How can I run an external command asynchronously from Python?

subprocess.Popen does exactly what you want.

from subprocess import Popen
p = Popen(['watch', 'ls']) # something long running
# ... do other stuff while subprocess is running
p.terminate()

(Edit to complete the answer from comments)

The Popen instance can do various other things like you can poll() it to see if it is still running, and you can communicate() with it to send it data on stdin, and wait for it to terminate.

If you want to run many processes in parallel and then handle them when they yield results, you can use polling like in the following:

from subprocess import Popen, PIPE
import time

running_procs = [
    Popen(['/usr/bin/my_cmd', '-i %s' % path], stdout=PIPE, stderr=PIPE)
    for path in '/tmp/file0 /tmp/file1 /tmp/file2'.split()]

while running_procs:
    for proc in running_procs:
        retcode = proc.poll()
        if retcode is not None: # Process finished.
            running_procs.remove(proc)
            break
        else: # No process is done, wait a bit and check again.
            time.sleep(.1)
            continue

    # Here, `proc` has finished with return code `retcode`
    if retcode != 0:
        """Error handling."""
    handle_results(proc.stdout)

The control flow there is a little bit convoluted because I'm trying to make it small -- you can refactor to your taste. :-)

This has the advantage of servicing the early-finishing requests first. If you call communicate on the first running process and that turns out to run the longest, the other running processes will have been sitting there idle when you could have been handling their results.

What I am wondering is if this [os.system()] is the proper way to accomplish such a thing?

No. os.system() is not the proper way. That's why everyone says to use subprocess.

For more information, read http://docs.python.org/library/os.html#os.system

The subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function. Use the subprocess module. Check especially the Replacing Older Functions with the subprocess Module section.

The accepted answer is very old.

I found a better modern answer here:

https://kevinmccarthy.org/2016/07/25/streaming-subprocess-stdin-and-stdout-with-asyncio-in-python/

and made some changes:

1. make it work on windows
2. make it work with multiple commands

import sys
import asyncio

if sys.platform == "win32":
    asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())


async def _read_stream(stream, cb):
    while True:
        line = await stream.readline()
        if line:
            cb(line)
        else:
            break


async def _stream_subprocess(cmd, stdout_cb, stderr_cb):
    try:
        process = await asyncio.create_subprocess_exec(
            *cmd, stdout=asyncio.subprocess.PIPE, stderr=asyncio.subprocess.PIPE
        )

        await asyncio.wait(
            [
                _read_stream(process.stdout, stdout_cb),
                _read_stream(process.stderr, stderr_cb),
            ]
        )
        rc = await process.wait()
        return process.pid, rc
    except OSError as e:
        # the program will hang if we let any exception propagate
        return e


def execute(*aws):
    """ run the given coroutines in an asyncio loop
    returns a list containing the values returned from each coroutine.
    """
    loop = asyncio.get_event_loop()
    rc = loop.run_until_complete(asyncio.gather(*aws))
    loop.close()
    return rc


def printer(label):
    def pr(*args, **kw):
        print(label, *args, **kw)

    return pr


def name_it(start=0, template="s{}"):
    """a simple generator for task names
    """
    while True:
        yield template.format(start)
        start += 1


def runners(cmds):
    """
    cmds is a list of commands to excecute as subprocesses
    each item is a list appropriate for use by subprocess.call
    """
    next_name = name_it().__next__
    for cmd in cmds:
        name = next_name()
        out = printer(f"{name}.stdout")
        err = printer(f"{name}.stderr")
        yield _stream_subprocess(cmd, out, err)


if __name__ == "__main__":
    cmds = (
        [
            "sh",
            "-c",
            """echo "$SHELL"-stdout && sleep 1 && echo stderr 1>&2 && sleep 1 && echo done""",
        ],
        [
            "bash",
            "-c",
            "echo 'hello, Dave.' && sleep 1 && echo dave_err 1>&2 && sleep 1 && echo done",
        ],
        [sys.executable, "-c", 'print("hello from python");import sys;sys.exit(2)'],
    )

    print(execute(*runners(cmds)))

It is unlikely that the example commands will work perfectly on your system, and it doesn't handle weird errors, but this code does demonstrate one way to run multiple subprocesses using asyncio and stream the output.