Add an `as X` option to the `for` template tag that captures loop output as a list

[meesterguyman]

Code of Conduct

• I agree to follow Django's Code of Conduct

Feature Description

It's currently possible to capture the output of many template tags by appending as X at the end. This captures the output as a singular value for use later in the template. I would like to propose that we add a similar option to the for template tag and capture the output of the loop as a list of individual values, each of which can be further manipulated and passed to other templates for display.

Problem

In generating responsive columns for newsletters, we're passing in a queryset of posts. I would like to be able generate the display contents for each column within the template, save the column displays as a list of displays, inject it into template context, and then pass this list into a columns.html template, which would be responsible for arranging these into a standardized column format. I would expect this to look something like the following:

{% for post in posts as post_display_list %}
  <div class="photo">{{ post.photo }}</div>
  <div class="title">{{ post.title }}</div>
  <div class="desc">{{ post.desc }}</div>
{% endfor %}
{% include 'newsletters/layout/columns.html' column_contents=post_display_list column_widths=200 %}

Presently, the only way I know to do what I'm after here cleanly is to generate post_display_list programmatically within a view and then pass it in as context, but this is not at all ideal. Because we're dealing with presentation here, it would be much better to have the HTML viewable inline with the surrounding template, so that for more complex cell layouts especially, it's easier to understand and update. By capturing the output as a list of values instead of one big string, it can be passed into another template for further manipulation.

I'm considering building this myself one way or another, but this seems an intuitive enough addition to the for tag that I would think it would be useful for everyone, so thought I'd mention it here as a feature request.

Request or proposal

proposal

Additional Details

No response

Implementation Suggestions

No response

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[martinberoiz]

I think you can do that with the with tag

{% with post_display_list=posts %}
{% for post in post_display_list %}
  <div class="photo">{{ post.photo }}</div>
  <div class="title">{{ post.title }}</div>
  <div class="desc">{{ post.desc }}</div>
{% endfor %}
{% include 'newsletters/layout/columns.html' column_contents=post_display_list column_widths=200 %}
{% end with %}

( I didn't try this example, not sure if it works)

[meesterguyman]

That will just print out the column contents for each post in place. The idea here is to capture the output of the for loop without printing it, so that it can be passed into another template as a list (as opposed to a singular value).

[UnknownPlatypus]

https://docs.djangoproject.com/en/6.0/ref/templates/builtins/#make-list could this help?

[meesterguyman]

That looks like it's meant to split strings into individual characters. What I'm proposing here would translate a queryset or other list of things into a separate list, with each item in that list modified for presentation. By adding this to a for loop within a template, these modifications can be housed entirely within the template, which is really where they belong. Not seeing any way to do this with built in tags.

[tim-schilling]

I have a quick clarification questions @meesterguyman, if posts is an iterable such as range(1000), post_display_list should effectively end up as list(range(1000))?

[meesterguyman]

If posts is a list of range 1000 and was put through this loop:

{% for post in posts as post_display_list %}
  <b>{{ post }}</b>
{% endfor %}

then post_display_list would end up being the following:
['<b>0</b>', '<b>1</b>', '<b>2</b>', '<b>3</b>' . . . ]

In short, every item of the list would have its corresponding HTML display, which could then be passed on to a template expecting a list of HTML displays.

[codingjoe]

@meesterguyman what's stopping you from using partialdef in those places?

[LilyFirefly]

I think you can achieve this with a custom simple_block_tag:

from django import template

register = template.Library()


@register.simple_block_tag(takes_context=True)
def withblock(context, content, name):
    context[name] = content
    return content  # or return ""

{% withblock name="post_display_list" %}
{% for post in posts %}
  <div class="photo">{{ post.photo }}</div>
  <div class="title">{{ post.title }}</div>
  <div class="desc">{{ post.desc }}</div>
{% endfor %}
{% endwithblock %}
{% include 'newsletters/layout/columns.html' column_contents=post_display_list column_widths=200 %}

[codingjoe]

I think you can achieve this with a custom simple_block_tag:

An inclusion-tag is probably well suited too.

[meesterguyman]

The block tag example above doesn't work, the reason being, the columns.html template is expecting post_display_list to be a list. The block tag above captures the entire thing as a string, which would require splitting the string up to process. The point of the construct that I've suggested is that the output of every iteration of the for loop is captured as its own separate list item, so that it's ready to be passed as a list into what's next.

The partialdef method would ALMOST work. Suppose that my columns.html template contained the following:

{% partialdef cell_contents %}
  <div class="photo">{{ post.photo }}</div>
  <div class="title">{{ post.title }}</div>
  <div class="desc">{{ post.desc }}</div>
{% partialdef %}

{% for post in posts %}
  <div class="column">{% partial cell_contents %}</div>
{% endfor %}

This would work for passing in a queryset of post instances. BUT it would only work for posts, since the partial def hard codes posts into its contents. What if I want to make columns out of something that isn't a post? I'd have to make a new columns-other.html template to accommodate that. And something else? Yet another template.

But what if we a more general partialdef like the following:

{% partialdef cell_contents %}
  <div class="photo">{{ photo }}</div>
  <div class="title">{{ title }}</div>
  <div class="desc">{{ desc }}</div>
{% partialdef %}

That would work for anything with a photo, title, and description. But what if I want the contents to be entirely different? Again, I'd have to multiply column templates.

What I've suggested with the for loop doesn't depend on definitions. It allows you to output and capture in a list literally anything from any kind of object and then pass it on. In my use case, this means a single standard columns.html template rather than a bunch of different per-model or per-layout column templates, all of which would have to be maintained separately.

I still think making this simple addition to the for loop is the way to go. It adds something that cannot be achieved in any other way, is intuitive to use, and will work for all varieties of different content.

[LilyFirefly]

Ah, try this:

from django import template

register = template.Library()


@register.simple_block_tag(takes_context=True)
def withappend(context, content, name):
    context.setdefault(name, []).append(content)
    return content

{% for post in posts %}
{% withappend name="post_display_list" %}
  <div class="photo">{{ post.photo }}</div>
  <div class="title">{{ post.title }}</div>
  <div class="desc">{{ post.desc }}</div>
{% endwithappend %}
{% endfor %}
{% include 'newsletters/layout/columns.html' column_contents=post_display_list column_widths=200 %}

[meesterguyman]

Looks like that will accomplish what I'm after. The question then becomes, syntactically, does it make intuitive sense to make this functionality native to the for loop. We have:

{% for post in posts %}{% withappend name="post_display_list" %}
  <div>{{ post.title }}</div>
{% endwithappend %}{% endfor %}

As opposed to:

{% for post in posts as post_display_list %}
  <div>{{ post.title }}</div>
{% endfor %}

They would do the same thing, but the latter is cleaner and wouldn't require any additional tags. Clearly not an urgent addition, but still, in my opinion, an intuitive one.