如何从Python异步运行外部命令？

Question

我需要从Python脚本异步运行shell命令。这意味着我希望我的Python脚本在外部命令发生时继续运行并执行它需要做的任何事情。

我读过这篇文章：

  

在Python中调用外部命令

然后我去做了一些测试，看起来像os.system()将完成工作，前提是我在命令末尾使用&，这样我就不必等待它返回了。我想知道的是，这是否是实现这一目标的正确方法？我试过commands.call()但它对我不起作用，因为它会阻止外部命令。

如果建议使用<=>，或者我应该尝试其他路线，请告诉我。

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

I've had good success with the asyncproc module, which deals nicely with the output from the processes. For example:

import os
from asynproc import Process
myProc = Process("myprogram.app")

while True:
    # check to see if process has ended
    poll = myProc.wait(os.WNOHANG)
    if poll is not None:
        break
    # print any new output
    out = myProc.read()
    if out != "":
        print out

Answer 5

Using pexpect [ http://www.noah.org/wiki/Pexpect ] with non-blocking readlines is another way to do this. Pexpect solves the deadlock problems, allows you to easily run the processes in the background, and gives easy ways to have callbacks when your process spits out predefined strings, and generally makes interacting with the process much easier.

Answer 6

I have the same problem trying to connect to an 3270 terminal using the s3270 scripting software in Python. Now I'm solving the problem with an subclass of Process that I found here:

http://code.activestate.com/recipes/440554/

And here is the sample taken from file:

def recv_some(p, t=.1, e=1, tr=5, stderr=0):
    if tr < 1:
        tr = 1
    x = time.time()+t
    y = []
    r = ''
    pr = p.recv
    if stderr:
        pr = p.recv_err
    while time.time() < x or r:
        r = pr()
        if r is None:
            if e:
                raise Exception(message)
            else:
                break
        elif r:
            y.append(r)
        else:
            time.sleep(max((x-time.time())/tr, 0))
    return ''.join(y)

def send_all(p, data):
    while len(data):
        sent = p.send(data)
        if sent is None:
            raise Exception(message)
        data = buffer(data, sent)

if __name__ == '__main__':
    if sys.platform == 'win32':
        shell, commands, tail = ('cmd', ('dir /w', 'echo HELLO WORLD'), '\r\n')
    else:
        shell, commands, tail = ('sh', ('ls', 'echo HELLO WORLD'), '\n')

    a = Popen(shell, stdin=PIPE, stdout=PIPE)
    print recv_some(a),
    for cmd in commands:
        send_all(a, cmd + tail)
        print recv_some(a),
    send_all(a, 'exit' + tail)
    print recv_some(a, e=0)
    a.wait()

Answer 7

Considering "I don't have to wait for it to return", one of the easiest solutions will be this:

subprocess.Popen( \
    [path_to_executable, arg1, arg2, ... argN],
    creationflags = subprocess.CREATE_NEW_CONSOLE,
).pid

But... From what I read this is not "the proper way to accomplish such a thing" because of security risks created by subprocess.CREATE_NEW_CONSOLE flag.

The key things that happen here is use of subprocess.CREATE_NEW_CONSOLE to create new console and .pid (returns process ID so that you could check program later on if you want to) so that not to wait for program to finish its job.