Subscribe to events in the entity lifecycle

This is a Bronze rule. In quality_scale.yaml, it is called entity-event-setup.

When an entity subscribes to events, for example to updates pushed by the library of the integration, it must subscribe in async_added_to_hass, and unsubscribe when it is removed. Do not subscribe in __init__.

Home Assistant calls async_added_to_hass after the entity platform has registered the entity. From that moment on, the entity has everything it needs, like self.hass and self.async_write_ha_state. A callback that fires before that point, and tries to write a state, fails.

Unsubscribing matters just as much. An entity that is removed, or whose config entry is unloaded, but is still subscribed, keeps receiving events. That leaks memory and can write states for an entity that no longer exists.

The easiest way is async_on_remove. It takes the unsubscribe function, and calls it when the entity is removed, so you do not have to keep track of it yourself:

class MySensor(SensorEntity):
    """A sensor that gets pushed updates."""

    def __init__(self, client: MyClient) -> None:
        """Initialize the sensor."""
        self.client = client

    async def async_added_to_hass(self) -> None:
        """Subscribe to updates when added to Home Assistant."""
        await super().async_added_to_hass()
        self.async_on_remove(
            self.client.events.subscribe("update", self._handle_update)
        )

    @callback
    def _handle_update(self, event: MyEvent) -> None:
        """Handle an update from the device."""
        self._attr_native_value = event.value
        self.async_write_ha_state()

If you need more control, unsubscribe in async_will_remove_from_hass instead. Entities based on CoordinatorEntity already do this for their coordinator, so you only need it for subscriptions of your own.

Checked in review.

If the entities of the integration do not subscribe to events, this rule does not apply.

• config-entry-unloading: support unloading the config entry.