Interactive input/output using Python

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None of the current answers worked for me. At the end, I’ve got this working:

import subprocess


def start(executable_file):
    return subprocess.Popen(
        executable_file,
        stdin=subprocess.PIPE,
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE)


def read(process):
    return process.stdout.readline().decode("utf-8").strip()


def write(process, message):
    process.stdin.write(f"{message.strip()}\n".encode("utf-8"))
    process.stdin.flush()


def terminate(process):
    process.stdin.close()
    process.terminate()
    process.wait(timeout=0.2)


process = start("./dummy.py")
write(process, "hello dummy")
print(read(process))
terminate(process)

Tested with this dummy.py script:

#!/usr/bin/env python3.6

import random
import time

while True:
    message = input()
    time.sleep(random.uniform(0.1, 1.0)) # simulates process time
    print(message[::-1])

The caveats are (all managed in the functions):

  • Input/output always lines with newline.
  • Flush child’s stdin after every write.
  • Use readline() from child’s stdout.

It’s a pretty simple solution in my opinion (not mine, I found it here: https://eli.thegreenplace.net/2017/interacting-with-a-long-running-child-process-in-python/). I was using Python 3.6.

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