What is a View ? What are context and request ?

Views provide a connection between an external actor and an object.

A View is typically a display component. Views are typically reponsible for creating HTML. Views can directly return HTML, but will often supply presentational logic and processed data to a Zope Page Template, which then contains the HTML.

Web developers will normally deal with a specialized View called a BrowserView?. This is just a View that is made for a web browser, as Zope 3 can also provide Views for other protocols, such as FTP or WebDAV?. In a BrowserView?, the external actor is a web browser request, and the object that the view connects is looked up using traversal and is called the context. Because the Web is the predominant focus of most Zope developers, often the term View is assumed to be a BrowserView?.

The constructor for a BrowserView? looks like this:

class BrowserView(Location):
    implements(IBrowserView)

    def __init__(self, context, request):
        self.context = context
        self.request = request

Context is the object that the View is acting upon. Often context will be a Content or Model object, but it may also be a Container or Site object or any object that Zope can publish.

Request is an HTTP Request. If the View is a BrowserView?, the Request will have a form attribute where all form data is already marshalled for the programmer.

Consider the URL http://localhost:8080/your-id/a-todo-list/get-cat-food. In Zope 3, your-id would be a Container component that also provided a IHomeFolder? interface, a-todo-list would be a To-Do Container component that also provided a IToDoList? interaface, and get-cat-food would be a ToDo?-Item Content or Model component that also provided a IToDoItem? interface. If you entered the URL http://localhost:8080/your-id/a-todo-list/get-cat-food into your web browser, then context would be an object that provided the IToDoItem? interface, while request would be an object that represented the web browser request for that web page. However, if the URL was just http://localhost:8080/your-id/ then context would be an object that represented your home folder.

You can look-up a View programmatically with a query:

view = component.queryMultiAdapter((object, request), name='index')

For more reading on Views, there is a section about them in the Plone Core Developer Reference that provides information on how Zope 3 Views are being used in Plone: http://plone.org/documentation/manual/plone-developer-reference/patterns/views