developers.home-assistant/docs/entity_index.md

title	sidebar_label
Entity	Introduction

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.

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

    @property
    def name(self):
        """Name of the device."""
        return 'My Switch'

    @property
    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.

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.

Name	Type	Default	Description
assumed_state	boolean	False	Return True if the state is based on our assumption instead of reading it from the device.
available	boolean	True	Indicate if Home Assistant is able to read the state and control the underlying device.
device_state_attributes	dict	None	Extra 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_picture	URL	None	Url of a picture to show for the entity.
name	string	None	Name of the entity
should_poll	boolean	True	Should 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_id	string	None	A 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.

Name	Type	Default	Description
force_update	boolean	False	Write 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.
hidden	boolean	False	Indicate if the entity should not be shown on the frontend.
icon	icon	None	Icon 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.

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.

Name	Type	Unit	Constant	Description
battery_charging	boolean	N/A	ATTR_BATTERY_CHARGING	Battery 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_level	integer	%	ATTR_BATTERY_LEVEL	Battery 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.

async_added_to_hass()

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 or subscribe to updates.

async_will_remove_from_hass()

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.