The maagic API does exactly that. Sadly, there is no example showing how to use that, so here’s a short one, albeit a bit concocted (using the dhcp introductory data model):
from confd import maapi, maagic
# create "local subnet" for all networks
loc_subnet = ['127.0.0.1', '255.255.255.0']
with maapi.single_write_trans('admin', 'system') as trans:
# start with the root node
root = maagic.get_root(trans)
# iterate over "shared-network" instances:
for network in root.dhcp.shared_networks.shared_network:
subnet_list = network.subnets.subnet
# if the local subnet is not there, create it
if loc_subnet not in subnet_list:
subnet_list.create(loc_subnet)
trans.apply()