HA has a registry that associates entities to device_ids. Roughly speaking, a physical device gets a device_id (assigned by HA, not “unique” in the sense that the same physical device will get a new id when reconnected). Then entities are logical abstractions sitting above a device, e.g., if a sensor provides light, humidity, and temp readings, that might be three sensor entities associated to the same device_id.
I’ve never had a need to access the device IDs until the new Nest integration (based on Google’s SDM API). There events (like camera_motion or camera_person, etc.) are fired with a device_id. I would like to listen to those events in AppDaemon, but for that I need a way of looking up the device_id of an entity.
I don’t know the HA philosophy on these levels of abstraction, so I understand a possible argument that it is “inappropriate” for AppDaemon to access the device registry. In that case, it’s the Nest integration that is problematic.