TemplateResponse and SimpleTemplateResponse

Standard HttpResponse objects are static structures. They are provided with a block of pre-rendered content at time of construction, and while that content can be modified, it isn’t in a form that makes it easy to perform modifications.

However, it can sometimes be beneficial to allow decorators or middleware to modify a response after it has been constructed by the view. For example, you may want to change the template that is used, or put additional data into the context.

TemplateResponse provides a way to do just that. Unlike basic HttpResponse objects, TemplateResponse objects retain the details of the template and context that was provided by the view to compute the response. The final output of the response is not computed until it is needed, later in the response process.

SimpleTemplateResponse objects

class SimpleTemplateResponse

Attributes

SimpleTemplateResponse.template_name

The name of the template to be rendered. Accepts a backend-dependent template object (such as those returned by get_template()), the name of a template, or a list of template names.

Example: ['foo.html', 'path/to/bar.html']

SimpleTemplateResponse.context_data

The context data to be used when rendering the template. It must be a dict.

Example: {'foo': 123}

SimpleTemplateResponse.rendered_content

The current rendered value of the response content, using the current template and context data.

SimpleTemplateResponse.is_rendered

A boolean indicating whether the response content has been rendered.

Methods

SimpleTemplateResponse.__init__(template, context=None, content_type=None, status=None, charset=None, using=None, headers=None)

Instantiates a SimpleTemplateResponse object with the given template, context, content type, HTTP status, and charset.

template
A backend-dependent template object (such as those returned by get_template()), the name of a template, or a list of template names.
context
A dict of values to add to the template context. By default, this is an empty dictionary.
content_type
The value included in the HTTP Content-Type header, including the MIME type specification and the character set encoding. If content_type is specified, then its value is used. Otherwise, 'text/html' is used.
status
The HTTP status code for the response.
charset
The charset in which the response will be encoded. If not given it will be extracted from content_type, and if that is unsuccessful, the DEFAULT_CHARSET setting will be used.
using
The NAME of a template engine to use for loading the template.
headers
A dict of HTTP headers to add to the response.
SimpleTemplateResponse.resolve_context(context)

Preprocesses context data that will be used for rendering a template. Accepts a dict of context data. By default, returns the same dict.

Override this method in order to customize the context.

SimpleTemplateResponse.resolve_template(template)

Resolves the template instance to use for rendering. Accepts a backend-dependent template object (such as those returned by get_template()), the name of a template, or a list of template names.

Returns the backend-dependent template object instance to be rendered.

Override this method in order to customize template loading.

SimpleTemplateResponse.add_post_render_callback()

Add a callback that will be invoked after rendering has taken place. This hook can be used to defer certain processing operations (such as caching) until after rendering has occurred.

If the SimpleTemplateResponse has already been rendered, the callback will be invoked immediately.

When called, callbacks will be passed a single argument – the rendered SimpleTemplateResponse instance.

If the callback returns a value that is not None, this will be used as the response instead of the original response object (and will be passed to the next post rendering callback etc.)

SimpleTemplateResponse.render()

Sets response.content to the result obtained by SimpleTemplateResponse.rendered_content, runs all post-rendering callbacks, and returns the resulting response object.

render() will only have an effect the first time it is called. On subsequent calls, it will return the result obtained from the first call.

TemplateResponse objects

class TemplateResponse

TemplateResponse is a subclass of SimpleTemplateResponse that knows about the current HttpRequest.

Methods

TemplateResponse.__init__(request, template, context=None, content_type=None, status=None, charset=None, using=None, headers=None)

Instantiates a TemplateResponse object with the given request, template, context, content type, HTTP status, and charset.

request
An HttpRequest instance.
template
A backend-dependent template object (such as those returned by get_template()), the name of a template, or a list of template names.
context
A dict of values to add to the template context. By default, this is an empty dictionary.
content_type
The value included in the HTTP Content-Type header, including the MIME type specification and the character set encoding. If content_type is specified, then its value is used. Otherwise, 'text/html' is used.
status
The HTTP status code for the response.
charset
The charset in which the response will be encoded. If not given it will be extracted from content_type, and if that is unsuccessful, the DEFAULT_CHARSET setting will be used.
using
The NAME of a template engine to use for loading the template.
headers
A dict of HTTP headers to add to the response.

The rendering process

Before a TemplateResponse instance can be returned to the client, it must be rendered. The rendering process takes the intermediate representation of template and context, and turns it into the final byte stream that can be served to the client.

There are three circumstances under which a TemplateResponse will be rendered:

  • When the TemplateResponse instance is explicitly rendered, using the SimpleTemplateResponse.render() method.
  • When the content of the response is explicitly set by assigning response.content.
  • After passing through template response middleware, but before passing through response middleware.

A TemplateResponse can only be rendered once. The first call to SimpleTemplateResponse.render() sets the content of the response; subsequent rendering calls do not change the response content.

However, when response.content is explicitly assigned, the change is always applied. If you want to force the content to be re-rendered, you can reevaluate the rendered content, and assign the content of the response manually:

# Set up a rendered TemplateResponse
>>> from django.template.response import TemplateResponse
>>> t = TemplateResponse(request, "original.html", {})
>>> t.render()
>>> print(t.content)
Original content

# Re-rendering doesn't change content
>>> t.template_name = "new.html"
>>> t.render()
>>> print(t.content)
Original content

# Assigning content does change, no render() call required
>>> t.content = t.rendered_content
>>> print(t.content)
New content

Post-render callbacks

Some operations – such as caching – cannot be performed on an unrendered template. They must be performed on a fully complete and rendered response.

If you’re using middleware, you can do that. Middleware provides multiple opportunities to process a response on exit from a view. If you put behavior in the response middleware, it’s guaranteed to execute after template rendering has taken place.

However, if you’re using a decorator, the same opportunities do not exist. Any behavior defined in a decorator is handled immediately.

To compensate for this (and any other analogous use cases), TemplateResponse allows you to register callbacks that will be invoked when rendering has completed. Using this callback, you can defer critical processing until a point where you can guarantee that rendered content will be available.

To define a post-render callback, define a function that takes a single argument – response – and register that function with the template response:

from django.template.response import TemplateResponse


def my_render_callback(response):
    # Do content-sensitive processing
    do_post_processing()


def my_view(request):
    # Create a response
    response = TemplateResponse(request, "mytemplate.html", {})
    # Register the callback
    response.add_post_render_callback(my_render_callback)
    # Return the response
    return response

my_render_callback() will be invoked after the mytemplate.html has been rendered, and will be provided the fully rendered TemplateResponse instance as an argument.

If the template has already been rendered, the callback will be invoked immediately.

Using TemplateResponse and SimpleTemplateResponse

A TemplateResponse object can be used anywhere that a normal django.http.HttpResponse can be used. It can also be used as an alternative to calling render().

For example, the following view returns a TemplateResponse with a template and a context containing a queryset:

from django.template.response import TemplateResponse


def blog_index(request):
    return TemplateResponse(
        request, "entry_list.html", {"entries": Entry.objects.all()}
    )
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