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

io, nonblocking, python, subprocess

Solution

`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

Problem

I'm using the subprocess module to start a subprocess and connect to its output stream (standard output). I want to be able to execute non-blocking reads on its standard output. Is there a way to make .readline non-blocking or to check if there is data on the stream before I invoke `.readline`? I'd like this to be portable or at least work under Windows and Linux. Here is how I do it for now (it's blocking on the `.readline` if no data is available): ``` p = subprocess.Popen('myprogram.exe', stdout = subprocess.PIPE) output_str = p.stdout.readline() ```

Original source

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