Add new devices automatically

This is a Gold rule. In quality_scale.yaml, it is called dynamic-devices.

When a new device shows up after the integration is set up, like a new light paired to a hub or a new device added to a cloud account, the integration creates its entities by itself. The user does not have to reload the integration or restart Home Assistant.

Users add a device to their hub or account, and expect it to show up in Home Assistant. Having to know that a reload is needed, and where to find it, is the kind of friction that Gold is about removing.

This is the other side of stale-devices: devices come and go, and Home Assistant follows.

With a coordinator, keep track of the devices the platform already knows. A listener on the coordinator runs after every update, and adds entities for the new devices:

async def async_setup_entry(
    hass: HomeAssistant,
    entry: MyConfigEntry,
    async_add_entities: AddConfigEntryEntitiesCallback,
) -> None:
    """Set up the sensors of My integration."""
    coordinator = entry.runtime_data
    known_devices: set[str] = set()

    @callback
    def _add_new_devices() -> None:
        new_devices = set(coordinator.data) - known_devices
        if not new_devices:
            return

        known_devices.update(new_devices)
        async_add_entities(
            MySensor(coordinator, device_id) for device_id in new_devices
        )

    _add_new_devices()
    entry.async_on_unload(coordinator.async_add_listener(_add_new_devices))

The listener is removed when the config entry unloads, through entry.async_on_unload.

In this example, known_devices only grows while the entry is loaded. When the integration also removes devices that are gone, like stale-devices describes, a device that comes back later is still in known_devices, and its entities are not added again until the entry reloads. Drop removed devices from known_devices as well, so they are treated as new when they return.

Checked in review.

Integrations with a single, fixed device per config entry, that lives and goes with the config entry, are exempt from this rule.

• stale-devices: remove stale devices.