How to output loop.counter in python jinja template?

Question

I want to be able to output the current loop iteration to my template.

According to the docs, there is a loop.counter variable that I am trying to use:

<ul>
{% for user in userlist %}
  <li>
      {{ user }} {{loop.counter}}
  </li>
      {% if loop.counter == 1 %}
          This is the First user
      {% endif %}
{% endfor %}
</ul>

But is being outputed to my template. What is the correct syntax?

Ardoin

Answer 1

The counter variable inside the loop is called loop.index in Jinja2.

>>> from jinja2 import Template

>>> s = "{% for element in elements %}{{loop.index}} {% endfor %}"
>>> Template(s).render(elements=["a", "b", "c", "d"])
1 2 3 4

In addition to loop.index, there is also

• loop.index0 (index starting at 0)
• loop.revindex (reverse index; ending at 1)
• loop.revindex0 (reverse index; ending at 0)
• Even more at http://jinja.pocoo.org/docs/templates/.

Swabber

What if there is nested-for loop? How will it work there? How to access outer for loop's index when inside of the nested for? – Ardenardency

It is possible to store the outer loop index in a variable. See: https://mcmap.net/q/109824/-get-loop-index-of-outer-loop – Ideomotor

Answer 2

Inside of a for-loop block, you can access some special variables, such as loop.index (but not loop.counter). From the official docs:

Variable	Description
loop.index	The current iteration of the loop. (1 indexed)
loop.index0	The current iteration of the loop. (0 indexed)
loop.revindex	The number of iterations from the end of the loop (1 indexed)
loop.revindex0	The number of iterations from the end of the loop (0 indexed)
loop.first	True if first iteration.
loop.last	True if last iteration.
loop.length	The number of items in the sequence.
loop.cycle	A helper function to cycle between a list of sequences.
loop.depth	Indicates how deep in a recursive loop the rendering currently is. Starts at level 1
loop.depth0	Indicates how deep in a recursive loop the rendering currently is. Starts at level 0
loop.previtem	The item from the previous iteration of the loop. Undefined during the first iteration.
loop.nextitem	The item from the following iteration of the loop. Undefined during the last iteration.
loop.changed(*val)	True if previously called with a different value (or not called at all).

Uturn

Answer 3

If you are using Django, use forloop.counter instead of loop.counter:

<ul>
{% for user in userlist %}
  <li>
      {{ user }} {{forloop.counter}}
  </li>
      {% if forloop.counter == 1 %}
          This is the First user
      {% endif %}
{% endfor %}
</ul>

Ascot

I am getting jinja2.exceptions.UndefinedError: 'forloop' is undefined – Torment

use loop with jinja; here is the documentation. forloop is for django templates. – Ascot

The question is clearly asking about jinja, why are we giving answers about Django templates here... – Infinitude

Answer 4

Real-life example:

{% for image in item['images'] %}
    {% set image_id = item_id ~ '-preview-' ~ loop.index0 %}
    <div id="{{ image_id }}" class="overlay">
        <a class="cancel" href="#{{ item_id }}"></a>
        <div class="popup">
            {% set src = image if image.startswith('http') else '/static/images/store/' ~ item_id ~ '/' ~ image %}
            <a href="{{ src }}"><img class="modal-img" src="{{ src }}"/></a>
        </div>
    </div>
{% endfor %}

Brahui