(Django) A simple template tag to add an 'active' class to anchor tags

active_tag.py

from django import template
from django.core.urlresolvers import reverse

register = template.Library()

@register.simple_tag
def add_active(request, name, by_path=False):
    """ Return the string 'active' current request.path is same as name
    
    Keyword aruguments:
    request  -- Django request object
    name     -- name of the url or the actual path
    by_path  -- True if name contains a url instead of url name
    """
    if by_path:
        path = name
    else:
        path = reverse(name)

    if request.path == path:
        return ' active '

    return ''

gistfile1.mustache

{% load active_tag %}
{% comment %} Add active class by url name {% endcomment %}
<a class="{% add_active request 'url_name' %}" href="{% url url_name %}">Cars</a>

{% comment %} Add active class by url path {% endcomment %}
<a class="{% add_active request '/some/django/path' 1 %}" href="{% url url_name %}">Cars</a>

Nowadays you can also use
@register.simple_tag(takes_context=True) and get the request, via the context.

Do small modification to your code so it can accept "kwargs" in url

def add_active(request, name, pk='', by_path=False):
          if by_path:
             path = name
          elif group:
             path = reverse(name, kwargs={'pk': pk})
          else :
             path = reverse(name)


          if request.path == path:
              return " active "
 
    return ""

<a class="{% add_active request 'url_name' 'pk' %}" href="{% url url_name pk %}">Cars</a>