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

        @param reactor: A reactor which will notify this L{Port} when
            its socket is ready for reading or writing. Defaults to
            L{None}, ie the default global reactor.
        @type reactor: L{interfaces.IReactorFDSet}

        @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.
        @type fd: L{int}

        @param addressFamily: The address family (sometimes called I{domain}) of
            the existing socket.  For example, L{socket.AF_INET}.
        @param addressFamily: L{int}

        @param protocol: A C{DatagramProtocol} instance which will be
            connected to the C{port}.
        @type proto: L{twisted.internet.protocol.DatagramProtocol}

        @param maxPacketSize: The maximum packet size to accept.
        @type maxPacketSize: L{int}

        @return: A new instance of C{cls} wrapping the socket given by C{fd}.
        @rtype: L{Port}
        """
        port = socket.fromfd(fd, addressFamily, cls.socketType)
        interface = port.getsockname()[0]
        self = cls(None, protocol, interface=interface, reactor=reactor,
                   maxPacketSize=maxPacketSize)
        self._preexistingSocket = port
        return self

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

        @param reactor: A reactor which will notify this L{Port} when
            its socket is ready for reading or writing. Defaults to
            L{None}, ie the default global reactor.
        @type reactor: L{interfaces.IReactorFDSet}

        @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.
        @type fd: L{int}

        @param addressFamily: The address family (sometimes called I{domain}) of
            the existing socket.  For example, L{socket.AF_INET}.
        @param addressFamily: L{int}

        @param protocol: A C{DatagramProtocol} instance which will be
            connected to the C{port}.
        @type proto: L{twisted.internet.protocol.DatagramProtocol}

        @param maxPacketSize: The maximum packet size to accept.
        @type maxPacketSize: L{int}

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