停止读取Python中没有挂起的进程输出？

Question

我有一个用于Linux的Python程序，几乎像这样：

import os
import time

process = os.popen("top").readlines()

time.sleep(1)

os.popen("killall top")

print process

程序挂在此行：

process = os.popen("top").readlines()

而这种情况发生在保持更新输出的工具中，例如“ Top”

我最好的尝试：

import os
import time
import subprocess

process = subprocess.Popen('top')

time.sleep(2)

os.popen("killall top")

print process

它比第一个更好（它已经发了声），但是返回了：

<subprocess.Popen object at 0x97a50cc>

第二次审判：

import os
import time
import subprocess

process = subprocess.Popen('top').readlines()

time.sleep(2)

os.popen("killall top")

print process

与第一个相同。由于“ readlines（）”而挂起

它的返回应该是这样的：

top - 05:31:15 up 12:12,  5 users,  load average: 0.25, 0.14, 0.11
Tasks: 174 total,   2 running, 172 sleeping,   0 stopped,   0 zombie
Cpu(s):  9.3%us,  3.8%sy,  0.1%ni, 85.9%id,  0.9%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   1992828k total,  1849456k used,   143372k free,   233048k buffers
Swap:  4602876k total,        0k used,  4602876k free,  1122780k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND            
31735 Barakat   20   0  246m  52m  20m S 19.4  2.7  13:54.91 totem              
 1907 root      20   0 91264  45m  15m S  1.9  2.3  38:54.14 Xorg               
 2138 Barakat   20   0 17356 5368 4284 S  1.9  0.3   3:00.15 at-spi-registry    
 2164 Barakat    9 -11  164m 7372 6252 S  1.9  0.4   2:54.58 pulseaudio         
 2394 Barakat   20   0 27212 9792 8256 S  1.9  0.5   6:01.48 multiload-apple    
 6498 Barakat   20   0 56364  30m  18m S  1.9  1.6   0:03.38 pyshell            
    1 root      20   0  2880 1416 1208 S  0.0  0.1   0:02.02 init               
    2 root      20   0     0    0    0 S  0.0  0.0   0:00.02 kthreadd           
    3 root      RT   0     0    0    0 S  0.0  0.0   0:00.12 migration/0        
    4 root      20   0     0    0    0 S  0.0  0.0   0:02.07 ksoftirqd/0        
    5 root      RT   0     0    0    0 S  0.0  0.0   0:00.00 watchdog/0         
    9 root      20   0     0    0    0 S  0.0  0.0   0:01.43 events/0           
   11 root      20   0     0    0    0 S  0.0  0.0   0:00.00 cpuset             
   12 root      20   0     0    0    0 S  0.0  0.0   0:00.02 khelper            
   13 root      20   0     0    0    0 S  0.0  0.0   0:00.00 netns              
   14 root      20   0     0    0    0 S  0.0  0.0   0:00.00 async/mgr          
   15 root      20   0     0    0    0 S  0.0  0.0   0:00.00 pm

并保存在变量“ process”中。我知道吗，我现在真的很困吗？

Answer 1

#!/usr/bin/env python
"""Start process; wait 2 seconds; kill the process; print all process output."""
import subprocess
import tempfile
import time

def main():
    # open temporary file (it automatically deleted when it is closed)
    #  `Popen` requires `f.fileno()` so `SpooledTemporaryFile` adds nothing here
    f = tempfile.TemporaryFile() 

    # start process, redirect stdout
    p = subprocess.Popen(["top"], stdout=f)

    # wait 2 seconds
    time.sleep(2)

    # kill process
    #NOTE: if it doesn't kill the process then `p.wait()` blocks forever
    p.terminate() 
    p.wait() # wait for the process to terminate otherwise the output is garbled

    # print saved output
    f.seek(0) # rewind to the beginning of the file
    print f.read(), 
    f.close()

if __name__=="__main__":
    main()

类似于尾巴的解决方案，仅打印输出的一部分
你可以在另一个线程中读取过程输出，并将所需数量的最后几行保存在队列中：

import collections
import subprocess
import time
import threading

def read_output(process, append):
    for line in iter(process.stdout.readline, ""):
        append(line)

def main():
    # start process, redirect stdout
    process = subprocess.Popen(["top"], stdout=subprocess.PIPE, close_fds=True)
    try:
        # save last `number_of_lines` lines of the process output
        number_of_lines = 200
        q = collections.deque(maxlen=number_of_lines) # atomic .append()
        t = threading.Thread(target=read_output, args=(process, q.append))
        t.daemon = True
        t.start()

        #
        time.sleep(2)
    finally:
        process.terminate() #NOTE: it doesn't ensure the process termination

    # print saved lines
    print ''.join(q)

if __name__=="__main__":
    main()

此变体必须q.append()是原子操作。否则，输出可能会损坏。

signal.alarm() 解
你可以用来 在指定的超时后signal.alarm()调用，process.terminate()而不用读入另一个线程。尽管它可能无法与subprocess模块很好地交互。基于@Alex Martelli的答案：

import collections
import signal
import subprocess

class Alarm(Exception):
    pass

def alarm_handler(signum, frame):
    raise Alarm

def main():
    # start process, redirect stdout
    process = subprocess.Popen(["top"], stdout=subprocess.PIPE, close_fds=True)

    # set signal handler
    signal.signal(signal.SIGALRM, alarm_handler)
    signal.alarm(2) # produce SIGALRM in 2 seconds

    try:
        # save last `number_of_lines` lines of the process output
        number_of_lines = 200
        q = collections.deque(maxlen=number_of_lines)
        for line in iter(process.stdout.readline, ""):
            q.append(line)
        signal.alarm(0) # cancel alarm
    except Alarm:
        process.terminate()
    finally:
        # print saved lines
        print ''.join(q)

if __name__=="__main__":
    main()

该方法仅适用于* nix系统。如果process.stdout.readline()不返回，它可能会阻塞。

threading.Timer 解
import collections
import subprocess
import threading

def main():
    # start process, redirect stdout
    process = subprocess.Popen(["top"], stdout=subprocess.PIPE, close_fds=True)

    # terminate process in timeout seconds
    timeout = 2 # seconds
    timer = threading.Timer(timeout, process.terminate)
    timer.start()

    # save last `number_of_lines` lines of the process output
    number_of_lines = 200
    q = collections.deque(process.stdout, maxlen=number_of_lines)
    timer.cancel()

    # print saved lines
    print ''.join(q),

if __name__=="__main__":
    main()

这种方法在Windows上也应适用。在这里，我用作process.stdout迭代。它可能会引入额外的输出缓冲，iter(process.stdout.readline, “”)如果不希望的话，你可以切换到该方法。如果进程没有终止，process.terminate()则脚本挂起。

没有线程，没有信号解决方案

import collections
import subprocess
import sys
import time

def main():
    args = sys.argv[1:]
    if not args:
        args = ['top']

    # start process, redirect stdout
    process = subprocess.Popen(args, stdout=subprocess.PIPE, close_fds=True)

    # save last `number_of_lines` lines of the process output
    number_of_lines = 200
    q = collections.deque(maxlen=number_of_lines)

    timeout = 2 # seconds
    now = start = time.time()    
    while (now - start) < timeout:
        line = process.stdout.readline()
        if not line:
            break
        q.append(line)
        now = time.time()
    else: # on timeout
        process.terminate()

    # print saved lines
    print ''.join(q),

if __name__=="__main__":
    main()

此变型既不使用线程，也不使用信号，但会在终端中产生乱码输出。如果process.stdout.readline()阻塞，它将阻塞。