2026.10.4
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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 the part of an automation that makes the automation start when something specific happens, for example, when the sun sets, a door opens, or a button is pressed. [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.

As a test 
{% if my_variable is defined %}
  Variable exists: {{ my_variable }}
{% endif %}
Result (stringA piece of text, like a name, message, or entity ID. In templates, wrap strings in quotes, like "living_room" or "lights are on".) 
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 = 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.
defined(
    value: Any,
) -> bool

Function parameters

The following parameters can be provided to this test.

value any Required

The variable to test. Returns true if the variable is defined in the current context.

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 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.

Guard trigger variable access

Only access triggerA trigger is the part of an automation that makes the automation start when something specific happens, for example, when the sun sets, a door opens, or a button is pressed. [Learn more] data when it is available.

TemplateA 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] 
{% if trigger is defined %}
  Triggered by: {{ trigger.entity_id }}
{% else %}
  No trigger context
{% endif %}
Result (stringA piece of text, like a name, message, or entity ID. In templates, wrap strings in quotes, like "living_room" or "lights are on".) 
No trigger context

Check for optional script variables

Handle optional variables in 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] gracefully.

TemplateA 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] 
{% if message is defined %}
  {{ message }}
{% else %}
  Default notification text
{% endif %}
Result (stringA piece of text, like a name, message, or entity ID. In templates, wrap strings in quotes, like "living_room" or "lights are on".) 
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.

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: