Django Render Block

Render the content of a specific block tag from a Django template. Works for arbitrary template inheritance, even if a block is defined in the child template but not in the parent. Generally it works like render_to_string from Django, but allows you to specify a block to render.

Features

• Render a specific block from a template
• Fully supports the Django templating engine
• Partially supports the Jinja2 engine: it does not currently process the extends tag.

Requirements

Django Render Block supports Django 4.2, 5.0, and 5.1 on Python 3.8, 3.9, 3.10, 3.11 and 3.12 (see the Django documentation for which versions of Python are supported by particular Django versions).

Examples

In test1.html:

{% block block1 %}block1 from test1{% endblock %}
{% block block2 %}block2 from test1{% endblock %}

In test2.html:

{% extends 'test1.html' %}
{% block block1 %}block1 from test2{% endblock %}

And from the Python shell:

>>> from render_block import render_block_to_string
>>> print(render_block_to_string('test2.html', 'block1'))
'block1 from test2'
>>> print(render_block_to_string('test2.html', 'block2'))
'block2 from test1'

It can also accept a context as a dict (just like render_to_string), in test3.html:

{% block block3 %}Render this {{ variable }}!{% endblock %}

And from Python:

>>> print(render_block_to_string('test3.html', 'block3', {'variable': 'test'}))
'Render this test!'

API Reference

The API is simple and attempts to mirror the built-in render_to_string API.

render_block_to_string(template_name, block_name, context=None, request=None)

template_name

The name of the template to load and render. If it’s a list of template names, Django uses select_template() instead of get_template() to find the template.

block_name

The name of the block to render from the above template.

context

A dict to be used as the template’s context for rendering. A Context object can be provided for Django templates.

context is optional. If not provided, an empty context will be used.

request

The request object used to render the template.

request is optional and works only for Django templates. If both context and request are provided, a RequestContext will be used instead of a Context.

Exceptions

Like render_to_string this will raise the following exceptions:

TemplateDoesNotExists

Raised if the template(s) specified by template_name cannot be loaded.

TemplateSyntaxError

Raised if the loaded template contains invalid syntax.

There are also two additional errors that can be raised:

BlockNotFound

Raised if the block given by block_name does not exist in the template.

UnsupportedEngine

Raised if a template backend besides the Django backend is used.

Testing the context used by render_block

If you write tests with the test client, you typically use the test client's get() and post() methods to test your view code. The return value of these methods is a Response object. It is not the same as the HttpResponse object returned by your view. It has some additional data, such as the context that was used while rendering a template.

The function render_block() returns a BlockOfTemplateResponse object, which has been prepared to make the context available to the response in tests. However, its notify_block_render() method must be mocked so that it sends a specific signal. This signal is handled by the test client to add the context to the Response object.

One way to mock the notify_block_render() method is to use the following setup and tear-down code in your test classes:

from unittest.mock import patch
from django.test.signals import template_rendered

class TestYourCode(TestCase):

    def setUp(self):
        self.mock_method = patch(
            "render_block.BlockOfTemplateResponse.notify_block_render"
        ).start()
        self.mock_method.side_effect = lambda template, context: template_rendered.send(
            sender=None, template=template, context=context
        )

    def tearDown(self):
        self.mock_method.stop()

Assuming a view exists that uses render_block() and you want to test the context that was passed as parameter to render_block(), you can access the context in your tests, like this:

response = client.get(reverse("logbook:messages_overview"))
assert response.status_code == 200
assert response.context["messages"] == ["Disk is full.", "Uninstalled unused apps."]

Contributing

If you find a bug or have an idea for an improvement to Django Render Block, please file an issue or provide a pull request! Check the list of issues for ideas of what to work on.

Attribution

This is based on a few sources:

• Originally Django Snippet 769
• Updated version Django Snippet 942
• A version of the snippets was ported as Django-Block-Render
• Additionally inspired by part of django-templated-email
• Also based on a StackOverflow answer 2687173