Call a function dynamically: apply

The apply template function calls another function with a value as its first argument, plus any extra arguments you give it. It lets you hand a function to places that normally expect a filter or test, like map, select, or reject.

You reach for apply when you want to run a custom function (one you built with as_function) or a built-in function across every item of a list. Most templates don’t need it. It comes in handy when you have reusable logic you want to apply to many values without writing a full for loop.

Usage

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

{{ apply(5, float) }}

5.0

{{ 5 | apply(float) }}

5.0

{% if "5" is apply(is_number) %}
  It's a number
{% endif %}

It's a number

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

apply(
    value: Any,
    fn: Callable,
    *args: Any,
    **kwargs: Any,
) -> Any

Function parameters

The following parameters can be provided to this function.

Using apply with map

apply is most useful when combined with map to transform lists of values. You pass any function to map("apply", fn) and it runs on each item in the list.

{{ [1, 2, 3] | map("apply", float) | list }}

[1.0, 2.0, 3.0]

Using apply as a test with select

When used as a test, apply lets you filter items using any function as a predicate.

{{ ["3", "abc", "7"] | select("apply", is_number) | list }}

["3", "7"]

Good to know

• Extra arguments are passed after the value, so apply(5, float, 0) calls float(5, 0) and uses 0 as the default.
• Works with built-in template functions, filters, and macros wrapped with as_function, but not arbitrary Python callables.

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.

Apply a custom function across sensor values

Use apply with as_function to run a custom macro over a list of values.

{% macro macro_celsius_to_f(value, returns) %}
  {{ returns(value * 9 / 5 + 32) }}
{% endmacro %}
{% set c_to_f = as_function(macro_celsius_to_f) %}
{{ [0, 20, 100] | map("apply", c_to_f) | list }}

[32.0, 68.0, 212.0]

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:

• Turn a macro into a function: as_function - Turns a template macro into a reusable function you can pass to map, select, and reject.

• Immediate if (ternary): iif - Shorthand for basic if/else logic in a single expression.