def _fromListeningDescriptor(cls, reactor, fd, factory):
        """
        Create a new L{Port} based on an existing listening I{SOCK_STREAM}
        socket.

        Arguments are the same as to L{Port.__init__}, except where noted.

        @param fd: An integer file descriptor associated with a listening
            socket.  The socket must be in non-blocking mode.  Any additional
            attributes desired, such as I{FD_CLOEXEC}, must also be set already.

        @return: A new instance of C{cls} wrapping the socket given by C{fd}.
        """
        port = socket.fromfd(fd, cls.addressFamily, cls.socketType)
        self = cls(port.getsockname(), factory, reactor=reactor)
        self._preexistingSocket = port
        return self

        Create a new L{Port} based on an existing listening I{SOCK_STREAM}
        socket.

        Arguments are the same as to L{Port.__init__}, except where noted.

        @param fd: An integer file descriptor associated with a listening
            socket.  The socket must be in non-blocking mode.  Any additional
            attributes desired, such as I{FD_CLOEXEC}, must also be set already.

        @return: A new instance of C{cls} wrapping the socket given by C{fd}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/unix.py