Check if an array is not empty in Jinja2

Question

I need to check if the variable texts is defined or not in index.html.

If the variable is defined and not empty then I should render the loop.

Answer 1

It's possible that texts could be defined but contain a single list element which is an empty string; For example:

texts = ['']

In this case, testing if texts is defined will produce a true result so you should test the first element instead:

{% if texts[0] != '' %}
    ..code here..
{% endif %}

You might also want to combine that with the |length filter to make sure it only has one element.

Answer 2

This is a neat and simple solution that worked well for me!

{% if texts is defined and texts[0] is defined %}
    ...
{% endif %}

Answer 3

I think your best bet is a combination of defined() check along with looking at the length of the array via length() function:

{% if texts is defined and texts|length > 0 %}
    ...
{% endif %}

Answer 4

As mentioned in the documentation, you could also write:

{% for text in texts %}
    <div class="post">
        <div class="post-title">{{text.subject}}</div>
        <pre class="post-content">{{text.content}}</pre>
    </div>
{% else %}
    <div>{{ error }}</div>
{% endfor %}

It handles both the case where texts is undefined, and the case where texts is empty.

Answer 5

Take a look at the documentation of Jinja2 defined(): http://jinja.pocoo.org/docs/templates/#defined

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

Is it clear enough? In your case it could look like this:

{% if texts is defined %}
    {% for text in texts %} 
        <div>{{ error }}</div>
        <div class="post">
            <div class="post-title">{{ text.subject }}</div>
            <pre class="post-content">{{ text.content }}</pre>
        </div>
    {% endfor %}
{% else %}
    Error!
{% endif %}

Answer 6

This is what worked for my use case in my Django app:

I needed to pass a queryset as context to an html template and display the block only if the queryset had values

Queryset:

events = Event.objects.filter(schedule_end__gte=date.today()).order_by('-created_at')

Passed context dictionary as follows:

{ "events" : events }

HTML template

{% if events %}
  <h3>Upcoming Events</h3>
  <ul>
    {% for event in events %}
    <li><h4>{{ event.title }}</h4></li>
    {% endfor %}
  </ul>
{% endif %}