from django import template
from main.models import Category

register = template.Library()

@register.tag(name="list_categories")
def list_categories(parser, token):
    return ListCategoriesNode()

class ListCategoriesNode(template.Node):
    def render(self, context):
        context['list_categories'] = Category.objects.all()
        return ''

{% load main_tags %}
<h2>категории</h2>
{% list_categories as cats %}
{% for cat in cats %}
<li>
<a href="{{ cat.get_absolute_url }}">{{ cat.title }}</a>
</li>
{% endfor %}

 
{% load main_tags %}
{% list_categories %}
{% for cat in list_categories %}
<li>
<a href="{{ cat.get_absolute_url }}">{{ cat.title }}</a>
</li>
{% endfor %}