if

Selects an argument depending on a condition.

For example:

{{ value|if:"yes":"no" }}

This is a shortcut for using the tag#if tag. The same can be expressed as follows:

{% if value %}yes{% else %}no{% endif %}

Note that falsy values (0, false, undefined or empty string) evaluate to false.

In templates, the compiler evaluates the condition once and only evaluates the selected argument. The other argument is not evaluated, so a model lookup in that branch does not run. Both arguments are required; use undefined when the false branch should produce no value.

Combine the filter with as to reuse the selected value:

{% if show_image|if:m.media[id]:undefined as medium %}
    {{ medium.mime|escape }}
{% endif %}

The media model is only queried when show_image is true. The block is rendered when the selected value is truthy, and medium contains that value.

This lazy evaluation applies to compiled templates. Direct Erlang calls to filter_if:'if'/4 follow normal Erlang evaluation: all arguments are evaluated before the function is called.

Elaborate examples

{% with is_i18n|if:r.translation[lang_code].body:r.body as body %}

So if is_i18n evaluates to true, body is assigned to r.translation[lang_code].body, else to r.body.

{% include "_language_attrs.tpl" id=pid class=(pid==id)|if:"active":"" %}

Add parameter class to the included template; when pid equals id, class is "active", otherwise an empty string.