get_list_value - Customtag (for Django)

get_list_value.py

from django import template
from django.template import Node, resolve_variable
from django.template.base import Variable

__author__ = 'martinsandstrom'

register = template.Library()

"""
This custom tag appends values onto a get var, with a delimiter. Like a list.
Such as this: marteinn.se/?cakes=strawberry,chocolate,apple

Import using this: {% load get_list_value %}

Usage:
{% toggle_get_list_value cakes='apple' %}
If value apple is found on get param cakes, remove it, otherwise add it.

{% add_get_list_value cakes='strawberry' %}
Add the value strawberry onto variable cakes.

{% remove_get_list_value cakes='apple' %}
Remove the value apple, if found that is.

I also added a filter for those occasions you would like to validate if
the value exist. (Although you should probably not do it in the template).
{{ request.GET|contains_get_list_value:"cakes,pie" }}

And something a little more advanced.
{% with request.GET|contains_get_list_value:"cakes,strawberry" as found %}
    <a href="{% toggle_get_list_value cakes='strawberry' %}"
       class="{% if found %}active{% endif %}">Strawberry</a>
{% endwith %}
"""


class GetListParameterManager(Node):
    toggle = False
    add = False
    remove = False

    def __init__(self, values, toggle=False, add=False, remove=False):
        self.values = values
        self.toggle = toggle
        self.add = add
        self.remove = remove

    def render(self, context):
        req = resolve_variable('request',context)
        params = req.GET.copy()
        for key, value in self.values.items():

            value = Variable(value).resolve(context)
            value_list = []

            if params.get(key):
                orig_value = params.get(key)
                value_list = orig_value.split(",")

            if (self.add or self.toggle) and not value in value_list:
                value_list.append(value)
            elif (self.remove or self.toggle) and value in value_list:
                value_list.remove(value)

            parsed_value=",".join(value_list)
            if parsed_value:
                params[key] = parsed_value
            else:
                params.pop(key)

        urlencoded_params = params.urlencode(safe=",")
        return '?%s' %  urlencoded_params

def _prepare_values(parses, token):
    from re import split
    contents = split(r'\s+', token.contents, 2)[1]
    pairs = split(r',', contents)

    values = {}

    for pair in pairs:
        s = split(r'=', pair, 2)
        values[s[0]] = s[1]

    return values

@register.tag
def add_get_list_value(parser, token):
    values = _prepare_values(parser, token)
    return GetListParameterManager(values, add=True)

@register.tag
def remove_get_list_value(parser, token):
    values = _prepare_values(parser, token)
    return GetListParameterManager(values, remove=True)

@register.tag
def toggle_get_list_value(parser, token):
    values = _prepare_values(parser, token)
    return GetListParameterManager(values, toggle=True)

@register.filter(name="contains_get_list_value")
def contains_get_list_value(request_params, get_list_raw):
    get_list = get_list_raw.split(",")
    name = get_list[0]
    value = get_list[1]

    if request_params.get(name):
        list = request_params.get(name).split(",")
        if value in list:
            return True
        return False

    return False