Views ===== A :term:`view` is a callable which is invoked when a request enters your application. :mod:`repoze.bfg's` primary job is to find and call a view when a :term:`request` reaches it. The view's return value must implement the :term:`WebOb` ``Response`` object interface. Defining a View as a Function ----------------------------- The easiest way to define a view is to create a function that accepts two arguments: :term:`context`, and :term:`request`. For example, this is a hello world view implemented as a function:: def hello_world(context, request): from webob import Response return Response('Hello world!') The :term:`context` and :term:`request` arguments can be defined as follows: context An instance of a model found via graph :term:`traversal` or :term:`URL dispatch`. request A WebOb request object representing the current WSGI request. A view must return an object that implements the :term:`WebOb` ``Response`` interface. The easiest way to return something that implements this interface is to return a ``webob.Response`` object. But any object that has the following attributes will work: status The HTTP status code (including the name) for the response. E.g. ``200 OK`` or ``401 Unauthorized``. headerlist A sequence of tuples representing the list of headers that should be set in the response. E.g. ``[('Content-Type', 'text/html'), ('Content-Length', '412')]`` app_iter An iterable representing the body of the response. This can be a list, e.g. ``['Hello world!']`` or it can be a filelike object, or any other sort of iterable. If a view happens to return something to the :mod:`repoze.bfg` publisher that does not implement this interface, the publisher will raise an error. Mapping Views to URLs ---------------------- You must associate a view with a URL by adding information to your :term:`application registry` via :term:`ZCML` in your ``configure.zcml`` file using a ``bfg:view`` declaration. .. sourcecode:: xml The above maps the ``.views.hello_world`` view function to :term:`context` objects which implement the ``.models.IHello`` interface when the *view name* is ``hello.html``. Note that values prefixed with a period (``.``)for the ``for`` and ``view`` attributes of a ``bfg:view`` (such as those above) mean "relative to the Python package directory in which this :term:`ZCML` file is stored". So if the above ``bfg:view`` declaration was made inside a ``configure.zcml`` file that lived in the ``hello`` package, you could replace the relative ``.models.IHello`` with the absolute ``hello.models.IHello``; likewise you could replace the relative ``.views.hello_world`` with the absolute ``hello.views.hello_world``. Either the relative or absolute form is functionally equivalent. It's often useful to use the relative form, in case your package's name changes. It's also shorter to type. You can also declare a *default view* for a model type: .. sourcecode:: xml A *default view* has no ``name`` attribute. When a :term:`context` is traversed and there is no *view name* in the request, the *default view* is the view that is used. You can also declare that a view is good for any model type by using the special ``*`` character in the ``for`` attribute: .. sourcecode:: xml This indicates that when :mod:`repoze.bfg` identifies that the *view name* is ``hello.html`` against *any* :term:`context`, this view will be called. View Security ------------- If a :term:`security policy` is active, any :term:`permission` attached to a ``bfg:view`` declaration will be consulted to ensure that the currently authenticated user possesses that permission against the context before the view function is actually called. Here's an example of specifying a permission in a ``bfg:view`` declaration: .. sourcecode:: xml When a security policy is enabled, this view will be protected with the ``add`` permission. See the :ref:`security_chapter` chapter to find out how to turn on a security policy.