Redirecting stdout to multiprocessing.Pipe throws error

Question

I would like to redirect stdout and stderr of a new process back to the parent process via pipe.

I have found this code in another thread , which uses the os.dup2() function to duplicate stdout .

import os
from multiprocessing import Process, Pipe

def spam(w):
    os.dup2(w.fileno(), 1)
    for i in range(3):
        print('eggs')


if __name__ == '__main__':
    r, w = Pipe()
    reader = os.fdopen(r.fileno(), 'r')
    p = Process(target=spam, args=(w,))
    p.start()

    for i in range(3):
        print('From pipe: %s' % reader.readline())

    reader.close()
    p.join()

With Python 2.7 everything works fine, but Python 3 gives me a invalid file descriptor error after the main process has read the messages from the pipe. I cannot figure out why this happens. I tried closing the file handler explicitly, but it did not work.

(I'm running this on a Linux machine.)

Answer 1

So, as it turns out not closing reader works fine.

import os
from multiprocessing import Process, Pipe


def spam(w):
    os.dup2(w.fileno(), 1)
    for i in range(3):
        print('eggs')


if __name__ == '__main__':
    r, w = Pipe()
    reader = os.fdopen(r.fileno(), 'r')
    p = Process(target=spam, args=(w,))
    p.start()
    for i in range(3):
        print('From pipe: %s' % reader.readline())
    p.join()