Flatten nested lists: flatten

The flatten template function takes a list that contains other lists (nested lists) and flattens them into a single, flat list. All items from all levels of nesting are brought up to the top level, which is useful for working with deeply nested data structures.

This is useful when you collect data from multiple sources that each return a list, and you end up with a list of lists. For example, you might gather the attributes from several sensorsSensors return information about a thing, for instance the level of water in a tank. [Learn more] that each return a list of values, or you might combine multiple groups and want a single flat list of all 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]. The optional levels parameter lets you control how many levels of nesting to flatten, so you can partially flatten a deeply nested structure if needed.

Usage

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

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

[1, 2, 3, 4, 5]

{{ [[1, 2], [3, [4, 5]]] | flatten }}

[1, 2, 3, 4, 5]

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 = are usually required. A few optional parameters have no default value, so the signature shows them without a = as well. To check whether a parameter is required, look at the parameter list below the signature. Each parameter is marked as Required or Optional.

The most common optional parameter without a default value is default. The default parameter sets the value a function returns when the function can’t produce a result, for example, when float gets text that isn’t a number. If you leave out default, there is no fallback value, so most functions raise an error and the template fails. The parameter list of each function says what happens. If you set default, the function returns your value instead.

round(
    value: Any,
    precision: int = 0,
    method: str = "common",
    default: Any,
) -> float | int | Any

flatten(
    value: list,
    levels: int | None = None,
) -> list

Function parameters

The following parameters can be provided to this function.

Controlling flatten depth

By default, flatten removes all nesting. Use the levels parameter to control how deep the flattening goes.

{{ [[1, [2, 3]], [4, [5, 6]]] | flatten(levels=1) }}

[1, [2, 3], 4, [5, 6]]

Good to know

• Without levels, every layer of nesting is removed, which can produce surprising results on deeply structured data.
• Any nested collection except text is unpacked, including tuples and sets. Text stays as a single item.
• A dictionary inside the list is replaced by its keys.
• none items stay in the list.

Try it yourself

Ready to test this? Open Settings > 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.

Flatten sensor attributes from multiple entities

When multiple sensorsSensors return information about a thing, for instance the level of water in a tank. [Learn more] each return a list in an attribute, combine and flatten them into one list.

{{
  [
    state_attr("sensor.room_a", "device_list"),
    state_attr("sensor.room_b", "device_list")
  ] | flatten
}}

["device_1", "device_2", "device_3", "device_4"]

Flatten and count unique items

Flatten a nested structure and then count the unique values.

{{ [[1, 2, 3], [2, 3, 4], [4, 5]] | flatten | unique | list | count }}

5

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:

• Randomly shuffle a list: shuffle - Randomly shuffles the items in a list.

• Merge dictionaries: combine - Merges multiple dictionaries into one.

• Expand groups into entities: expand - Expands groups and zones into a list of individual entity state objects.