Subprocess.Popen: cloning stdout and stderr both to terminal and variables

popen, python, python-3.x, subprocess

Solution

You could spawn threads to read the stdout and stderr pipes, write to a common queue, and append to lists. Then use a third thread to print items from the queue.

import time
import Queue
import sys
import threading
import subprocess
PIPE = subprocess.PIPE


def read_output(pipe, funcs):
    for line in iter(pipe.readline, ''):
        for func in funcs:
            func(line)
            # time.sleep(1)
    pipe.close()

def write_output(get):
    for line in iter(get, None):
        sys.stdout.write(line)

process = subprocess.Popen(
    ['random_print.py'], stdout=PIPE, stderr=PIPE, close_fds=True, bufsize=1)
q = Queue.Queue()
out, err = [], []
tout = threading.Thread(
    target=read_output, args=(process.stdout, [q.put, out.append]))
terr = threading.Thread(
    target=read_output, args=(process.stderr, [q.put, err.append]))
twrite = threading.Thread(target=write_output, args=(q.get,))
for t in (tout, terr, twrite):
    t.daemon = True
    t.start()
process.wait()
for t in (tout, terr):
    t.join()
q.put(None)
print(out)
print(err)

The reason for using the third thread -- instead of letting the first two threads both print directly to the terminal -- is to prevent both print statements from occurring concurrently, which can result in sometimes garbled text.

The above calls `random_print.py`, which prints to stdout and stderr at random:

import sys
import time
import random

for i in range(50):
    f = random.choice([sys.stdout,sys.stderr])
    f.write(str(i)+'\n')
    f.flush()
    time.sleep(0.1)

This solution borrows code and ideas from J. F. Sebastian, here.

Here is an alternative solution for Unix-like systems, using `select.select`:

import collections
import select
import fcntl
import os
import time
import Queue
import sys
import threading
import subprocess
PIPE = subprocess.PIPE

def make_async(fd):
    # https://stackoverflow.com/a/7730201/190597
    '''add the O_NONBLOCK flag to a file descriptor'''
    fcntl.fcntl(
        fd, fcntl.F_SETFL, fcntl.fcntl(fd, fcntl.F_GETFL) | os.O_NONBLOCK)

def read_async(fd):
    # https://stackoverflow.com/a/7730201/190597
    '''read some data from a file descriptor, ignoring EAGAIN errors'''
    # time.sleep(1)
    try:
        return fd.read()
    except IOError, e:
        if e.errno != errno.EAGAIN:
            raise e
        else:
            return ''

def write_output(fds, outmap):
    for fd in fds:
        line = read_async(fd)
        sys.stdout.write(line)
        outmap[fd.fileno()].append(line)

process = subprocess.Popen(
    ['random_print.py'], stdout=PIPE, stderr=PIPE, close_fds=True)

make_async(process.stdout)
make_async(process.stderr)
outmap = collections.defaultdict(list)
while True:
    rlist, wlist, xlist = select.select([process.stdout, process.stderr], [], [])
    write_output(rlist, outmap)
    if process.poll() is not None:
        write_output([process.stdout, process.stderr], outmap)
        break

fileno = {'stdout': process.stdout.fileno(),
          'stderr': process.stderr.fileno()}

print(outmap[fileno['stdout']])
print(outmap[fileno['stderr']])

This solution uses code and ideas from Adam Rosenfield's post, here.

Problem

Original source

Related problems

• live output from subprocess command
• Displaying subprocess output to stdout and redirecting it
• Wrap subprocess' stdout/stderr
• Stop reading process output in Python without hang?
• How can I print and display subprocess stdout and stderr output without distortion?