mget wrapper that batches keys per shard and execute as few
        mgets as necessary to fetch the keys from all the shards involved.

        This method should be invoked on a TwemRedis instance as if it
        were being invoked directly on a StrictRedis instance.

def mget(self, args):
        """
        mget wrapper that batches keys per shard and execute as few
        mgets as necessary to fetch the keys from all the shards involved.

        This method should be invoked on a TwemRedis instance as if it
        were being invoked directly on a StrictRedis instance.
        """
        key_map = collections.defaultdict(list)
        results = {}
        for key in args:
            shard_num = self.get_shard_num_by_key(key)
            key_map[shard_num].append(key)

        # TODO: parallelize
        for shard_num in key_map.keys():
            shard = self.get_shard_by_num(shard_num)
            results[shard_num] = shard.mget(key_map[shard_num])
        return results