capture_tty.py

import os
import pty
import select
import subprocess


# Example:
# tty_capture(["python", "test.py"], b"abc\n")
# (b'stdin: True, stdout: True, stderr: True\r\n',
#  b'read from stdin: abc\r\n')

def tty_capture(cmd, bytes_input):
    """Capture the output of cmd with bytes_input to stdin,
    with stdin, stdout and stderr as TTYs."""
    mo, so = pty.openpty()  # provide tty to enable line-buffering
    me, se = pty.openpty()  
    mi, si = pty.openpty()  
    fdmap = {mo: 'stdout', me: 'stderr', mi: 'stdin'}

    p = subprocess.Popen(
        cmd,
        bufsize=1, stdin=si, stdout=so, stderr=se, 
        close_fds=True)
    os.write(mi, bytes_input)

    timeout = .04  # seconds
    res = {'stdout': b'', 'stderr': b''}
    while True:
        ready, _, _ = select.select([mo, me], [], [], timeout)
        if ready:
            for fd in ready:
                data = os.read(fd, 512)
                if not data:
                    break
                res[fdmap[fd]] += data
        elif p.poll() is not None:  # select timed-out
            break  # p exited
    for fd in [si, so, se, mi, mo, me]:
        os.close(fd)  # can't do it sooner: it leads to errno.EIO error
    p.wait()
    return res['stdout'], res['stderr']

test.py

import sys

print("stdin: {}, stdout: {}, stderr: {}"
      .format(sys.stdin.isatty(), sys.stdout.isatty(), sys.stderr.isatty()))
line = sys.stdin.readline()
sys.stderr.write("read from stdin: {}".format(line))
sys.stderr.flush()