multiprocessing: How can I ʀᴇʟɪᴀʙʟʏ redirect stdout from a child process?

The solution you suggest is a good one: create your processes manually such that you have explicit access to their stdout/stderr file handles. You can then create a socket to communicate with the sub-process and use multiprocessing.connection over that socket (multiprocessing.Pipe creates the same type of connection object, so this should give you all the same IPC functionality).

Here’s a two-file example.

master.py:

import multiprocessing.connection
import subprocess
import socket
import sys, os

## Listen for connection from remote process (and find free port number)
port = 10000
while True:
    try:
        l = multiprocessing.connection.Listener(('localhost', int(port)), authkey="secret")
        break
    except socket.error as ex:
        if ex.errno != 98:
            raise
        port += 1  ## if errno==98, then port is not available.

proc = subprocess.Popen((sys.executable, "subproc.py", str(port)), stdout=subprocess.PIPE, stderr=subprocess.PIPE)

## open connection for remote process
conn = l.accept()
conn.send([1, "asd", None])
print(proc.stdout.readline())

subproc.py:

import multiprocessing.connection
import subprocess
import sys, os, time

port = int(sys.argv[1])
conn = multiprocessing.connection.Client(('localhost', port), authkey="secret")

while True:
    try:
        obj = conn.recv()
        print("received: %s\n" % str(obj))
        sys.stdout.flush()
    except EOFError:  ## connection closed
        break

You may also want to see the first answer to this question to get non-blocking reads from the subprocess.