dup2 recover original filedescriptor

Question

I am trying to clone the stdout of a process into my program with dup2, and it works, except when I duplicate stdin I can not get it back to its original state again.

My code is as follows.

from os import fork, wait, pipe, execvp, dup2, close
from sys import stdin

def get_input(data):
    r,w = pipe()
    pid = fork()
    old = 0
    if pid > 0:
        wait()
        close(w)
        old = dup2(r,0)
        for line in stdin:
            print('data - ', line.strip())

    else:
        close(r)
        dup2(w,1)
        execvp(data.split()[0],data.split())

while True:
    get_input(input())

I get and EOFError on the second iteration in the while loop because the stdin still is the pipe.

I tried closing the pipe channel and I tried "rebuilding" the filedescriptor by getting the old_fd but it doesnt change that I get the error.

Answer 1

So after a while i realized that there was no reason to duplicate the file descriptor to stdin, because I don't need to pass any data out of the process.

So I could just use the systemcall read which will read the byes of the file descriptor given to it.

Fist I imported read from os:

from os import read

and then I deleted everything in the parent process and added the following:

chunk = ''
while (tmp := read(r,1024).decode()) != "":
    chunk += tmp
close(r)

And now I have all the output of stdout captured as a string inside my python program.