handler.py

from charms.reactive.context import relation_endpoint


@when('charm.installed', 'relation_endpoint.db.ready')
def handler():
    render_config(hostnames=relation_endpoint.user_db.hostnames)
    relation_endpoint.admin_db.hostnames

something.py

class CassandraClient(RelationEndpoint):
    @property
    def _ready_units(self):
        ready_units = []
        for relation in self.relations:
            for unit in relation.units:
                if unit.received_data.foo == 'foo':
                    ready_units.append(unit)
        return ready_units
    
    @property
    def _unready_units(self):
        return filter(lambda u: u.received_data.foo != 'foo',
                      (u for r in self.relations for u in r.units))
    
    @property
    def hostnames(self):
        return [u.received_data.hostname for u in self._ready_units]

    @when('relation_endpoint.{relation_endpoint_name}.changed')
    def changed(self):
        if self._ready_units:
            set_flag('relation_endpoint.{relation_endpoint_name}.ready')

Something we lost here is the docstrings for the flags. We'll need some convention for that again.