Template tag for Django : Regroup list of lists by index, exactly like the core {% regroup %} tag.

regroup_tag.py

from itertools import groupby
from django import template
from django.template.base import TemplateSyntaxError


register = template.Library()


class RegroupListNode(template.Node):
    def __init__(self, target, expression, var_name):
        self.target, self.expression = target, expression
        self.var_name = var_name

    def render(self, context):
        obj_list = self.target.resolve(context, True)
        if obj_list is None:
            # target variable wasn't found in context; fail silently.
            context[self.var_name] = []
            return ''
        sortkeyfn = lambda s: s[0]
        obj_list.sort(key=sortkeyfn)
        result = []
        for key, valuesiter in groupby(obj_list, key=sortkeyfn):
            result.append(dict(grouper=key, list=list(valuesiter)))
        context[self.var_name] = result
        return ''


def regrouplist(parser, token):
    """
    Just like `django.template.defaulttags.regroup`
    but for list of lists.

    Let's say i have these datas : 

        table_datas = [
            ['Mr', 'Jacques', 'CHIRAC'],
            ['Mr', 'Bill', 'CLINTON'],
            ['Mme', 'Carla', 'BRUNI AU SOLEIL']
        ]

    I use this to regroup on index 0 : 

        {% regrouplist table_datas by 0 as gender_list %}

    To get something like :

        "Mr":
            ['Mr', 'Jacques', 'CHIRAC'],
            ['Mr', 'Bill', 'CLINTON'],
        "Mme":
            ['Mme', 'Carla', 'BRUNI AU SOLEIL']

    """
    firstbits = token.contents.split(None, 3)
    if len(firstbits) != 4:
        raise TemplateSyntaxError("'regrouplist' tag takes five arguments")
    target = parser.compile_filter(firstbits[1])
    if firstbits[2] != 'by':
        raise TemplateSyntaxError("second argument to 'regrouplist' tag must be 'by'")
    lastbits_reversed = firstbits[3][::-1].split(None, 2)
    if lastbits_reversed[1][::-1] != 'as':
        raise TemplateSyntaxError("next-to-last argument to 'regrouplist' tag must"
                                  " be 'as'")

    expression = parser.compile_filter(lastbits_reversed[2][::-1])

    var_name = lastbits_reversed[0][::-1]
    return RegroupListNode(target, expression, var_name)
regrouplist = register.tag(regrouplist)