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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.

As a filter

{{ my_variable | is_defined }}

Result (any)

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 = 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.

How to read a signature

Here is the signature of the round filter as an example:

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

round is the name of the function.
Between the parentheses are the parameters. Each parameter has a name, followed by a colon and the type of value the parameter expects. The type only tells you what kind of value to pass. For default: Any, you pass a value of any type, such as 0 or "unknown", not the word Any.
= 0 and = "common" are default values. You can leave out precision and method, and round uses these default values.
default has no =, but the parameter list marks default as optional. If you leave out default and round gets a value that isn’t a number, round raises an error.
-> shows the type of value the function returns.
Inside a signature, | means “or”. This function returns a decimal number, a whole number, or the value you passed as default.

round is a filter, so the first parameter, value, is the value before the |. For example, {{ 3.14159 | round(2) }} returns 3.14.

The types you’ll see most often:

str: text, such as "hello"
int: a whole number, such as 5
float: a decimal number, such as 3.14
bool: true or false
list: a list of values, such as [1, 2, 3]. list[str] is a list of texts.
dict: a set of keys and values, such as {"name": "Kitchen"}
datetime and timedelta: a date and time, and a duration. For details, refer to Dates and times.
None: no value
Any: any type of value

Some signatures use special forms:

*args means you can pass any number of values, separated by commas. For example, average(1, 2, 3).
value | name(...) means the function is a filter. The value before the | is passed to the filter as its first parameter. Some filter signatures, like the round example, show this value as the first parameter between the parentheses instead.

is_defined(
    value: Any,
) -> Any

Function parameters

The following parameters can be provided to this filter.

value any Required

The value to check. If the value is an Undefined object, an error is raised. Otherwise, the value is returned unchanged.

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.

Template: Raises an error if my_var does not exist

{{ my_var | is_defined }}

Tip

This filter is most useful in scriptsA script is a saved list of steps that runs when you start it, for example, from a dashboard, with Assist, or from an automation. Unlike an automation, a script has no triggers, so it doesn’t start by itself. [Learn more] and automationsAutomations in Home Assistant allow you to automatically respond to things that happen in and around your home. [Learn more] where variables are passed in from outside. It helps you catch cases where a required variable was not provided.

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 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.

Validate a script variable

Ensure that a required variable was passed into a scriptA script is a saved list of steps that runs when you start it, for example, from a dashboard, with Assist, or from an automation. Unlike an automation, a script has no triggers, so it doesn’t start by itself. [Learn more] before using it.

Action

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.

Template

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

Result (string)

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.

Tip

AI assistants like ChatGPT or Claude can also explain or fix templates when you describe what you want in plain language.

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.

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