Add a lazy TemplateResponse

[Russell Keith-Magee]

Proposal from Simon Willison:

Summary: Add a TemplateResponse class - a response that contains a template and context, but doesn't evaluate the template until it is required (or explicitly 'baked'). This allows middlewares, decorators, or view wrappers to change the template.

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The original proposal comes from ​this django-users thread, in the context of making CSRF easier to use. Simon said:

I've been experimenting recently with a TemplateResponse class which lets you do this:

def view(request): 
    return TemplateResponse(request, 'my_template.html', {'foo': 'bar'}) 

It's an HttpResponse class that's designed to be lazily evaluated. This makes it really useful for subclassing - you can do things like this:

class MyGenericView(object): 
   def view(self, request): 
       # Do something complicated 
       return TemplateResponse(request, 'my_template.html', {'foo': 'bar'}) 
class MyCustomisedView(object): 
   def view(self, request): 
       response = super(MyCustomisedView, self).view(request) 
       response.context['foo'] = 'baz' 
       if is_mobile_phone(request): 
           response.template = 'my_mobile_template.html' 
       return response 

This pattern would be particularly valuable for customising the admin, which currently uses nasty extra_context argument hacks to achieve something that isn't nearly as useful.

My current implementation of TemplateResponse is ​here - there's a SimpleTemplateResponse as well which doesn't use RequestContext (but has a more verbose name to discourage its use).

[Jannis Leidel]

For the record I'm +1 on that, really like the idiom.

[Russell Keith-Magee]

(In [14850]) Fixed #12815 -- Added TemplateResponse, a lazy-evaluated Response class. Thanks to Simon Willison for the original idea, and to Mikhail Korobov and Ivan Sagalaev for their assistance, including the draft patch from Mikhail.