Python subprocessのstdoutとstderrの両方をリアルタイムで取得

test_stream.py

#!/usr/bin/python3

import sys
import time

print("start")

i = 1
while True:
    if i % 5 == 0:
        print(i, flush=True, file=sys.stderr)
    else:
        print(i, flush=True)
    i += 1
    time.sleep(1.0)

test_stream_client.py

#!/usr/bin/python3
import subprocess
import sys
from subprocess import PIPE
from threading import Thread


def read_stream(in_file, out_file):
    for line in in_file:
        print(line.strip(), file=out_file, flush=True)

p = subprocess.Popen("./test_stream.py", stdin=sys.stdin, stdout=PIPE, stderr=PIPE, universal_newlines=True)
Thread(target=read_stream, args=(p.stdout, sys.stdout)).start()
Thread(target=read_stream, args=(p.stderr, sys.stderr)).start()

# p = subprocess.Popen("./test_stream.py", stdin=sys.stdin, stdout=PIPE, stderr=subprocess.STDOUT, universal_newlines=True)
# Thread(target=read_stream, args=(p.stdout, sys.stdout)).start()