In Jinja2, How Do You Test If a Variable Is Undefined?

{% 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.

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

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 }

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) -%}

{% 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)

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 %}