What is a practical difference between check_call check_output call, and Popen methods in the subprocess module?

Question

Honestly, I just don\'t understand the lingo of \"non-zero\" status to really interpret what\'s going on or what that means (it wasn\'t even defined) on help pages. What are som

Answer 1

To expand on @otorrillas's answer with some examples.

Say you have a short script that you only care whether it succeeds or not*, and you just want to do it now and wait for the result. Say, for example, you want to send a single ping to a remote machine. call is the one for you:

res = call(['ping', '127.0.0.1', '-c', '1', '-W', '1'])
# res is 0 if the ping succeeded, non-zero if it failed. 

Now say that you have a command you want to assume is going to succeed, and is going to give you some output you want to use for something. check_output is for you. Perhaps a subversion command:

svn_output = check_output(['svn', 'info', path_to_my_working_copy])
# svn_output now contains the svn info output. If it failed, an 
# exception is thrown which more easily allows you to give 
# responsibility of that failure to whoever passed us the wrong 
# path_to_my_working_copy.

In general, if your use case does not simply fall under one of those categories, you are probably going to wind up using Popen.

One simple use case might be, say you have a daemon process that you want to start, but then run alongside of your python process. Now you use Popen:

my_proc = Popen(['my-daemon'])

# We're going to go and do something else now, but want to make sure
# my_proc dies when we do.
import atexit
atexit.register(my_proc.kill)

NB: If you use Popen raw, you must make sure you terminate the process, possibly using an atexit as I have illustrated.

* "non-zero" exit status just means the process failed. A famous computer science quote attributed to Tolstoy is "Happy processes are all alike; every unhappy process is unhappy in its own way", i.e. there's only one way for a process to be happy: to return 0. Everything else is unhappy, but there are lots of ways to be unhappy.

Answer 2

The main difference is that, while popen is a non-blocking function (meaning you can continue the execution of the program without waiting the call to finish), both call and check_output are blocking.

The other difference is in what they return:

• popen returns a Popen object.
• call returns the returncode attribute.
• check_output returns the output of the command execution.

The methods call and check_output are, in fact, blocking wrappers of popen, using a Popen object. For example, you can get the returncode attribute by calling Popen.returncode().