Variable context between two blocks in Django templates?

Question

I have two blocks that call the same method with same variables. I want to call the method only once, but the result is then outsite the scope of the block tags. I've tried calling this method in the parent template header.html and with a with tag, but nothing seems to work.

This is the layout:

{% extends "header.html" %}

{% load navigation_tags %}

{% block header %}
    {% get_section site=site as section %}
    {% include "foobar.html" with section=section %}
{% endblock header %}

{% block navigation %}
    <nav>
        <div class="container">
            {% get_section site=site as section %}
            {% navigation section.slug %}
        </div>
    </nav>
{% endblock navigation %}

navigation_tags.py

@register.assignment_tag
def get_parent_section(site):
    if site.id == settings.FOOBAR_SITE_ID:
        section = Section.objects.get(id=settings.FOOBAR_SECTION_ID)
    else:
        # This is also a section instance.
        return site.default_section

Answer 1

As mentioned by 2pacho in another answer and Fernando Cezar in a comment, the easiest way to share values between different sections is to set it in the template context. If you are using the render shortcut function, you can pass a dict as the context parameter to add a value to the rendering context of the template. That would be a good place to add it and this would be the easiest place to put it.

return render(request, 'template.html', {'section': get_parent_section(site)})

However, if for some reason, you can't include it in the context, you can use a decorator to add memoization to your function, so that it will cache the computation results and return it immediately when called with the same parameters. You can use functools.lru_cache to do so, or it's Django backport at django.utils.lru_cache.lru_cache if you are using Python 2.x.

@register.assignment_tag
@functools.lru_cache()
def get_parent_section(site):
    if site.id == settings.FOOBAR_SITE_ID:
        section = Section.objects.get(id=settings.FOOBAR_SECTION_ID)
    else:
        # This is also a section instance.
        return site.default_section