How to detect when subprocess asks for input in Windows

Question

I have a subprocess that either quits with a returncode, or asks something and waits for user input.

I would like to detect when the process asks the question and quit i

Answer 1

My idea to find out if the subprocess reads user input is to (ab)use the fact that file objects are stateful: if the process reads data from its stdin, we should be able to detect a change in the stdin's state.

The procedure is as follows:

1. Create a temporary file that'll be used as the subprocess's stdin
2. Write some data to the file
3. Start the process
4. Wait a little while for the process to read the data (or not), then use the tell() method to find out if anything has been read from the file

This is the code:

import os
import time
import tempfile
import subprocess

# create a file that we can use as the stdin for the subprocess
with tempfile.TemporaryFile() as proc_stdin:
    # write some data to the file for the subprocess to read
    proc_stdin.write(b'whatever\r\n')
    proc_stdin.seek(0)

    # start the thing
    cmd = ["python","ask.py"]
    proc = subprocess.Popen(cmd, stdin=proc_stdin, stdout=subprocess.PIPE)

    # wait for it to start up and do its thing
    time.sleep(1)

    # now check if the subprocess read any data from the file
    if proc_stdin.tell() == 0:
        print("it didn't take input")
    else:
        print("it took input")

Ideally the temporary file could be replaced by some kind of pipe or something that doesn't write any data to disk, but unfortunately I couldn't find a way to make it work without a real on-disk file.