.. _views_chapter: 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: .. code-block:: python :linenos: 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 may 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. .. code-block:: xml :linenos: The above maps the ``.views.hello_world`` view function to :term:`context` objects which are instances (or subclasses) of the Python class represented by ``.models.Hello`` when the *view name* is ``hello.html``. .. note:: 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.Hello`` with the absolute ``hello.models.Hello``; 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: .. code-block:: xml :linenos: 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: .. code-block:: xml :linenos: This indicates that when :mod:`repoze.bfg` identifies that the *view name* is ``hello.html`` against *any* :term:`context`, this view will be called. .. note:: If you're allergic to reading and writing :term:`ZCML`, or you're just more comfortable defining your view declarations using Python, you may use the :term:`repoze.bfg.convention` package. This package provides a decorator named ``bfg_view`` that can be used to associate ``for``, ``name``, ``permission`` and ``request_type`` information with a function that acts as a BFG view instead of needing to rely on ZCML for the same task. You only need to add a single ZCML stanza to your ``configure.zcml`` for :term:`repoze.bfg.convention` to find all views decorated in this fashion. Using Model Interfaces ---------------------- Instead of registering your views ``for`` a Python *class*, you can instead register a view for an :term:`interface`. Since an interface can be attached arbitrarily to any instance (as opposed to its identity being implied by only its class), associating a view with an interface can provide more flexibility for sharing a single view between two or more different implementations of a model type. For example, if two model object instances of different Python class types share the same interface, you can use the same view against each of them. In order to make use of interfaces in your application during view dispatch, you must create an interface and mark up your classes or instances with interface declarations that refer to this interface. To attach an interface to a *class*, you define the interface and use the ``zope.interface.implements`` function to associate the interface with the class. .. code-block:: python :linenos: from zope.interface import Interface from zope.interface import implements class IHello(Interface): """ A marker interface """ class Hello(object): implements(IHello) To attach an interface to an *instance*, you define the interface and use the ``zope.interface.alsoProvides`` function to associate the interface with the instance. This function mutates the instance in such a way that the interface is attached to it. .. code-block:: python :linenos: from zope.interface import Interface from zope.interface import alsoProvides class IHello(Interface): """ A marker interface """ class Hello(object): pass def make_hello(): hello = Hello() alsoProvides(hello, IHello) return hello Regardless of how you associate an interface with an instance or class, the resulting ZCML to associate that interface with a view is the same. Assuming the above code that defines an ``IHello`` interface lives in the root of your application, and its module is named "models.py", the below interface declaration will associate the ``.views.hello_world`` view with models that implement (aka provide) this interface. .. code-block:: xml :linenos: Any time a model that is determined to be the :term:`context` provides this interface, and a view named ``hello.html`` is looked up against it as per the URL, the ``.views.hello_world`` view will be invoked. Note that views registered against a class take precedence over views registered for an interfaces when an ambiguity arises. If a view is registered for both the class type of the context and an interface implemented by the context, the view registered for the class will "win". See :term:`Interface` in the glossary to find more information about interfaces. The ``bfg:view`` ZCML Element ----------------------------- The ``bfg:view`` ZCML element has these possible attributes: view The Python dotted-path name to the view callable. for A Python dotted-path name representing the Python class that the :term:`context` must be an instance of, *or* the :term:`interface` that the :term:`context` must provide in order for this view to be found and called. name The *view name*. Read and understand :ref:`traversal_chapter` to understand the concept of a view name. permission The name of a *permission* that the user must possess in order to call the view. See :ref:`view_security_section` for more information about view security and permissions. request_type A Python dotted-path name representing the :term:`interface` that the :term:`request` must have in order for this view to be found and called. See :ref:`view_request_types_section` for more information about view security and permissions. .. _view_request_types_section: View Request Types ------------------ You can optionally add a *request_type* attribute to your ``bfg:view`` declaration, which indicates what "kind" of request the view should be used for. For example: .. code-block:: xml :linenos: Where the code behind ``.interfaces.IJSONRequest`` might look like: .. code-block:: python :linenos: from repoze.bfg.interfaces import IRequest class IJSONRequest(IRequest): """ An marker interface for representing a JSON request """ This is an example of simple "content negotiation", using JSON as an example. To make sure that this view will be called when the request comes from a JSON client, you can use an ``INewRequest`` event subscriber to attach the ``IJSONRequest`` interface to the request if and only if the request headers indicate that the request has come from a JSON client. Since we've indicated that the ``request_type`` in our ZCML for this particular view is ``.interfaces.IJSONRequest``, the view will only be called if the request provides this interface. You can also use this facility for "skinning" a by using request parameters to vary the interface(s) that a request provides. By attaching to the request an arbitrary interface after examining the hostname or any other information available in the request within an ``INewRequest`` event subscriber, you can control view lookup precisely. For example, if you wanted to have two slightly different views for requests to two different hostnames, you might register one view with a ``request_type`` of ``.interfaces.IHostnameFoo`` and another with a ``request_type`` of ``.interfaces.IHostnameBar`` and then arrange for an event subscriber to attach ``.interfaces.IHostnameFoo`` to the request when the HTTP_HOST is ``foo`` and ``.interfaces.IHostnameBar`` to the request when the HTTP_HOST is ``bar``. The appropriate view will be called. You can also form an inheritance hierarchy out of ``request_type`` interfaces. When :mod:`repoze.bfg` looks up a view, the most specific view for the interface(s) found on the request based on standard Python method resolution order through the interface class hierarchy will be called. .. note:: see :ref:`events_chapter` for more information about event subscribers, and how to provide requests with differing request types. .. _view_security_section: 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: .. code-block:: xml :linenos: When a security policy is enabled, this view will be protected with the ``add`` permission. The view will not be called if the user does not possess the ``add`` permission relative to the current :term:`context`. Instead an HTTP ``Unauthorized`` status will be returned to the client. .. note:: See the :ref:`security_chapter` chapter to find out how to turn on a security policy. Using a View to Do A HTTP Redirect ---------------------------------- You can issue an HTTP redirect from within a view by returning a slightly different response. .. code-block:: python :linenos: from webob.exc import HTTPFound def myview(context, request): return HTTPFound(location='http://example.com') All exception types from the :mod:`webob.exc` module implement the ``IResponse`` interface; any can be returned as the response from a view. See :term:`WebOb` for the documentation for this module; it includes other response types for Unauthorized, etc.