If you start Python by pythonw then sys.stdout and sys.stderr are set to None. If you also use multiprocessing then when the child process finishes BaseProcess._bootstrap calls sys.stdout.flush() and sys.stderr.flush() finally. This causes the process return code to be not zero (it is 1).

This looks to be a duplicate of https://bugs.python.org/issue28326

Unfortunately this is NOT a duplicate of https://bugs.python.org/issue28326. That issue is about a closed output stream. In that case sys.stdout and sys.stderr are file like objects which have been closed.

This issue is about sys.stdout and sys.stderr being None! This is because pythonw was used not python.

@Pox, nevertheless, the fix committed in https://github.com/python/cpython/pull/4073 should also fix this issue. Do you disagree?

Pox, please retest with either 3.6.4 or 3.7.0a3 (or .0a4 when released, soon).  Both were released after the merge that should fix this.

Retested it with a freshly installed 3.6.4 version. Used the following code to test:

import sys
import multiprocessing


def foo():
    return 'bar'


if __name__ == '__main__':
    proc = multiprocessing.Process(target=foo)
    proc.start()
    proc.join()
    with open('process_exit_code.txt', 'w') as f:
        f.write(sys.version)
        f.write('\nprocess exit code: ')
        f.write(str(proc.exitcode))

It is very important to run the script with pythonw, not just with python. This is the content of the resulting process_exit_code.txt file on my machine:
3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)]
process exit code: 1

As it can be seen the problem was not fixed. The process exit code should be 0. By default the new multiprocessing process created uses the same interpreter as the creator process, so it uses pythonw too.

I'm not using Windows, so I'm unable to test using pythonw.  You'll have to provide a fix, or someone else will have to.

I have already uploaded a patch file but it is not in the required format. Also I realize that most of the confusion was because I forgot to provide the OS version. Perhaps it would be good to have a separate field for that.

I will upload a patch as it is described in the developer guide.

New changeset e756f66c83786ee82f5f7d45931ae50a6931dd7f by Antoine Pitrou in branch 'master':
bpo-31804: Fix multiprocessing.Process with broken standard streams (#6079)
https://github.com/python/cpython/commit/e756f66c83786ee82f5f7d45931ae50a6931dd7f

New changeset ff5d21331ec6cefec6ba5b78d256d8dbcd67a069 by Antoine Pitrou (Miss Islington (bot)) in branch '3.7':
bpo-31804: Fix multiprocessing.Process with broken standard streams (GH-6079) (GH-6080)
https://github.com/python/cpython/commit/ff5d21331ec6cefec6ba5b78d256d8dbcd67a069

New changeset 069b8d20be8018fbd49ed5aaf64c4caba311e48f by Antoine Pitrou in branch '3.6':
[3.6] bpo-31804: Fix multiprocessing.Process with broken standard streams (GH-6079) (GH-6081)
https://github.com/python/cpython/commit/069b8d20be8018fbd49ed5aaf64c4caba311e48f

This is all fixed in the Python 3 branches now.  I won't bother with Python 2 as it has quite a different code structure and backporting would take too much of my time.

Pox TheGreat, thanks for reporting and the initial patch!