Test if defined: defined

The defined test checks whether a variable has been defined in the current template context. It returns true if the variable exists and false if it is undefined.

This test is commonly used to guard against errors when working with variables that may or may not be present. For example, triggerA trigger is a set of values or conditions of a platform that are defined to cause an automation to run. [Learn more] variables are only available inside an automationAutomations in Home Assistant allow you to automatically respond to things that happen in and around your home. [Learn more] context, and optional variables passed to scripts may not always be set. Testing with defined before accessing a variable prevents “undefined” errors and lets you branch your logic accordingly.

Usage

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

{% if my_variable is defined %}
  Variable exists: {{ my_variable }}
{% endif %}

Variable exists: hello

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.

defined(
    value: Any,
) -> bool

Function parameters

The following parameters can be provided to this test.

Good to know

• Checks whether a variable exists in the current context, not whether it has a non-empty value.
• A variable set to None passes this test. Use is not none to filter those out too.

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.

Guard trigger variable access

Only access triggerA trigger is a set of values or conditions of a platform that are defined to cause an automation to run. [Learn more] data when it is available.

{% if trigger is defined %}
  Triggered by: {{ trigger.entity_id }}
{% else %}
  No trigger context
{% endif %}

No trigger context

Check for optional script variables

Handle optional variables in a scriptScripts are components that allow you to specify a sequence of actions to be executed by Home Assistant when turned on. [Learn more] gracefully.

{% if message is defined %}
  {{ message }}
{% else %}
  Default notification text
{% endif %}

Default notification text

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:

• Test if undefined: undefined - Tests whether a variable is undefined.

• Test if none: none - Tests whether a value is None.

• Set a default value: default - Provides a default value if a variable is undefined or none.

• Require a value is defined: is_defined - Forces a template error if the value is undefined, ensuring variables exist before use.