YANG models not only define configuration and operational data. They also define remote procedure calls or commands. Cisco IOS XR 6.1.2 starts introducing models for those RPCs or commands. Those models define how to roll back config, test SNMP traps or generate custom syslog messages. An RPC model specifies whether the RPC takes input data and whether it produces output data.
In YDK 0.5.1, we introduced the Executor service which allows you to execute RPCs. This service supports a single operation (execute_rpc) and is currently documented at:
Watch the ydk-py-samples repository for a series of upcoming sample apps that execute RPCs. Stay tuned!