Home Assistant — Entity Lifecycle and Registries

Entity Lifecycle

1. Platform discovery: async_setup_entry called on platform
2. Entity creation: Entities instantiated, passed to async_add_entities
3. Registration: HA assigns entity_id, registers in entity registry
4. First update: async_added_to_hass called, initial state written
5. Updates: Coordinator triggers _handle_coordinator_update
6. Removal: async_will_remove_from_hass for cleanup

class MyEntity(CoordinatorEntity):
    async def async_added_to_hass(self) -> None:
        await super().async_added_to_hass()
        # Restore previous state
        if (last_state := await self.async_get_last_state()) is not None:
            self._attr_native_value = last_state.state

    async def async_will_remove_from_hass(self) -> None:
        await super().async_will_remove_from_hass()
        # Cleanup resources

Device and Entity Registries

Device registry groups entities under physical/logical devices:

from homeassistant.helpers.device_registry import DeviceInfo

@property
def device_info(self) -> DeviceInfo:
    return DeviceInfo(
        identifiers={(DOMAIN, self._serial)},  # Stable, unique tuple
        name="My Device",
        manufacturer="Acme",
        model="Widget Pro",
        sw_version="1.2.3",
        configuration_url="http://192.168.1.100",
    )

identifiers must be stable across restarts — this is how HA knows entities belong together and avoids creating duplicate device entries.