Interfaces and why they matter

Interfaces are a bit techie and something a non-developer would probably rather not think about. However, they are an important part of component wiring, so it is as well to know a bit about what they are and do.

Interfaces as Markers

ZCML attributes often refer to interfaces rather than actual classes - for instance the example below wires up the presentation viewlet for content types that have the IATDocument interface.


In effect this is saying that the presentation viewlet is available for any content type which is ATDocument-like or behaves like an ATDocument. So, in this case, the interface is a marker.

The convenience of this is that a content type can have one (or more) interfaces, and several content types can share the same one. If you develop a new content type and mark it with the IATDocument interface, you can use this presentation viewlet with it - no extra wiring required.

Components and Interfaces

Components themselves can be marked with an interface - the technical term is "provides". Note that in the presentation viewlet example, the viewlet manager is referred to by its interface, not its name:


To track down the actual component, look in the configure.zcml file in the same directory as the interfaces. For instance, in plone/app/layout/viewlets/configure.zcml you'll see the interface has been wired up with a Python class to create a viewlet manager component:


How to spot an interface

It is usually fairly easy to spot a reference to an interface. By convention, their names will be prefixed with an "I", and they will live in an interface or interfaces namespace. If you investigate or in any egg or product, you won't find very much code, but you'll often find useful information – effectively it is documentation about what a component providing (i.e. marked by) that interface should do. For example:

class IAboveContentBody(IViewletManager):
    """A viewlet manager that sits above the content body in view templates    """

If you've used the plone3_theme paster template, you'll find you have a ready-made file to which you can add your own interfaces if you need to create them.