Give 'firstof' tag ability to assign result to context

[Tim Valenta]

When you need to access fields on objects in a {% firstof %} fashion, semantically 'optional' fields can cause issues:

    Today the teacher is {% firstof substitute.name teacher.name %},
    and drives a {% firstof substitute.car teacher.car %}

If substitute doesn't have a value for car , then it will fall back to teacher.car .

The real issue is that {% firstof %} cannot preserve its value for later multiple inspections. If the {% firstof %} tag could assign its result to a context variable, it would solve this problem:

    {% firstof substitute teacher as teacher_for_today %}
    Today the teacher is {{ teacher_for_today }}
    {% if teacher_for_today.car %}
        and drives a {{ teacher_for_today.car }}
    {% endif %}

I am unsure that this is possible to achieve without the as syntax. One would have to code a template tag or filter for the sole purpose of duplicating the exact functionality that I propose be added to {% firstof %} .

Attached patch for functionality and documentation on the subject.

[Tim Valenta]

Patch to defaulttags.py and templatetag docs

[Alex Robbins]

I worked on this ticket during the sprint on 12/12/09.
There was initially some concern that this ticket would accomplish the same thing as the proposed capture tag. They would do exactly the same thing though.

Jacob is -0 on this ticket since we are making the tag a switch statement, but didn't want to veto it without discussion.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Aymeric Augustin]

I don't know what the "capture" tag discussed 3 years ago was, but AFAIK it didn't come into existence.

This feature can be useful and it's still quite restricted -- it isn't a general switch statement.

[mark0978@…]

There is another instance where the AS keyword would be very useful

Trying to write a template to include something for xyz: in json, but i can't put {% firstof %} output thru escapejs as it currently stands.

xyz: '{% firstof a b %}',

with AS, we could do:

{% firstof a b AS c %}
xyz: '{{ c|escapejs }}',

Right now to do this you end up with a logic block in the template.

  {% if a %}{{ a|escapejs }}{% else %}{{ b|escapejs }}{% endif %}

which isn't horrible until you only want a or b if EITHER of them is defined and then it looks like:

  {% if a or b %}xyz: '{% if a %}{{ a|escapejs }}{% else %}{{ b|escapejs }}{% endif %}',{% endif %}

while it could be as simple as:

{% firstof a b AS c %}{% if c %}xyz: '{{ c|escapejs }}',{% endif %}

[Tim Graham <timograham@…>]

In 75bc5bc6:

Fixed #12199 -- Added the ability to use "as" with the firstof template tag.