Non Blocking Subprocess.Call

Blocking and Non Blocking subprocess calls

Popen is nonblocking. call and check_call are blocking.
You can make the Popen instance block by calling its wait or communicate method.

If you look in the source code, you'll see call calls Popen(...).wait(), which is why it is blocking.
check_call calls call, which is why it blocks as well.

Strictly speaking, shell=True is orthogonal to the issue of blocking. However, shell=True causes Python to exec a shell and then run the command in the shell. If you use a blocking call, the call will return when the shell finishes. Since the shell may spawn a subprocess to run the command, the shell may finish before the spawned subprocess. For example,

import subprocess
import time

proc = subprocess.Popen('ls -lRa /', shell=True)
time.sleep(3)
proc.terminate()
proc.wait()

Here two processes are spawned: Popen spawns one subprocess running the shell. The shell in turn spawns a subprocess running ls. proc.terminate() kills the shell, but the subprocess running ls remains. (That is manifested by copious output, even after the python script has ended. Be prepared to kill the ls with pkill ls.)

A non-blocking read on a subprocess.PIPE in Python

fcntl, select, asyncproc won't help in this case.

A reliable way to read a stream without blocking regardless of operating system is to use Queue.get_nowait():

import sys
from subprocess import PIPE, Popen
from threading  import Thread

try:
    from queue import Queue, Empty
except ImportError:
    from Queue import Queue, Empty  # python 2.x

ON_POSIX = 'posix' in sys.builtin_module_names

def enqueue_output(out, queue):
    for line in iter(out.readline, b''):
        queue.put(line)
    out.close()

p = Popen(['myprogram.exe'], stdout=PIPE, bufsize=1, close_fds=ON_POSIX)
q = Queue()
t = Thread(target=enqueue_output, args=(p.stdout, q))
t.daemon = True # thread dies with the program
t.start()

# ... do other things here

# read line without blocking
try:  line = q.get_nowait() # or q.get(timeout=.1)
except Empty:
    print('no output yet')
else: # got line
    # ... do something with line

Python subprocess multiple non blocking communicates

I now switched from using subprocess to using pexpect.
My syntax is now as follows:

child = pexpect.spawn('rosrun ros_pkg ros_node')
command = child.sendline('new command')
output = child.read_nonblocking(10000, timeout=1)
....
logic
....
command = child.sendline('new command')
output = child.read_nonblocking(10000, timeout=1)

Many thanks to novel_yet_trivial on reddit: https://www.reddit.com/r/learnpython/comments/2o2viz/subprocess_popen_multiple_times/

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