developers.home-assistant/docs/core/entity.md

title	sidebar_label
Entity	Introduction

For a generic introduction of entities, see entities architecture.

Basic implementation

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

from homeassistant.components.switch import SwitchEntity


class MySwitch(SwitchEntity):
    def __init__(self):
        self._is_on = False

    @property
    def name(self):
        """Name of the entity."""
        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 a 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.

:::tip 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.
attribution	string	None	The branding text required by the API provider.
available	boolean	True	Indicate if Home Assistant is able to read the state and control the underlying device.
device_class	string	None	Extra 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.
entity_category	string	None	Classification of a non-primary entity. Set to config for an entity which allows changing the configuration of a device, for example a switch entity making it possible to turn the background illumination of a switch on and off. Set to diagnostic for an entity exposing some configuration parameter or diagnostics of a device but does not allow changing it, for example a sensor showing RSSI or MAC-address.
entity_picture	URL	None	Url of a picture to show for the entity.
extra_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.
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.

:::warning Entities that generate a significant amount of state changes can quickly increase the size of the database when the extra_state_attributes also change frequently. Minimize the number of extra_state_attributes for these entities by removing non-critical attributes or creating additional sensor entities. :::

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
entity_registry_enabled_default	boolean	True	Indicate if the entity should be enabled or disabled when first added to the entity registry. This includes fast-changing diagnostic entities or, assumingly less commonly used entities. For example, a sensor exposing RSSI or battery voltage should typically be set to False; to prevent unneeded (recorded) state changes or UI clutter by these entities.
entity_registry_visible_default	boolean	True	Indicate if the entity should be hidden or visible when first added to the entity registry.
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.
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 entities according to its device_class. This should be used only in the case where there either is no matching device_class or where the icon used for the device_class would be confusing or misleading.

System properties

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

Name	Type	Default	Description
enabled	boolean	True	Indicate 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.

Property implementation

Property function

Writing property methods for each property is just a couple of lines of code, for example

class MySwitch(SwitchEntity):

    @property
    def icon(self) -> str | None:
        """Icon of the entity."""
        return "mdi:door"

    ...

Entity class or instance attributes

Alternatively, a shorter form is to set Entity class or instance attributes according to either of the following patterns:

class MySwitch(SwitchEntity):

    _attr_icon = "mdi:door"

    ...

class MySwitch(SwitchEntity):

    def __init__(self, icon: str) -> None:
        self._attr_icon = icon

    ...

This does exactly the same as the first example but relies on a default implementation of the property in the base class. The name of the attribute starts with _attr_ followed by the property name. For example, the default device_class property returns the _attr_device_class class attribute.

Not all entity classes support the _attr_ attributes for their entity specific properties, please refer to the documentation for the respective entity class for details.

:::tip If an integration needs to access its own properties it should access the property (self.name), not the class or instance attribute (self._attr_name). :::

Example

The below code snippet gives an example of best practices for when to implement property functions, and when to use class or instance attributes.

class SomeEntity():
    _attr_device_clas = DEVICE_CLASS_TEMPERATURE  # This will be common to all instances of SomeEntity
    def __init__(self, device):
        self._device = device
        self._attr_available = False  # This overrides the default
        self._attr_name = device.get_friendly_name()

        # The following should be avoided:
        if some_complex_condition and some_other_condition and something_is_none_and_only_valid_after_update and device_available:
           ...

    def update(self)
        if self.available  # Read current state, no need to prefix with _attr_
            # Update the entity
            self._device.update()

        if error:
            self._attr_available = False  # Set property value
            return
        # We don't need to check if device available here
        self._attr_is_on = self._device.get_state()  # Update "is_on" property

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, subscribe to updates or set callback/dispatch function/listener.

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.