code: def fulfill_transaction(transaction, *, private_keys):
      
      if not isinstance(private_keys, (list, tuple)):
          private_keys = [private_keys]
  
      # NOTE: Needed for the time being. See
      # https://github.com/bigchaindb/bigchaindb/issues/797
      if isinstance(private_keys, tuple):
          private_keys = list(private_keys)
  
      transaction_obj = Transaction.from_dict(transaction)
      try:
          signed_transaction = transaction_obj.sign(private_keys)
      except KeypairMismatchException as exc:
          raise MissingPrivateKeyError('A private key is missing!') from exc
  
      return signed_transaction.to_dict()
docs: Fulfills the given transaction.
  
      Args:
          transaction (dict): The transaction to be fulfilled.
          private_keys (:obj:`str` | :obj:`list` | :obj:`tuple`): One or
              more private keys to be used for fulfilling the
              transaction.
  
      Returns:
          dict: The fulfilled transaction payload, ready to be sent to a
          BigchainDB federation.
  
      Raises:
          :exc:`~.exceptions.MissingPrivateKeyError`: If a private
              key is missing.