Set symmetric difference: symmetric_difference

The symmetric_difference template function returns a list of items that are in either of the two lists, but not in both. This is the set symmetric difference operation: it gives you the items that are unique to each list, excluding any items that the two lists share.

This is useful when you want to find what has changed between two collections. For example, you might compare today’s list of active devicesA device is a model representing a physical or logical unit that contains entities. with yesterday’s to see which devices were added or removed. Or you might compare the members of two groups to find which entitiesAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more] are exclusive to each group. It effectively combines the results of two difference calls in both directions. Duplicates are removed from the result since it operates as a set operation.

Usage

Here’s how to use this template function. Copy any example and adjust it to your setup.

{{ symmetric_difference([1, 2, 3, 4], [3, 4, 5, 6]) }}

[1, 2, 5, 6]

{{ [1, 2, 3, 4] | symmetric_difference([3, 4, 5, 6]) }}

[1, 2, 5, 6]

Function signature

The signature is a technical summary of this template function. It shows the name of the function, the values (called parameters) it accepts, and what type of data each parameter expects (for example, a piece of text or a number).

Function parameters that have a = with a value after them are optional. If you leave them out, the default value shown is used automatically. Function parameters without a default are required.

symmetric_difference(
    value: list,
    other: list,
) -> list

Function parameters

The following parameters can be provided to this function.

Good to know

• Duplicates within either input list are collapsed: the result treats both inputs as sets.
• The order of items in the result is not guaranteed, so do not rely on a specific ordering.

Try it yourself

Ready to test this? Open Developer tools > Template, paste the example into the Template editor, and watch the result update on the right. Edit the values to see how the function adapts to your own entitiesAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more].

More examples

Real scenarios where this function comes up in automations and templates. Copy any example and adapt it to your setup.

Find devices exclusive to each group

Determine which entitiesAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more] are in one group but not the other, and vice versa.

{% set morning = ["light.kitchen", "light.hallway", "light.bedroom"] %}
{% set evening = ["light.hallway", "light.bedroom",
                  "light.porch", "light.living_room"] %}
{{ symmetric_difference(morning, evening) }}

["light.kitchen", "light.porch", "light.living_room"]

Detect changes in a list

Compare a previous and current list to find items that were added or removed.

{% set previous = ["sensor.temp_a", "sensor.temp_b", "sensor.temp_c"] %}
{% set current = ["sensor.temp_b", "sensor.temp_c", "sensor.temp_d"] %}
{{ symmetric_difference(previous, current) }}

["sensor.temp_a", "sensor.temp_d"]

Still stuck?

The Home Assistant community is quick to help: join Discord for real-time chat, post on the community forum with your template and expected result, or share on our subreddit /r/homeassistant.

Related template functions

These functions work well alongside this one:

• Set intersection: intersect - Returns items common to both lists (set intersection).

• Set difference: difference - Returns items in the first list but not in the second (set difference).

• Set union: union - Returns all unique items from both lists (set union).

• Convert to set: set - Converts an iterable to a set, removing duplicate values.