def write(self, datagram, addr=None):
        """
        Write a datagram.

        @param addr: should be a tuple (ip, port), can be None in connected
        mode.
        """
        if self._connectedAddr:
            assert addr in (None, self._connectedAddr)
            try:
                return self.socket.send(datagram)
            except socket.error as se:
                no = se.args[0]
                if no == errno.WSAEINTR:
                    return self.write(datagram)
                elif no == errno.WSAEMSGSIZE:
                    raise error.MessageLengthError("message too long")
                elif no in (errno.WSAECONNREFUSED, errno.WSAECONNRESET,
                            ERROR_CONNECTION_REFUSED, ERROR_PORT_UNREACHABLE):
                    self.protocol.connectionRefused()
                else:
                    raise
        else:
            assert addr != None
            if (not isIPAddress(addr[0]) and not isIPv6Address(addr[0])
                    and addr[0] != "<broadcast>"):
                raise error.InvalidAddressError(
                    addr[0],
                    "write() only accepts IP addresses, not hostnames")
            if isIPAddress(addr[0]) and self.addressFamily == socket.AF_INET6:
                raise error.InvalidAddressError(
                    addr[0], "IPv6 port write() called with IPv4 address")
            if isIPv6Address(addr[0]) and self.addressFamily == socket.AF_INET:
                raise error.InvalidAddressError(
                    addr[0], "IPv4 port write() called with IPv6 address")
            try:
                return self.socket.sendto(datagram, addr)
            except socket.error as se:
                no = se.args[0]
                if no == errno.WSAEINTR:
                    return self.write(datagram, addr)
                elif no == errno.WSAEMSGSIZE:
                    raise error.MessageLengthError("message too long")
                elif no in (errno.WSAECONNREFUSED, errno.WSAECONNRESET,
                            ERROR_CONNECTION_REFUSED, ERROR_PORT_UNREACHABLE):
                    # in non-connected UDP ECONNREFUSED is platform dependent,
                    # I think and the info is not necessarily useful.
                    # Nevertheless maybe we should call connectionRefused? XXX
                    return
                else:
                    raise

        Write a datagram.

        @param addr: should be a tuple (ip, port), can be None in connected
        mode.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/iocpreactor/udp.py