Add optional {% default %} clause to {% for %} template tag

[Jannis Leidel]

As ​requested by Simon I propose an {% else %} tag for the {% for %} template tag which could be used like this:

from django.template import *
>>> t1 = Template("""
    {% load mytags %}
    {% for athlete in athlete_list %}
        {{ athlete }} 
    {% else %}
        No athlete in list!
    {% endfor %}
    """)
>>> c1 = Context(
    {'athlete_list':
        ['me', 'myself', 'I']
    })
>>> t1.render(c1)
u'me myself I '
>>> c2 = Context({})
>>> t1.render(c2)
u'No athlete in list!'

For a standalone version look at ​http://www.djangosnippets.org/snippets/546/

[Jannis Leidel]

First implementation, including documentation

[Rob Hudson <treborhudson@…>]

The same be accomplished by this:

{% if athlete_list %}
  {% for athlete in athlete_list %}
    ...
  {% endfor %}
{% else %}
  Nothing in list
{% endif %}

[anonymous]

Such behaviour can be confusing as it is not similar to python

for object in objects:
    if object:
        print 'Found'
        break
else:
    print 'Not found'

pattern.

[Adrian Holovaty]

The idea gets a +1 from me.

[Matthias Pronk <django@…>]

I wonder how useful this new syntax is, as you can't use it for the following (pretty common) pattern:

{% if athlete_list %}
  <ul>
  {% for athlete in athlete_list %}
  <li>{{ athlete.name }}</li>
  {% endfor %}
  </ul>
{% else %}
  <p>Nothing in list</p>
{% endif %}

(Because there is no place where you can put the <ul> tags.)

Plus it doesn't comply with the Python syntax (as Anonymous@20:51 says).

[Jannis Leidel]

Thanks Adrian! Matthias, I think it's still worthwhile and useful if you use:

<ul>
  {% for athlete in athlete_list %}
    <li>{{ athlete.name }}</li>
  {% else %}
    <li class="emptylist">Nothing in list</li>
  {% endfor %}
</ul>

I don't see a reason why compliance with the Python syntax is required for a template tag.

[Simon Willison]

There's a good discussion about this on my blog, where Fredrik points out that this would be semantically different from how Python handles else: blocks attached to for statements. As such, the extra block would need a different name - {% default %} was suggested.

[Simon Willison]

Forgot the link: ​http://simonwillison.net/2008/Jan/16/django/#comments

[Jannis Leidel]

Second implementation, including documentation

[Jannis Leidel]

Ok, this sounds like a reasonable compromise. I changed it to having an optional {% default %} clause.

[Jannis Leidel]

The working example is:

>>> from django.template import *
>>> t = Template("""
    {% for athlete in athlete_list %}
        {{ athlete }} 
    {% default %}
        No athlete in list!
    {% endfor %}
    """)
>>> c1 = Context(
    {'athlete_list':
        ['me', 'myself', 'I']
    })
>>> t.render(c1)
u'me myself I '
>>> c2 = Context({})
>>> t.render(c2)
u'No athlete in list!'

[Chris Beaven]

Still a reference to else in the patch (in a docstring), and needs tests.

I'd suggest dropping the "As you can see, " in the docs and also to mention that the default node is displayed if the given array is empty or not found (emphasis for the bit that I'm suggesting adding).

Fix those and it's good to go.

[Jannis Leidel]

Third implementation, including documentation and tests, fixing some issues, thanks SmileyChris!

[Jannis Leidel]

Ouh, am I actually allowed to change the ticket properties?

[Chris Beaven]

If you're confident that you've fixed the issue, sure. It doesn't change it's status so that is alright.

Final thoughts:

• Perhaps make nodelist_default optional in case someone is using ForNode for thier own devices? def __init__(self, loopvars, sequence, is_reversed, nodelist_loop, nodelist_default=None): (and then move the empty NodeList generation into their too)

• You should have a test for the actual working case (i.e. non-empty list)

• You need an empty line after "New in Django development version"

[Jannis Leidel]

Ok, sounds all reasonable, except one thing: Do you mean with "the empty NodeList generation" the following?

class ForNode(Node):
    ...
    def render(self, context):
        nodelist = NodeList() # <-- this line?
    ...

[Chris Beaven]

Yea, that logic obviously needs to move if nodelist_default could be None in __init__

[Jannis Leidel]

Fourth implementation, including documentation and tests. Do you think this is sufficient? Maby I still don't see your point :/

[Jannis Leidel]

Is the last patch what you meant?

[Chris Beaven]

Close but not quite, you need something like this:

lass ForNode(Node):
    def __init__(self, loopvars, sequence, is_reversed, nodelist_loop, nodelist_default=None):
        self.loopvars, self.sequence = loopvars, sequence
        self.is_reversed = is_reversed
        self.nodelist_loop = nodelist_loop
        if nodelist_default is None:
            self.nodelist_default = NodeList()
        else:
            self.nodelist_default = nodelist_default

[Jannis Leidel]

fifth implementation with optional nodelist_default, including documentation and tests.

[Jannis Leidel]

Updated patch to Django r9032 (​http://github.com/jezdez/django/tree/ticket6398)

[Jacob]

(In [9530]) Fixed #6398: added an optional {% empty %} clause to the {% for %} template tag. The contents of this clause are rendered if the list iterated over turns out to be empty. Thanks, Jannis Leidel.

Astute readers will notice that the patch originally called this default; after consideration I decided that empty is a very slightly better color for this particular bikeshed.

[Stein Magnus Jodal]

As far as I can see, the conversion from 'default' to 'empty' in the patch is not complete. At least, the tests seems broken, as they use "{% default %}" and not "{% empty %}".

Check out the following in #9530:

• django/trunk/django/template/defaulttags.py lines 682 and 684 (and optionally 'nodelist_default' at 685, 688-689), and
• django/trunk/tests/regressiontests/templates/tests.py lines 487-489.

[Russell Keith-Magee]

This problem was fixed in [9532], which was committed a few hours after [9530].

[(none)]

Milestone post-1.0 deleted