Extend "for" tag to allow unpacking of lists

[Chris Beaven]

As discussed here: ​http://groups.google.com/group/django-developers/browse_thread/thread/5c0de4476c12029f

Basically, rather than having to do this:

{% for post in posts %}
 {{ post.0 }} - {{ post.1 }}
{% endfor %}

We should be able to do this:

{% for name, description in posts %}
 {{ name }} - {{ description }}
{% endfor %}

[Honza Král <Honza.Kral@…>]

first crude version of the patch

[Honza Král <Honza.Kral@…>]

the patch is more a proof of concept thing...
with the patch:

In [1]: from django import template

In [2]: t=template.Template('''{% for a,b in items %} {{ a }} -> {{ b }} {% endfor %}''')

In [3]: t.render( template.Context( { 'items' : [ ('aa',2), (3,4)] } ) )
Out[3]: ' aa -> 2  3 -> 4 '

In [4]: t=template.Template('''{% for a, b in items %} {{ a }} -> {{ b }} {% endfor %}''')

In [5]: t.render( template.Context( { 'items' : [ ('aa',2), (3,4)] } ) )
Out[5]: ' aa -> 2  3 -> 4 '

In [6]: t=template.Template('''{% for a , b in items %} {{ a }} -> {{ b }} {% endfor %}''')

In [7]: t.render( template.Context( { 'items' : [ ('aa',2), (3,4)] } ) )
Out[7]: ' aa -> 2  3 -> 4 '

what remains to be done:

• cleanup (error handling in particular)
• tests
• documentation

[Honza Král <Honza.Kral@…>]

sorry, a minor problem with the previous version

[Chris Beaven]

new and improved :)

[arnodel@…]

for_tag.3.patch (line 125)

context.update(dict(zip(self.loopvars, item)))

will push a new context dict into the current context, so it will need to be popped later.
It could be changed to:

context.dicts[0].update(zip(self.loopvars, item))

which puts the variables into the current context dict.

BTW I think this is a bug in the Context class:
Context.update(x) is defined as self.dicts = [x] + self.dicts
I think it should be defined as self.dicts[0].update(x)

That would allow this line in your patch to be:

context.update(zip(self.loopvars, item))

--
Arnaud

[arnodel@…]

Sorry

[Chris Beaven]

It looks like you're correct. According to the docstring of Context.update it "Pushes an entire dictionary's keys and values onto the context." but currently it doesn't. I'll open another ticket regarding this.

[Chris Beaven]

This patch doesn't need improvement, it just relies on bug #3529 being fixed.

[Chris Beaven]

If patch 2 in #3529 is chosen, this patch should be updated

from:
    context.update(dict(zip(self.loopvars, item))) 
to:
    context.update(dict(zip(self.loopvars, item)), push=False) 

[Chris Beaven]

Gosh, I can't even remember my own patch already had tests and docs :P

[Chris Beaven]

small bug fix and tighter format checking

[Chris Beaven]

The bug fix was variable values from the previous loop could sneak on to the context if the current set of items was shorter than the loopvars list - see test for-tag-unpack09.

The format checking has been tightened so that keyword arguments must be comma separated -- either {% for a,b in c %} or {% for a, b in c %} will work.

This patch now has no reliance on bug #3529 since it actually uses the push behaviour now. It was however brought up in discussion that this tag could be more efficient if context.update also accepted 2-length tuples, but that's outside of the scope of this ticket.

[Chris Beaven]

Oops, broke previous version just before uploading

[Russell Keith-Magee]

Slightly improved for loop unpacking implementation

[Russell Keith-Magee]

I've made a few additions to the patch. For the most part, it's documentation cleanup; but I have added a few more test cases for whitespace around the commas in the unpack list, and made the parser a little more forgiving with those whitespace cases (using RE's, rather than just str.replace).

Other than that, I'm happy for this to go in. I'll ping this on the list again, but if there's no objections, I'm happy for it to go into trunk.

[Russell Keith-Magee]

(In [5443]) Fixed #3523 -- Added list unpacking to for loops in templates. Thanks to SmileyChris and Honza Kral for their work.