Conditional content of template blocks

[while0pass]

It will be very good if there is some possibility to mark sort of "conditional content" of the template blocks ({% block .. %}), that is the content that is displayed only if in a child template the block has some content. For instance, we have 2 templates:

parent.html

<table>
  <tr>
     <td>{% block firstcol %}{% endblock %}</td>
     <td>{% block secondcol %}{% endblock %}</td>
     {% block thirdcol %}<td>{% blockcontent %}</td>{% endblock %}
  </tr>
</table>

child.html

{% extends 'parent.html' %}

{% block firstcol %} 1 {% endblock %}
{% block firstcol %} 2 {% endblock %}

We should have such an output text:

<table>
  <tr>
     <td> 1 </td>
     <td> 2 </td>
  </tr>
</table>

but not the following:

<table>
  <tr>
     <td> 1 </td>
     <td> 2 </td>
     <td></td>
  </tr>
</table>

[while0pass]

typo: second "firstcol" should be "secondcol", like this:

child.html

{% extends 'parent.html' %}

{% block firstcol %} 1 {% endblock %}
{% block secondcol %} 2 {% endblock %}

[Andy Durdin]

Patch against current trunk

[Andy Durdin]

This scratches an itch for me too, so I thought I’d implement it.

My patch in ticket9173.diff (against r16253) implements a new tag, ifnotempty, with the following behaviour:

    Renders only if at least one variable or template tag within also renders.

    This allows a variable or template tag to be surrounded by markup, with
    the markup omitted if the variable or template tag does not render.

    An `{% else %}` block may also be used.

    In the following example, the section will not be rendered unless another
    template extends this one and overrides the `more_information` block::

        {% ifnotempty %}
            <section>
                <h1>More information</h1>
                {% block more_information %}{% endblock %}
            </section>
        {% endifnotempty %}

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Jacob]

I'm wholly unconvinced by the syntax proposed so far; it seems quite complicated and non-intuitive. However, I'm not opposed to the idea in general. So marking "someday" -- if someone comes up with syntax that makes sense and is easy to explain, then we might consider this.

[FunkyBob]

I ended up implementing a {% wrapif %} tag, which will render the [optional] head and tail blocks if the body renders as a non-blank [not all whitespace] string.

{% wrapif %}
<td>
{% body %}
{% block whatever %}{% endblock %}
{% tail %}
</td>
{% endwrapif %}

If body is omitted, the first block is assumed to be the body, and the head empty.
If tail is omitted, it is considered empty.

I mostly did this to help move container tags out of for loops, where they'd be inside a {% if forloop.first %} and thus cause a test every iteration.

[Markus Amalthea Magnuson]

I was thinking about this the other day. How about something like:

parent.html

<table>
  <tr>
    <td>{% block firstcol %}{% endblock %}</td>
    <td>{% block secondcol %}{% endblock %}</td>
    {% block thirdcol if block.child %}<td>{{ block.child }}</td>{% endblock %}
  </tr>
</table>

Which would introduce a {{ block.child }} thing similar to {{ block.super }}

And also conditional blocks in general, in which you can specify:

{% block title if post.title %}My title{% endif%}

Which is the equivalent of, but shorter than:

{% block title %}
  {% if post.title %}
    {{ post.title }}
  {% else %}
    {{ block.super }}
  {% endif %}
{% endblock %}

What do you think of all the above?