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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def decode_body(cls, header, f):
 """Generates a `MqttSubscribe` packet given a `MqttFixedHeader`. This method asserts that header.packet_type is `subscribe`. Parameters header: MqttFixedHeader f: file Object with a read method. Raises ------ DecodeError When there are extra bytes at the end of the packet. Returns ------- int Number of bytes consumed from ``f``. MqttSubscribe Object extracted from ``f``. """

        assert header.packet_type == MqttControlPacketType.subscribe

        decoder = mqtt_io.FileDecoder(mqtt_io.LimitReader(f, header.remaining_len))
        packet_id, = decoder.unpack(mqtt_io.FIELD_PACKET_ID)

        topics = []
        while header.remaining_len > decoder.num_bytes_consumed:
            num_str_bytes, name = decoder.unpack_utf8()
            max_qos, = decoder.unpack(mqtt_io.FIELD_U8)
            try:
                sub_topic = MqttTopic(name, max_qos)
            except ValueError:
                raise DecodeError('Invalid QOS {}'.format(max_qos))
            topics.append(sub_topic)

        assert header.remaining_len == decoder.num_bytes_consumed

        return decoder.num_bytes_consumed, MqttSubscribe(packet_id, topics)