Home Assistant Core Architecture

Home Assistant runs on a single-threaded asyncio event loop. All code shares this loop — blocking it freezes automations, the UI, and entity updates.

The hass Object

Every integration receives HomeAssistant instance (hass) — the central hub for all core systems:

from homeassistant.core import HomeAssistant

async def async_setup_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
    # hass.bus       — Event bus for pub/sub communication
    # hass.states    — State machine for entity states
    # hass.services  — Service registry for actions
    # hass.config    — System config (location, units, timezone)
    ...