Stop reading process output in Python without hang?
I have a Python program for Linux almost looks like this one :
import os
import time
process = os.popen("top").readlines()
time.sleep(1)
os.popen("killall top")
print process
the program hangs in this line :
process = os.popen("top").readlines()
and that happens in the tools that keep update outputting like "Top"
my best trials :
import os
import time
import subprocess
process = subprocess.Popen('top')
time.sleep(2)
os.popen("killall top")
print process
it worked better than the first one (it's kelled ), but it returns :
<subprocess.Popen object at 0x97a50cc>
the second trial :
import os
import time
import subprocess
process = subprocess.Popen('top').readlines()
time.sleep(2)
os.popen("killall top")
print process
the same as the first one. It hanged due to "readlines()"
Its returning should be like this :
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
and save in the variable "process". Any I idea guys, I'm really stuck now ?
Answer
#!/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()
Tail-like Solutions that print only the portion of the output
You could read the process output in another thread and save the required number of the last lines in a queue:
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()
This variant requires q.append() to be atomic operation. Otherwise the output might be corrupted.
signal.alarm() solution
You could use signal.alarm() to call the process.terminate() after specified timeout instead of reading in another thread. Though it might not interact very well with the subprocess module. Based on @Alex Martelli's answer:
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()
This approach works only on *nix systems. It might block if process.stdout.readline() doesn't return.
threading.Timer solution
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()
This approach should also work on Windows. Here I've used process.stdout as an iterable; it might introduce an additional output buffering, you could switch to the iter(process.stdout.readline, "") approach if it is not desirable. if the process doesn't terminate on process.terminate() then the scripts hangs.
No threads, no signals solution
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()
This variant use neither threads, no signals but it produces garbled output in the terminal. It will block if process.stdout.readline() blocks.