def _setupChild(self, masterfd, slavefd):
        """
        Set up child process after C{fork()} but before C{exec()}.

        This involves:

            - closing C{masterfd}, since it is not used in the subprocess

            - creating a new session with C{os.setsid}

            - changing the controlling terminal of the process (and the new
              session) to point at C{slavefd}

            - duplicating C{slavefd} to standard input, output, and error

            - closing all other open file descriptors (according to
              L{_listOpenFDs})

            - re-setting all signal handlers to C{SIG_DFL}

        @param masterfd: The master end of a PTY file descriptors opened with
            C{openpty}.
        @type masterfd: L{int}

        @param slavefd: The slave end of a PTY opened with C{openpty}.
        @type slavefd: L{int}
        """
        os.close(masterfd)
        os.setsid()
        fcntl.ioctl(slavefd, termios.TIOCSCTTY, '')

        for fd in range(3):
            if fd != slavefd:
                os.close(fd)

        os.dup2(slavefd, 0) # stdin
        os.dup2(slavefd, 1) # stdout
        os.dup2(slavefd, 2) # stderr

        for fd in _listOpenFDs():
            if fd > 2:
                try:
                    os.close(fd)
                except:
                    pass

        self._resetSignalDisposition()

        Set up child process after C{fork()} but before C{exec()}.

        This involves:

            - closing C{masterfd}, since it is not used in the subprocess

            - creating a new session with C{os.setsid}

            - changing the controlling terminal of the process (and the new
              session) to point at C{slavefd}

            - duplicating C{slavefd} to standard input, output, and error

            - closing all other open file descriptors (according to
              L{_listOpenFDs})

            - re-setting all signal handlers to C{SIG_DFL}

        @param masterfd: The master end of a PTY file descriptors opened with
            C{openpty}.
        @type masterfd: L{int}

        @param slavefd: The slave end of a PTY opened with C{openpty}.
        @type slavefd: L{int}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/process.py