Python's subprocess.Popen object hangs gathering child output when child process does not exit

Question

When a process exits abnormally or not at all, I still want to be able to gather what output it may have generated up until that point.

The obvious solution to this

Answer 1

I had the exact same problem. I ended up fixing the issue (after scouring Google and finding many related problems) by simply setting the following parameters when calling subprocess.Popen (or .call):

stdout=None

and

stderr=None

There are many problems with these functions but in my specific case I believe stdout was being filled up by the process I was calling and then resulting in a blocking condition. By setting these to None (opposed to something like subprocess.PIPE) I believe this is avoided.

Hope this helps someone.

Answer 2

There are good tips in another stackoverflow question: How do I get 'real-time' information back from a subprocess.Popen in python (2.5)

Most of the hints in there work with pipe.readline() instead of pipe.communicate() because the latter only returns at the end of the process.

Answer 3

Here's a POSIX way of doing it without the temporary file. I realize that subprocess is a little superfluous here, but since the original question used it...

import subprocess
import os
import time
import signal
import sys

pr, pw = os.pipe()
pid = os.fork () 

if pid: #parent
    os.close(pw)
    cmd = ["bash"]
    finish = time.time() + 3
    p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=pr, close_fds=True)
    while p.poll() is None:
        time.sleep(0.05)
        if finish < time.time():
            os.kill(p.pid, signal.SIGTERM)
            print "timed out and killed child, collecting what output exists so far"
            out, err = p.communicate()
            print "got it: ", out
            sys.exit(0)

else: #child
    os.close(pr)
    child_script = """
    #!/bin/bash
    while [ 1 ]; do
        ((++i))
        echo "output line $i"
        sleep 1
    done
    """
    os.write(pw, child_script)

Answer 4

Problem is that bash doesn't answer to CTRL-C when not connected with a terminal. Switching to SIGHUP or SIGTERM seems to do the trick:

cmd = ["bash", 'childProc.sh']
p = subprocess.Popen(cmd, stdout=subprocess.PIPE, 
                          stderr=subprocess.STDOUT, 
                          close_fds=True)
time.sleep(3)
print 'killing pid', p.pid
os.kill(p.pid, signal.SIGTERM)
print "timed out and killed child, collecting what output exists so far"
out  = p.communicate()[0]
print "got it", out

Outputs:

killing pid 5844
timed out and killed child, collecting what output exists so far
got it output line 0
output line 1
output line 2