ObjectDispatch and TGController

The TGController is the basic controller class that provides an easy method for nesting of controller classes to map URL hierarchies. There are however a few methods which provide ways to implement custom dispatching and some entry points that will make easy for the developer to track the progress of request dispatch.

Dispatching Entry Points

Dispatching entry points are methods that get executed while the dispatch is moving forward, this permits to run custom code which is related to the controller we are dispatching and not a specific method itself:

class Controller(BaseController):
    def _before(self, *remainder, **params):
        # Executed before running any method of Controller

    def _after(self, *remainder, **params):
        # Executed after running any method of Controller

    def _visit(self, *remainder, **params):
        # Executed when visiting a controller during dispatch.
  • _before gets executed whenever the dispatch process

    decides that the request has to be served by a method of the controller, before calling the method itself. It is executed before any method requirement specified through @require has been evaluated, but after the controller allow_only has been evaluated.

  • _after gets executed after the request has been dispatched

    to one of the controller methods.

  • _visit gets executed whenever the controller is visited

    during the dispatch process. Actual request target might be a subcontroller and not the controller itself. Might be called multiple times and gets executed before allow_only has been evaluated.

The Default Method

The developer may decide to provied a _default method within their controller which is called when the dispatch mechanism cannot find an appropriate method in your controllers to call. This _default method might look something like this:

class WikiController(BaseController):

  @expose('mytgapp.wiki.new)
  def _default(self, *args):
    """
      Return a page to prompt the user to create a new wiki page."""
    """
    return dict(new_page_slug=args)s

The Lookup Method

_lookup and _default are called in identical situations: when “normal” object traversal is not able to find an exposed method, it begins popping the stack of “not found” handlers. If the handler is a “_default” method, it is called with the rest of the path as positional parameters passed into the default method.

The not found handler stack can also contain “lookup” methods, which are different, as they are not actual controllers.

A lookup method takes as its argument the remaining path elements and returns an object (representing the next step in the traversal) and a (possibly modified) list of remaining path elements. So a blog might have controllers that look something like this:

class BlogController(BaseController):
    @expose()
    def _lookup(self, year, month, day, id, *remainder):
        dt = date(int(year), int(month), int(day))
        blog_entry = BlogEntryController(dt, int(id))
        return blog_entry, remainder

class BlogEntryController(object):
    def __init__(self, dt, id):
        self.entry = model.BlogEntry.get_by(date=dt, id=id)

    @expose(...)
    def index(self):
        ...

    @expose(...)
    def edit(self):
        ...

    @expose()
    def update(self):
        ....

So a URL request to .../2007/6/28/0/edit would map first to the BlogController’s _lookup method, which would lookup the date, instantiate a new BlogEntryController object (blog_entry), and pass that blog_entry object back to the object dispatcher, which uses the remainder do continue dispatch, finding the edit method. And of course the edit method would have access to self.entry, which was looked up and saved in the object along the way.

In other situations, you might have a several-layers-deep “_lookup” chain, e.g. for editing hierarchical data (/client/1/project/2/task/3/edit).

The benefit over “_default” handlers is that you return an object that acts as a sub-controller and continue traversing rather than being a controller and stopping traversal altogether. This allows you to use actual objects with data in your controllers.

Plus, it makes RESTful URLs much easier than they were in TurboGears 1.

Subclassing Controllers

When overriding a parent controller method you will usually have to expose it again and place any validation or event hook it previously had.

While this is possible, it is not the best way to add additional behavior to existing controllers. If they are implemented in an external library or application, you will have to look at the code of the library, see any template it exposed, any hook it registered and place them again.

If the library will change in any future release your code will probably stop working.

To avoid this behavior and the issues it raises since TurboGears 2.2 it is possible to subclass controllers inheriting the configuration the parent methods had.

The inherit parameter of the tg.decorators.expose decorator enables this behavior:

class OriginalController(TGController):
    @expose('mylib.templates.index')
    def index(self):
        return dict()

    @expose('mylib.templates.about')
    def about(self):
        return dict()

    @expose('json')
    def data(self):
        return {'v':5}

class MyCustomizedController(OriginalController):
    @expose(inherit=True)
    def index(self, *args, **kw):
        dosomething()
        return super(MyCustomizedController, self).index(*args, **kw)

    @expose('myapp.templates.newabout', inherit=True)
    def about(self):
        return super(MyCustomizedController, self).about(*args, **kw)

    def _before_render_data(remainder, params, output):
        output['child_value'] = 'CHILDVALUE'

    @expose(inherit=True)
    @before_render(_before_render_data)
    def data(self, *args, **kw):
        return super(MyCustomizedController, self).data(*args, **kw)

Mount Points and Dispatch

Since TurboGears 2.1.4 it is possible to ask for various informations about the request dispatchment and controllers mount points.

Those informations can be useful when writing controllers that you plan to reuse in multiple applications or mount points, making possible for example to generate all the urls knowing where they are mounted.

For statically mounted controllers the exposed informations are:

  • The mount_point property of a controller. If statically mounted it will return where the controller is mounted. This is the url to call when you want to access that controller.
  • The mount_steps property of a controller. If statically mounted it will return the complete list of parents of that controller.

In the case you are dispatching the request yourself, for example through a _lookup method, the mount_point and mount_steps informations won’t be available. In this case you can rely on some other functions exposed by TG:

  • The tg.request.controller_state object keeps track of all the steps provided to dispatch the request.
  • The tg.dispatched_controller() method when called inside a request will return the last statically mounted controller. This can be useful to detect which controller finished the request dispatch using the _lookup method.

The application RootController can usually be retrieved from tg.config['application_root_module'].RootController