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.

From the Jinja2 template designer documentation:

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

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