TurboGears 2.3 Hidden Gems #1 – New Response Management

TurboGears2.3 has been a major improvement for the framework, most of its code got rewritten to achieve less dependencies, cleaner codebase a cleaner API and a faster framework. This resulted in reduction to only 7 dependencies in minimal mode and a 3x faster codebase.

While those are the core changes for the release, there are a lot of side effects that users can exploit at their benefit. This is the reason why I decided to start this set of posts to describe some of those hidden gems and explain users how to achieve the best from the new release.

The first change I’m going to talk about is how the response management got refactored and simplified. While this has some direct benefits it also provided some interesting side effects it makes sense to explore.

How TurboGears on Pylons did it

TurboGears tried to abstract a lot of response complexity through tg.response object and as there were not many reasons to override TGController.__call__ it was common that the response object body was always set by TurboGears itself.

Due to the fact that Pylons controllers were somehow compliant to WSGI itself the TGController was then in charge of calling the start_response function by actually providing all the headers user set into tg.response

response = self._dispatch_call()
 
# Here the response body got set, removed for brevity

if hasattr(response, 'wsgi_response'):
  # Copy the response object into the testing vars if we're testing
  if 'paste.testing_variables' in environ:
      environ['paste.testing_variables']['response'] = response
  if log_debug:
      log.debug("Calling Response object to return WSGI data")

  return response(environ, self.start_response)

While this made sense for Pylons, where you are expected to subclass the controller to perform advanced customizations, it was actually something unexposed to TurboGears users.

TurboGears made possible to change application behaviour using hooks and controller_wrappers. So the use for subclassing the TGController was actually strictly related to custom dispatching methods, which was usually better solved by specializing the TGController._dispatch method (tgext.routes is a simple enough example of this).

Cleaning Up Things

This lead to a curious situation where the TGController needed to speak with TGApp through WSGI to make Pylons happy, so it needed to call start_response and return the response iterator itself. TGApp was supposed to be the WSGI application, but in fact most of the real work was happening into TGController, in the end we had two WSGI applications: both TGController and TGApp were callable that spoke WSGI.

The 2.3 rewrite has been a great occasion to solve this ambiguity by providing a clear communication channel between TGController and TGApp by assigning each one a specific responsibility.

Communication Channel

In TG2.3 only the TGApp is now in charge of exposing the WSGI application interface. The TGController is expected to get a TurboGears Request Context object and provide back a TurboGears Response object. The TGApp will then use the provided response object to submit headers and response body.

The TGController code got much more straightforward and the whole testing and call response part was moved to the TGApp itself:

try:
    response = self._perform_call(context)
except HTTPException as httpe:
    response = httpe

# Here the response body got set, removed for brevity

return response

This has been possible without breaking backward compatibility thanks to the fact that the only subclassing of TGController common in TurboGears world was the BaseController class implemented by most applications.

The BaseController usually acts just as a pass-through between TGApp and TGController to setup some shortcuts to authentication data and other helpers for each request. So the fact that the parameters received by BaseController.__call__ changed didn’t cause an huge issue as they were just forwarded to TGController.__call__

A little side effect

One of the interesting effects of this change is that your controllers are now enabled to return any instance of webob.Response.

In previous versions it was possible to return practically only webob. WSGIHTTPException subclasses (as they exposed a wsgi_response property which was consumed by Pylons), so it was possible to return an HTTPFound instance to force a redirect, but it was not possible to return a plain response.

A consequence of the new change is enabling your controller to call third party WSGI applications by using tg.request.get_reponse with a given application. The returned response can be directly provided as the return value of your controller.

This behaviour also makes easier to write reusable components that don’t need to rely on tg.response and change it. Your application can forward the request to them and proxy back the response they return.

Part #2 will cover Application Wrappers, which greatly benefit from the new response management.

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