    def _gen_condition(cls, initial, new_public_keys):
        """Generates ThresholdSha256 conditions from a list of new owners.

            Note:
                This method is intended only to be used with a reduce function.
                For a description on how to use this method, see
                :meth:`~.Output.generate`.

            Args:
                initial (:class:`cryptoconditions.ThresholdSha256`):
                    A Condition representing the overall root.
                new_public_keys (:obj:`list` of :obj:`str`|str): A list of new
                    owners or a single new owner.

            Returns:
                :class:`cryptoconditions.ThresholdSha256`:
        """
        try:
            threshold = len(new_public_keys)
        except TypeError:
            threshold = None

        if isinstance(new_public_keys, list) and len(new_public_keys) > 1:
            ffill = ThresholdSha256(threshold=threshold)
            reduce(cls._gen_condition, new_public_keys, ffill)
        elif isinstance(new_public_keys, list) and len(new_public_keys) <= 1:
            raise ValueError('Sublist cannot contain single owner')
        else:
            try:
                new_public_keys = new_public_keys.pop()
            except AttributeError:
                pass
            # NOTE: Instead of submitting base58 encoded addresses, a user
            #       of this class can also submit fully instantiated
            #       Cryptoconditions. In the case of casting
            #       `new_public_keys` to a Ed25519Fulfillment with the
            #       result of a `TypeError`, we're assuming that
            #       `new_public_keys` is a Cryptocondition then.
            if isinstance(new_public_keys, Fulfillment):
                ffill = new_public_keys
            else:
                ffill = Ed25519Sha256(
                    public_key=base58.b58decode(new_public_keys))
        initial.add_subfulfillment(ffill)
        return initial
Generates ThresholdSha256 conditions from a list of new owners.

Note:
This method is intended only to be used with a reduce function.
For a description on how to use this method, see
:meth:`~.Output.generate`.

Args:
initial (:class:`cryptoconditions.ThresholdSha256`):
A Condition representing the overall root.
new_public_keys (:obj:`list` of :obj:`str`|str): A list of new
owners or a single new owner.

Returns:
:class:`cryptoconditions.ThresholdSha256`:
bigchaindb/common/transaction.py