How to use the crossbar.bridge.mqtt._utils.build_string function in crossbar

# Client ID
        b.append(build_string(self.client_id))

        if self.flags.will:
            b.append(build_string(self.will_topic))

            # Will message is a uint16 prefixed bytestring
            b.append(pack('uint:16', len(self.will_message)).bytes)
            b.append(self.will_message)

        if self.flags.username:
            b.append(build_string(self.username))

        # Technically this should be binary data but we will only accept UTF-8
        if self.flags.password:
            b.append(build_string(self.password))

        return b"".join(b)

def serialise(self):
        """
        Assemble this into an on-wire message part.
        """
        b = []

        # Topic filter, as UTF-8
        b.append(build_string(self.topic_filter))

        # Reserved section + max QoS
        b.append(pack('uint:6, uint:2', 0, self.max_qos).bytes)

        return b"".join(b)

def _make_payload(self):
        """
        Build the payload from its constituent parts.
        """
        b = []

        # Topic Name
        b.append(build_string(self.topic_name))

        if self.packet_identifier:
            if self.qos_level > 0:
                # Session identifier
                b.append(pack('uint:16', self.packet_identifier).bytes)
            else:
                raise SerialisationFailure(self, "Packet Identifier on non-QoS 1/2 packet")
        else:
            if self.qos_level > 0:
                raise SerialisationFailure(self, "QoS level > 0 but no Packet Identifier")

        # Payload (bytes)
        b.append(self.payload)

        return b"".join(b)

b.append(build_string("MQTT"))

        # Protocol Level (4 == 3.1.1)
        b.append(pack('uint:8', 4).bytes)

        # CONNECT flags
        b.append(self.flags.serialise())

        # Keep Alive time
        b.append(pack('uint:16', self.keep_alive).bytes)

        # Client ID
        b.append(build_string(self.client_id))

        if self.flags.will:
            b.append(build_string(self.will_topic))

            # Will message is a uint16 prefixed bytestring
            b.append(pack('uint:16', len(self.will_message)).bytes)
            b.append(self.will_message)

        if self.flags.username:
            b.append(build_string(self.username))

        # Technically this should be binary data but we will only accept UTF-8
        if self.flags.password:
            b.append(build_string(self.password))

        return b"".join(b)

def _make_payload(self):
        """
        Build the payload from its constituent parts.
        """
        b = []

        # Protocol name (MQTT)
        b.append(build_string("MQTT"))

        # Protocol Level (4 == 3.1.1)
        b.append(pack('uint:8', 4).bytes)

        # CONNECT flags
        b.append(self.flags.serialise())

        # Keep Alive time
        b.append(pack('uint:16', self.keep_alive).bytes)

        # Client ID
        b.append(build_string(self.client_id))

        if self.flags.will:
            b.append(build_string(self.will_topic))

# Keep Alive time
        b.append(pack('uint:16', self.keep_alive).bytes)

        # Client ID
        b.append(build_string(self.client_id))

        if self.flags.will:
            b.append(build_string(self.will_topic))

            # Will message is a uint16 prefixed bytestring
            b.append(pack('uint:16', len(self.will_message)).bytes)
            b.append(self.will_message)

        if self.flags.username:
            b.append(build_string(self.username))

        # Technically this should be binary data but we will only accept UTF-8
        if self.flags.password:
            b.append(build_string(self.password))

        return b"".join(b)

b = []

        # Protocol name (MQTT)
        b.append(build_string("MQTT"))

        # Protocol Level (4 == 3.1.1)
        b.append(pack('uint:8', 4).bytes)

        # CONNECT flags
        b.append(self.flags.serialise())

        # Keep Alive time
        b.append(pack('uint:16', self.keep_alive).bytes)

        # Client ID
        b.append(build_string(self.client_id))

        if self.flags.will:
            b.append(build_string(self.will_topic))

            # Will message is a uint16 prefixed bytestring
            b.append(pack('uint:16', len(self.will_message)).bytes)
            b.append(self.will_message)

        if self.flags.username:
            b.append(build_string(self.username))

        # Technically this should be binary data but we will only accept UTF-8
        if self.flags.password:
            b.append(build_string(self.password))

        return b"".join(b)