Require a value is defined: is_defined

The is_defined filter checks that a value is not undefined and forces a template error if it is. If the value is defined, it passes through unchanged. This is a safety mechanism to catch typos and missing variables early rather than having them silently produce empty strings.

By default, Home Assistant is configured to be lenient with undefined variables, meaning a misspelled variable name produces an empty string instead of an error. While this is convenient for smaller templatesA template is an automation definition that can include variables for the action or data from the trigger values. This allows automations to generate dynamic actions. [Learn more], it can make debugging very difficult. If you write {{ sensr.temperature }} instead of {{ sensor.temperature }}, you get a blank value with no indication that anything is wrong. The is_defined filter lets you opt in to strict checking for specific variables, so you get an immediate error that points to the problem.

Usage

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

{{ my_variable | is_defined }}

the value of my_variable

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.

is_defined(
    value: Any,
) -> Any

Function parameters

The following parameters can be provided to this filter.

How it works

When you reference a variable that does not exist in a template, it becomes an Undefined object. Normally, this silently renders as an empty string. The is_defined filter detects this and raises an error instead, making the problem visible.

{{ my_var | is_defined }}

Good to know

• This raises an error on undefined input, which is the opposite of most filters. Use it intentionally to enforce a variable’s presence.
• A value set to None is considered defined and passes through without error.

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.

Validate a script variable

Ensure that a required variable was passed into a scriptScripts are components that allow you to specify a sequence of actions to be executed by Home Assistant when turned on. [Learn more] before using it.

action:
  - action: notify.mobile
    data:
      message: >
        Reminder: {{ message | is_defined }}

If the message variable was not provided when calling the script, this raises a clear error instead of sending an empty notification.

Guard multiple variables

Apply is_defined to each variable you depend on at the start of a template.

{% set name = target_name | is_defined %}
{% set room = target_room | is_defined %}
{{ name }} is in the {{ room }}

Alice is in the Kitchen

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:

• Check if entity has a value: has_value - Tests if an entity exists and has a valid state (not unavailable or unknown).

• Get the type of a value: typeof - Returns the type name of a value as a string.

• Test if value is numeric: is_number - Tests whether a value can be converted to a finite number.