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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 _send_offset_requests(self, timestamps):
 """Fetch offsets for each partition in timestamps dict. This may send request to multiple nodes, based on who is Leader for partition. Arguments: timestamps (dict):
 {TopicPartition: int} mapping of fetching timestamps. Returns: Future: resolves to a mapping of retrieved offsets """

        timestamps_by_node = collections.defaultdict(dict)
        for partition, timestamp in six.iteritems(timestamps):
            node_id = self._client.cluster.leader_for_partition(partition)
            if node_id is None:
                self._client.add_topic(partition.topic)
                log.debug("Partition %s is unknown for fetching offset,"
                          " wait for metadata refresh", partition)
                return Future().failure(Errors.StaleMetadata(partition))
            elif node_id == -1:
                log.debug("Leader for partition %s unavailable for fetching "
                          "offset, wait for metadata refresh", partition)
                return Future().failure(
                    Errors.LeaderNotAvailableError(partition))
            else:
                timestamps_by_node[node_id][partition] = timestamp

        # Aggregate results until we have all
        list_offsets_future = Future()
        responses = []
        node_count = len(timestamps_by_node)

        def on_success(value):
            responses.append(value)
            if len(responses) == node_count:
                offsets = {}
                for r in responses:
                    offsets.update(r)
                list_offsets_future.success(offsets)

        def on_fail(err):
            if not list_offsets_future.is_done:
                list_offsets_future.failure(err)

        for node_id, timestamps in six.iteritems(timestamps_by_node):
            _f = self._send_offset_request(node_id, timestamps)
            _f.add_callback(on_success)
            _f.add_errback(on_fail)
        return list_offsets_future