Popen([...], stderr=PIPE) ignores input() message from spawned python program

Question

I have one file running the other through the Popen().

# a.py
import subprocess

p = subprocess.Popen(['python3','/home/scotty/b.py'], 
stderr=subprocess.PIPE)
p.wait()

# b.py
input('???')

When I run a.py "???" doesn't appear but the prompt still works... why? And how can I fix it?

If I remove stderr=subprocess.PIPE then "???" does show up.

According to the docs the output of input "is written to standard output" and I'm not touching standard output.

Answer 1

It seems that in your exemple, everything happens as if opening a pipe for stderr but not for stdout has the effect of redirecting stdout to stderr because in the tests that I made, stderr receives the input's prompt.

Below is a code that works for me in linux. It is probably overcomplicated because I use socketpairs, which is most certainly not necessary, but at least it is robust and it works.

##################################
# a.py

import subprocess
import socket
import select

esock, echildsock = socket.socketpair()
osock, ochildsock = socket.socketpair()
p = subprocess.Popen(['python3','b.py'], 
    stderr=echildsock.fileno(),
    stdout=ochildsock.fileno())

while p.poll() is None:
    r, w, x = select.select([esock, osock],[],[], 1.0)
    if not r:
        continue # timed out
    for s in r:
        print('stdout ready' if s is osock else 'stderr ready')
        data = s.recv(1024)
        print('received', data.decode('utf8'))
osock.shutdown(socket.SHUT_RDWR)
osock.close()
esock.shutdown(socket.SHUT_RDWR)
esock.close()

##################################
# b.py

res = input('???')
print('in b:', res)