Here's something that you should remember about using pipes and fork(2) in Python 3: By default, O_CLOEXEC is passed to the pipe(2) system from the CPython runtime.

This means, that reading the read end of the side in the parent process after you forked will not work. Thefore you should explicitly change fctl flags and remove os.O_CLOEXEC:

  fcntl.fcntl(readfd, fcntl.F_SETFL, fcntl.fcntl(readfd, fcntl.F_GETFL) & ~os.O_CLOEXEC)