In Jinja2, how do you test if a variable is undefined?

Question

Converting from Django, I'm used to doing something like this:

{% if not var1 %} {% endif %}

and having it work if I didn't put var1 into the context. Jinja2 gives me an undefined error. Is there an easy way to say {% if var1 == None %} or similar?

score 482 · Accepted Answer · edited Jul 29 '14 at 00:16

482

From the Jinja2 template designer documentation:

{% if variable is defined %}
    value of variable: {{ variable }}
{% else %}
    variable is not defined
{% endif %}

edited Jul 29 '14 at 00:16

Day

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answered Oct 01 '10 at 20:14

Garrett

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1

I believe this to be the case but my search terms often do not reflect that. – dannyman Dec 15 '16 at 18:50
40

Additionally, you can use `{% if variable is not defined %}` to test the inverse. – pbaranay Feb 22 '17 at 15:37
1

@dannyman [Since 0.2.0](https://docs.ansible.com/ansible-container/container_yml/template.html#jinja-templating) – Samuel Harmer May 11 '17 at 20:09
11

`{% if variable is defined and variable %}` would also check for emptiness – Vivek May 17 '19 at 09:00
1

@Vivek your answer ist the best. Usually I give my variables empty values `test: ~` and your solution could recognise that. THANK YOU :) – Khaled Nov 18 '22 at 10:21

score 55 · Answer 2 · edited Dec 31 '19 at 17:19

55

{% if variable is defined %} is true if the variable is None.

Since not is None is not allowed, that means that

{% if variable != None %}

is really your only option.

edited Dec 31 '19 at 17:19

Community

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answered May 21 '15 at 14:05

Carsten Agger

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If `variable` is always evaluated to `True` when not None, `{% if variable != None %}` is equivalent to `{% if variable %}`. – azalea Oct 10 '19 at 01:05
6

If you want to check for `None` use lowercase `none` `{% if variable is not none %}` – Felipe Alvarez Oct 16 '19 at 06:29
2

@FelipeAlvarez Do you have a link to documentation on this? Thanks in advance. – ryanwebjackson Nov 06 '20 at 15:39
3

@ryanwebjackson This is the [builtin test](https://jinja.palletsprojects.com/en/2.11.x/templates/#none) called `none`. – kennysong Jan 08 '21 at 06:44

score 21 · Answer 3 · answered Sep 18 '14 at 15:01

You could also define a variable in a jinja2 template like this:

{% if step is not defined %}
{% set step = 1 %}
{% endif %}

And then You can use it like this:

{% if step == 1 %}
<div class="col-xs-3 bs-wizard-step active">
{% elif step > 1 %}
<div class="col-xs-3 bs-wizard-step complete">
{% else %}
<div class="col-xs-3 bs-wizard-step disabled">
{% endif %}

Otherwise (if You wouldn't use {% set step = 1 %}) the upper code would throw:

UndefinedError: 'step' is undefined

score 12 · Answer 4 · edited Mar 24 '13 at 23:20

12

In the Environment setup, we had undefined = StrictUndefined, which prevented undefined values from being set to anything. This fixed it:

from jinja2 import Undefined
JINJA2_ENVIRONMENT_OPTIONS = { 'undefined' : Undefined }

edited Mar 24 '13 at 23:20

Nathan

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answered Oct 01 '10 at 20:13

freyley

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Lubomir Varga · Answer 5 · 2018-07-20T04:27:46.060

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Consider using default filter if it is what you need. For example:

{% set host = jabber.host | default(default.host) -%}

or use more fallback values with "hardcoded" one at the end like:

{% set connectTimeout = config.stackowerflow.connect.timeout | default(config.stackowerflow.timeout) | default(config.timeout) | default(42) -%}

edited Jul 20 '18 at 04:27

answered Mar 08 '16 at 12:18

Lubomir Varga

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score 7 · Answer 6 · answered Sep 18 '22 at 08:38

7

You can use kind of Jinja Elvis operator

{{ 'OK' if variable is defined else 'N/A' }}

or additionally check emptiness

{{ 'OK' if (variable is defined and variable) else 'N/A' }}

Jinja templates - Template Designer Documentation

answered Sep 18 '22 at 08:38

Michal Zmuda

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score 1 · Answer 7 · answered Feb 12 '14 at 01:14

{% if variable is defined %} works to check if something is undefined.

You can get away with using {% if not var1 %} if you default your variables to False eg

class MainHandler(BaseHandler):
    def get(self):
        var1 = self.request.get('var1', False)

score 1 · Answer 8 · answered Jun 06 '21 at 06:49

I had an issue like this in Ansible. Ended up having to do a test on both @Garret and @Carsten / @azalea answers, so:

{% if variable is defined and variable %}
    value of variable: {{ variable }}
{% else %}
    variable is not defined or is falsy
{% endif %}

score -3 · Answer 9 · answered Sep 29 '22 at 15:58

-3

You can do this :

- debug: 
    msg: "Executed only if variable is defined"
  when:
    - variable is defined

answered Sep 29 '22 at 15:58

Néhémie

1

Welcome to SO, it would be useful for anyone reading your answer if you provided more context on what each line is doing and why. – GoodJuJu Oct 07 '22 at 08:29
1

This is for Ansible; it has _nothing_ to do with Jinja. – bfontaine Mar 02 '23 at 10:51

In Jinja2, how do you test if a variable is undefined?

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