Each device is represented in Home Assistant as an entity. An entity abstracts away the internal working of Home Assistant. As an integrator you don't have to worry about how services or the state machine work. Instead, you extend an entity class and implement the necessary properties and methods for the device type that you're integrating.

Architecture Overview of Home Assistant


Configuration is provided by the configuration.yaml file or by a Config Entry.


Examples of components: light, switch.

The component is responsible for defining the Abstract Entity Class and services to control the entities.

Entity Component

The Entity Component is responsible for:

  • Distributing the configuration to the platforms
  • Forward config entries and discoveries
  • Collect entities for service calls
  • Optionally maintain a group of all entities

Entity Platform

The Entity Platform manages all entities for the platform and polls them for updates if necessary.

When adding entities, the Entity Platform will query the Entity Registry to make sure that the entities to be added have the correct entity IDs.

Entity Registry

The Entity Registry will track entities and allows users to store extra settings for an entity.


Examples of platforms: light.hue, switch.wemo.

Platform uses configuration to query the external device/service and add entities to the entity platform.

Basic implementation

Below is an example switch entity that keeps track of their state in memory.

from homeassistant.components.switch import SwitchDevice
class MySwitch(SwitchDevice):
def __init__(self):
self._is_on = False
def name(self):
"""Name of the device."""
return "My Switch"
def is_on(self):
"""If the switch is currently on or off."""
return self._is_on
def turn_on(self, **kwargs):
"""Turn the switch on."""
self._is_on = True
def turn_off(self, **kwargs):
"""Turn the switch off."""
self._is_on = False

That's all there is to it to build a switch entity! Continue reading to learn more or check out the video tutorial.

Updating the entity

An entity represents a device. There are various strategies to keep your entity in sync with the state of the device, the most popular one being polling.


With polling, Home Assistant will ask the entity from time to time (depending on the update interval of the component) to fetch the latest state. Home Assistant will poll an entity when the should_poll property returns True (the default value). You can either implement your update logic using update() or the async method async_update(). This method should fetch the latest state from the device and store it in an instance variable for the properties to return it.

Subscribing to updates

When you subscribe to updates, your code is responsible for letting Home Assistant know that an update is available. Make sure you have the should_poll property return False.

Whenever you receive new state from your subscription, you can tell Home Assistant that an update is available by calling schedule_update_ha_state() or async callback async_schedule_update_ha_state(). Pass in the boolean True to the method if you want Home Assistant to call your update method before writing the update to Home Assistant.

Generic properties

The entity base class has a few properties that are common among all entities in Home Assistant. These can be added to any entity regardless of the type. All these properties are optional and don't need to be implemented.


Properties should always only return information from memory and not do I/O (like network requests). Implement update() or async_update() to fetch data.

assumed_statebooleanFalseReturn True if the state is based on our assumption instead of reading it from the device.
availablebooleanTrueIndicate if Home Assistant is able to read the state and control the underlying device.
device_classstringNoneExtra classification of what the device is. Each domain specifies their own. Device classes can come with extra requirements for unit of measurement and supported features.
device_state_attributesdictNoneExtra information to store in the state machine. It needs to be information that further explains the state, it should not be static information like firmware version. See below for details of standard attributes.
entity_pictureURLNoneUrl of a picture to show for the entity.
namestringNoneName of the entity
should_pollbooleanTrueShould Home Assistant check with the entity for an updated state. If set to False, entity will need to notify Home Assistant of new updates by calling one of the schedule update methods.
unique_idstringNoneA unique identifier for this entity. Needs to be unique within a platform (ie light.hue). Should not be configurable by the user or be changeable. Learn more.

Advanced properties

The following properties are also available on entities. However, they are for advanced use only and should be used with caution.

force_updatebooleanFalseWrite each update to the state machine, even if the data is the same. Example use: when you are directly reading the value from a connected sensor instead of a cache. Use with caution, will spam the state machine.
hiddenbooleanFalseIndicate if the entity should not be shown on the frontend. Deprecated. Will be removed in 0.107.0
iconiconNoneIcon to use in the frontend. Icons start with mdi: plus an identifier. You probably don't need this since Home Assistant already provides default icons for all devices.
entity_registry_enabled_defaultbooleanTrueIndicate if the entity should be enabled or disabled when it is first added to the entity registry.

System properties

The following properties are used and controlled by Home Assistant, and should not be overridden by integrations.

enabledbooleanTrueIndicate if entity is enabled in the entity registry. It also returns True if the platform doesn't support the entity registry. Disabled entities will not be added to Home Assistant.

Standard attributes

The following device_state_attributes are considered standard and should follow the convention below. The constant should be imported from homeassistant/const.py.

battery_chargingbooleanN/AATTR_BATTERY_CHARGINGBattery charging status of the entity, shown as a boolean true or false. If charging is not supported, then this attribute should not be created.
battery_levelinteger%ATTR_BATTERY_LEVELBattery level of the entity, shown as an integer percentage between 0-100.

Lifecycle hooks

Use these lifecycle hooks to execute code when certain events happen to the entity. All lifecycle hooks are async methods.


Called when an entity has their entity_id and hass object assigned, before it is written to the state machine for the first time. Example uses: restore the state, subscribe to updates or set callback/dispatch function/listener.


Called when an entity is about to be removed from Home Assistant. Example use: disconnect from the server or unsubscribe from updates.

Changing the entity model

If you want to add a new feature to an entity or any of its subtypes (light, switch, etc), you will need to propose it first in our architecture repo. Only additions will be considered that are common features among various vendors.

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