Turn a macro into a function: as_function

The as_function template function takes a template macro and turns it into a reusable function that produces a value. A macro on its own can only output text, which limits what you can do with its result. Wrapping it with as_function gives you a proper function you can pass to map, select, reject, or store in a variable for later use.

You reach for as_function when you’ve written a piece of reusable logic (usually in a macro) and want to run it across a list of values, or use it as a custom test. For most templates, built-in filters and functions are enough. The macro you wrap must take a special returns argument. Inside the macro, you call returns(value) to say “this is what the function should hand back”.

Usage

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

{% macro macro_double(value, returns) %}
  {{ returns(value * 2) }}
{% endmacro %}
{{ as_function(macro_double)(5) }}

10

{% macro macro_double(value, returns) %}
  {{ returns(value * 2) }}
{% endmacro %}
{{ macro_double | as_function }}

<function double>

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

as_function(
    macro: Macro,
) -> Callable

Function parameters

The following parameters can be provided to this function.

Writing a compatible macro

The macro must accept a returns parameter. Call returns() with the value you want the function to produce. The macro name is conventionally prefixed with macro_; this prefix is stripped from the resulting function’s name.

{% macro macro_add_ten(value, returns) %}
  {{ returns(value + 10) }}
{% endmacro %}
{% set add_ten = as_function(macro_add_ten) %}
{{ add_ten(5) }}

15

Using with map

Once converted, the function works seamlessly with map to transform lists.

{% macro macro_format_temp(value, returns) %}
  {{ returns(value | round(1) ~ "°C") }}
{% endmacro %}
{% set format_temp = as_function(macro_format_temp) %}
{{ [21.456, 19.8, 22.123] | map("apply", format_temp) | list }}

['21.5°C', '19.8°C', '22.1°C']

Good to know

• The macro must include a returns parameter and end by calling returns(value). Without that call, the function produces None.
• The conventional macro_ name prefix is stripped from the resulting function, so macro_double becomes double.

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.

Custom filter for select

Create a function that tests whether a value is within a range, then use it with select to filter a list.

{% macro macro_in_range(value, low, high, returns) %}
  {{ returns(low <= value <= high) }}
{% endmacro %}
{% set in_range = as_function(macro_in_range) %}
{{ [15, 22, 30, 18, 25] | select("apply", in_range, 20, 26) | list }}

[22, 25]

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:

• Call a function dynamically: apply - Calls a function with a value, letting you pass any function into places where a filter is expected.

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