live output from subprocess command

Question

I\'m using a python script as a driver for a hydrodynamics code. When it comes time to run the simulation, I use subprocess.Popen to run the code, collect the

Answer 1

Solution 1: Log stdout AND stderr concurrently in realtime

A simple solution which logs both stdout AND stderr concurrently, line-by-line in realtime into a log file.

import subprocess as sp
from concurrent.futures import ThreadPoolExecutor


def log_popen_pipe(p, stdfile):

    with open("mylog.txt", "w") as f:

        while p.poll() is None:
            f.write(stdfile.readline())
            f.flush()

        # Write the rest from the buffer
        f.write(stdfile.read())


with sp.Popen(["ls"], stdout=sp.PIPE, stderr=sp.PIPE, text=True) as p:

    with ThreadPoolExecutor(2) as pool:
        r1 = pool.submit(log_popen_pipe, p, p.stdout)
        r2 = pool.submit(log_popen_pipe, p, p.stderr)
        r1.result()
        r2.result()

Solution 2: A function read_popen_pipes() that allows you to iterate over both pipes (stdout/stderr), concurrently in realtime

import subprocess as sp
from queue import Queue, Empty
from concurrent.futures import ThreadPoolExecutor


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


def read_popen_pipes(p):

    with ThreadPoolExecutor(2) as pool:
        q_stdout, q_stderr = Queue(), Queue()

        pool.submit(enqueue_output, p.stdout, q_stdout)
        pool.submit(enqueue_output, p.stderr, q_stderr)

        while True:

            if p.poll() is not None and q_stdout.empty() and q_stderr.empty():
                break

            out_line = err_line = ''

            try:
                out_line = q_stdout.get_nowait()
                err_line = q_stderr.get_nowait()
            except Empty:
                pass

            yield (out_line, err_line)

# The function in use:

with sp.Popen(["ls"], stdout=sp.PIPE, stderr=sp.PIPE, text=True) as p:

    for out_line, err_line in read_popen_pipes(p):
        print(out_line, end='')
        print(err_line, end='')

    p.poll()