The ever useful and neat subprocess module· 12 mins · 2,400 words · #python #python-subprocess #tutorial · An article by Shrikant Sharat Kandula
Python’s subprocess module is one of my favourite modules in the standard library. If you have
ever done some decent amount of coding in python, you might have encountered it. This module is used
for dealing with external commands, intended to be a replacement to the old
and the like.
The most trivial use might be to get the output of a small shell command like
ps. Not that
this is the best way to get a list of files in a directory (think
os.listdir), but you
get the point.
I am going to put my notes and experiences about this module here. Please note, I wrote this with Python 2.7 in mind. Things are slightly different in other versions (even 2.6). If you find any errors or suggestions, please let me know.
A simple usage
For the sake of providing context, lets run the
ls command from subprocess and get its output
import subprocess ls_output = subprocess.check_output(['ls'])
I’ll cover getting output from a command in detail later. To give more command line arguments,
The first item in the list is the executable and rest are its command line arguments (
equivalent). No quirky shell quoting and complex nested quote rules to digest. Just a plain python
However, not having shell quoting implies you don’t also have the shell niceties. Like piping for one. The following won’t work the way one would expect it to.
subprocess.check_output(['ls', '|', 'wc', '-l'])
ls command gets its first command as
| and I have no idea what ls would do with it.
Perhaps complain that no such file exists. So, instead, we have to use the
shell boolean argument.
More later down in the article.
If there’s just one thing in the subprocess module that you should be concerned with, its the
Popen class. The other functions like
Popen internally. Here’s the signature from the docs.
class subprocess.Popen(args, bufsize=0, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=False, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0)
I suggest you read the docs for this class. As with all python docs, its really good.
Running via the shell
Subprocess can also run command-line instructions via a shell program. This is usually
on Linux and
cmd on windows.
subprocess.call('ls | wc -l', shell=True)
Notice that in this case we pass a string, not a list. This is because we want the shell to interpret the whole of our command. You can even use shell style quoting if you like. It is up to the shell to decide how to best split the command line into executable and command line arguments.
On windows, if you pass a list for args, it will be turned into a string using the same rules as the MS C runtime. See the doc-string for
subprocess.list2cmdlinefor more on this. Whereas on unix-like systems, even if you pass a string, its turned into a list of one item :).
The behaviour of the
shell argument can sometimes be confusing so I’ll try to clear it a bit here.
Something I wished I had when I first encountered this module.
Firstly, lets consider the case where
shell is set to
False, the default. In this case, if
args is a string, it is assumed to be the name of the executable file. Even if it contains spaces.
Consider the following.
This won’t work because subprocess is looking for an executable file called
ls -l, but obviously
can’t find it. However, if
args is a list, then the first item in this list is considered as the
executable and the rest of the items in the list are passed as command line arguments to the
does what you think it will.
Second case, with
shell set to
True, the program that actually gets executed is the OS default
/bin/sh on Linux and
cmd.exe on windows. This can be changed with the
When using the shell,
args is usually a string, something that will be parsed by the shell
args string is passed as a command line argument to the shell (with a
-c option on
Linux) such that the shell will interpret it as a shell command sequence and process it accordingly.
This means you can use all the shell builtins and goodies that your shell offers.
subprocess.call('ls -l', shell=True)
is similar to
$ /bin/sh -c 'ls -l'
In the same vein, if you pass a list as
shell set to
True, all items in the list are
passed as command line arguments to the shell.
subprocess.call(['ls', '-l'], shell=True)
is similar to
$ /bin/sh -c ls -l
which is the same as
$ /bin/sh -c ls
/bin/sh takes just the argument next to
-c as the command line to execute.
Getting the return code (aka exit status)
If you want to run an external command and its return code is all you’re concerned with, the
check_call functions are what you’re looking for. They both
return the return code after running the command. The difference is,
check_call raises a
CalledProcessError if the return code is non-zero.
If you’ve read the docs for these functions, you’ll see that its not recommended to use
stderr=PIPE. And if you don’t, the
stderr of the command are just
redirected to the parent’s (Python VM in this case) streams.
If that is not what you want, you have to use the
proc = Popen('ls')
The moment the
Popen class is instantiated, the command starts running. You can wait for it and
after its done, access the return code via the
proc.wait() print proc.returncode
If you are trying this out in a python REPL, you won’t see a need to call
you can just wait yourself in the REPL till the command is finished and then access the
>>> proc = Popen('ls') >>> file1 file2 >>> print proc.returncode None >>> # wat?
The command is definitely finished. Why don’t we have a return code?
>>> proc.wait() 0 >>> print proc.returncode 0
The reason for this is the
returncode is not automatically set when a process ends. You have to
.poll to realize if the program is done and set the
The simplest way to get the output of a command, as seen previously, is to use the
output = subprocess.check_output('ls')
check_ prefix in the function name? Ring any bell? That’s right, this function will
CalledProcessError if the return code is non-zero.
This may not always be the best solution to get the output from a command. If you do get a
CalledProcessError from this function call, unless you have the contents of
stderr you probably
have little idea what went wrong. You’ll want to know what’s written to the command’s
Reading error stream
There are two ways to get the error output. First is redirecting
stdout and only being
stdout. This can be done by setting the
stderr argument to
Second is to create a
Popen object with
stderr set to
stdout argument) and read from its
stderr attribute which is a readable file-like
object. There is also a convenience method on
Popen class, called
.communicate, which optionally
takes a string to be sent to the process’s
stdin and returns a tuple of
However, all of these assume that the command runs for some time, prints out a couple of lines of output and exits, so you can get the output(s) in strings. This is sometimes not the case. If you want to run a network intensive command like an svn checkout, which prints each file as and when downloaded, you need something better.
The initial solution one can think of is this.
proc = Popen('svn co svn+ssh://myrepo', stdout=PIPE) for line in proc.stdout: print line
This works, for the most part. But, again, if there is an error, you’ll want to read
It would be nice to read
stderr simultaneously. Just like a shell seems to be doing.
Alas, this remains a not so straightforward problem as of today, at least on non-Linux systems.
On Linux (and where its supported), you can use the
select module to keep an eye on
multiple file-like stream objects. But this isn’t available on windows. A more platform independent
solution that I found works well, is using threads and a
from subprocess import Popen, PIPE from threading import Thread from Queue import Queue, Empty io_q = Queue() def stream_watcher(identifier, stream): for line in stream: io_q.put((identifier, line)) if not stream.closed: stream.close() proc = Popen('svn co svn+ssh://myrepo', stdout=PIPE, stderr=PIPE) Thread(target=stream_watcher, name='stdout-watcher', args=('STDOUT', proc.stdout)).start() Thread(target=stream_watcher, name='stderr-watcher', args=('STDERR', proc.stderr)).start() def printer(): while True: try: # Block for 1 second. item = io_q.get(True, 1) except Empty: # No output in either streams for a second. Are we done? if proc.poll() is not None: break else: identifier, line = item print identifier + ':', line Thread(target=printer, name='printer').start()
Fair bit of code. This is a typical producer-consumer thing. Two threads producing lines of output
(one each from
stderr) and pushing them into a queue. One thread watching the queue
and printing the lines until the process itself finishes.
Passing an environment
env argument to
Popen (and others) lets you customize the environment of the command being
run. If it is not set, or is set to
None, the current process’s environment is used, just as
You might not agree with me, but I feel there are some subtleties with this argument that should have been mentioned in the documentation.
Merge with current environment
One is that if you provide a mapping to
env, whatever is in this mapping is all that’s available
to the command being run. For example, if you don’t give a
TOP_ARG in the
env mapping, the
command won’t see a
TOP_ARG in its environment. So, I frequently find myself doing this
p = Popen('command', env=dict(os.environ, my_env_prop='value'))
This makes sense once you realize it, but I wish it were at least hinted at in the documentation.
Another one, is to do with Unicode (Surprise surprise!). And windows. If you use
unicodes in the
env mapping, you get an error saying you can only use strings in the environment mapping. The
worst part about this error is that it only seems to happen on windows and not on Linux. If its an
error to use
unicodes in this place, I wish it break on both platforms.
This issue is very painful if you’re like me and use
unicode all the time.
from __future__ import unicode_literals
That line is present in all my python source files. The error message doesn’t even bother to mention
that you have
unicodes in your
env so it’s very hard to understand what’s going wrong.
Execute in a different working directory
This is handled by the
cwd argument. You set the location of the directory which you want as the
working directory of the program you are launching.
The docs do mention that the working directory is changed before the command even starts running.
But that you can’t specify program’s path relative to the
cwd. In reality, I found that you
can do this.
Either I’m missing something with this or the docs really are inaccurate. Anyway, this works
Prints out all the files in
/bin. Of course, the following doesn’t work when the working directory
So, if you are giving something explicitly to
cwd and are using a relative path for the
executable, this is something to keep in mind.
Killing and dieing
or for some dramatic umphh!
will do the trick to end the process. As noted in the documentation, the former sends a
and later sends a
SIGKILL on unix, but both do some native windows-y thing on windows.
Auto-kill on death
The processes you start in your python program, stay running even after your program exits. This is
usually what you want, but when you want all your sub processes killed automatically on exit with
Ctrl+C or the like, you have to use the
procs =  @atexit.register def kill_subprocesses(): for proc in procs: proc.kill()
And add all the
Popen objects created to the
procs list. This is the only solution I found that
Launch commands in a terminal emulator
On one occasion, I had to write a script that would launch multiple svn checkouts and then run many ant builds (~20-35) on the checked out projects. In my opinion, the best and easiest way to do this is to fire up multiple terminal emulator windows each running an individual checkout/ant-build. This allows us to monitor each process and even cancel any of them by simply closing the corresponding terminal emulator window.
This is pretty trivial actually. On Linux, you can use
xterm for this.
Popen(['xterm', '-e', 'sleep 3s'])
On windows, its not as straight forward. The first solution for this would be
Popen(['cmd', '/K', 'command'])
cmdto run the command and keep the command window from closing. You may use
/Cinstead to close the command window after the command finishes.
As simple as it looks, it has some weird behavior. I don’t completely understand it, but I’ll try to
explain what I have. When you try to run a python script with the above
Popen call, in a command
window like this
you don’t see a new command window pop up. Instead, the sub command runs in the same command window. I have no idea what happens when you run multiple sub commands this way. (I have only limited access to windows).
If instead you run it in something like an IDE or IDLE (F5), you have a new command window open up. I believe one each for each command you run this way. Just the way you expect.
But I gave up on
cmd.exe for this purpose and learnt to use the
mintty utility that
comes with cygwin (I think 1.7+).
mintty is awesome. Really. Its been a while since I felt
that way about a command line utility on windows.
Popen(['mintty', '--hold', 'error', '--exec', 'command'])
This. A new
mintty console window opens up running the command and it closes automatically, if
the command exits with zero status (that’s what
--hold error does). Otherwise, it stays on. Very
The subprocess module is a very useful thing. Spend some time understanding it better. This is my attempt at helping people with it, and turned out to be way longer than I’d expected. If there are any inaccuracies in this, or if you have anything to add, please leave a comment.