'.format(command))
try:
    x = subprocess.check_output(command)
except subprocess.CalledProcessError as exc:
    if exc.returncode == 124:
        print "Command timed out"
    else:
        raise

If you are using another OS (or you don't want to use timeout) then you can run your subprocess in a separate thread and have your main thread time it out if required. See this other question, Using module 'subprocess' with timeout, for details about how to do that.

Answer 2

Python 2.7 does not support timeout parameter. You can instead use EasyProcess. This is a layer on top of subprocess module and pretty easy to use.