Store runtime data in runtime_data

This is a Bronze rule. In quality_scale.yaml, it is called runtime-data.

Data that the integration needs while a config entry is loaded, like the API client or the coordinator, must be stored in entry.runtime_data. Not in hass.data. Give the config entry a type that says what is in runtime_data. When the integration also follows strict-typing, that typed config entry is required, and you use it everywhere you handle the entry.

runtime_data belongs to one config entry, and it is gone when the entry is unloaded. There is no shared dictionary to keep clean, and no keys that collide with other integrations. Because the config entry is typed, your editor and mypy know exactly what entry.runtime_data is, and catch mistakes before your users do. And every integration stores its data the same way, which makes the code easier to read for the next person.

Define the typed config entry, and assign runtime_data in async_setup_entry:

type MyConfigEntry = ConfigEntry[MyClient]


async def async_setup_entry(hass: HomeAssistant, entry: MyConfigEntry) -> bool:
    """Set up My integration from a config entry."""
    client = MyClient(entry.data[CONF_HOST])
    entry.runtime_data = client

    await hass.config_entries.async_forward_entry_setups(entry, PLATFORMS)
    return True

A platform reads it back, fully typed:

async def async_setup_entry(
    hass: HomeAssistant,
    entry: MyConfigEntry,
    async_add_entities: AddConfigEntryEntitiesCallback,
) -> None:
    """Set up the sensors."""
    client = entry.runtime_data

A coordinator that gets the config entry should use the typed entry too:

class MyCoordinator(DataUpdateCoordinator[MyData]):
    """Fetch data from the device."""

    config_entry: MyConfigEntry

When more than one object needs to live in runtime_data, put them in a dataclass and use that as the type.

• Hassfest, when the rule is done: async_setup_entry in __init__.py must assign entry.runtime_data.
• Hassfest, when strict-typing is also done: the config entry must be typed, with a type alias whose name ends in ConfigEntry. This is checked for async_setup_entry, async_unload_entry, async_migrate_entry, async_remove_entry, and async_remove_config_entry_device in __init__.py, for async_setup_entry of every platform, for the diagnostics functions, and for async_get_options_flow.
• Pylint, for every integration with a config flow: home-assistant-use-runtime-data warns when the integration stores data in hass.data[DOMAIN].
• Pylint: home-assistant-coordinator-untyped-config-entry warns when a coordinator does not use the typed config entry of its integration.

If the integration does not have any runtime data to store, this rule does not apply.

• strict-typing: use strict typing.
• test-before-setup: check the connection during setup.