new.py

print("Gib number")
x = input()
print('numba is ', x)

launcher.py

import subprocess as sp

p = sp.Popen(['python', 'new.py'], stdout=sp.PIPE, stderr=sp.STDOUT, stdin=sp.PIPE, text=True)

while True:
    line = p.stdout.readline()
    print('line is', line)
    p.stdin.write('10')


p.wait()

Why can’t the launcher.py read from new.py‘s stdout on second iteration?

However, If I use pwntools to launch the same file, it succeeds, albeit with an error on sendline on second iteration, which is understandable because presumably the stdin of the new.py is closed by then.

_launcher.py

from pwn import process

p = process(['python', 'new.py'], stderr=process.PTY, stdout=process.PTY)

while True:
    output = p.recv()
    print('line is', output)
    p.sendline(10)  # Sends the answer with a newline


p.wait()

What mechanism does pwntools‘s recv and sendline use and how can I launch the new.py without having to use pwntools?

Please provide any relevant readings as well.