How to test for a List in Jinja2?

Question

As far as I can see, there is no way to test if an object is a List instance in Jinja2.

Is that correct and has anyone implemented a custom test/extensio

Answer 1

In my setup, I'd like for a value to either be a string or list of strings coming into the Jinja template. So really what I cared about wasn't string vs list, but single item vs multiple items. This answer might help if your use case is similar.

Since there isn't a built-in test for "is list?" that also rejects strings, I borrowed a pattern from API design and wrapped the single objects in a list on the Python side then checked list length on the Jinja side.

Python:

context = { ... }

# ex. value = 'a', or ['a', 'b']
if not isinstance(value, list):
    value = [value]

context['foo'] = value

Jinja:

{% if foo|length == 1 %}
  single-item list
{% elif foo|length > 1 %}
  multi-item list
{% endif %}

And if all you want to do is add an item separator for display purposes, you can skip the explicit length check and just {{ value|join(', ') }}.