The Zope 3 Book/Interfaces

Introduction
Interfaces are objects that specify (document) the external behavior of objects that "provide" them. An interface specifies behavior through:


 * Informal documentation in a doc string
 * Attribute definitions
 * Invariants, which are conditions that must hold for objects that provide the interface

Some of the motivations for using interfaces are:


 * Avoid monolithic design by developing small, exchangeable pieces
 * Model external responsibility, functionality, and behavior
 * Establish contracts between pieces of functionality
 * Document the API

The classic software engineering book Design Patterns by the Gang of Four recommends that you Program to an interface, not an implementation. Defining a formal interface is helpful in understanding a system. Moreover, interfaces bring you all the benefits of Zope Component Architecture.

In some modern programming languages: Java, C#, VB.NET etc, interfaces are an explicit aspect of the language. Since Python lacks interfaces, Zope implements them as a meta-class to inherit from.

Types of Contract

 * "I can do X": Describing the ability to do something is the classical definition of an API. Those abilities are defined and implemented as methods.


 * "I have X": This statement declares the availability of data, which is classically associated with schemas. The data is stored in attributes and properties.


 * "You can do X with me": Here we describe the behavior of an object. Classically there is no analog. However, MIME-types are great example of behavior declaration. This is implemented using empty "marker interfaces" as they describe implicit behavior.

The distinction between those three types of contracts was first pointed out in this form by Philipp von Weitershausen.

Understanding those distinctions is very important, since other programming languages do not necessarily use all three of these notions. In fact, often only the first one is used.

Defining Interfaces

 * Python has no concept of interfaces
 * Not a problem
 * Interfaces are just objects
 * "Abuse" the class statement to create an interface
 * Syntax proposed in PEP 245

Jim Fulton does not see this as a problem, since it makes interfaces first class citizens. In Java, for example, interfaces are special types of objects that can only serve as interfaces in their intended, limited scope.

An interface from the zope.interface package, on the other hand, defines the interface by implementing a meta-class, a core concept of Python. Thus, interfaces are merely using an existing Python pattern.

An example
Here is a classic hello world style example:

In the above class, you defined a goodmorning method. If you call the goodmorning method from an object created using this class, it will return Good morning, ...!

Here host is the actual object your code uses. If you want to examine implementation details you need to access the class Host, either via the source code or an API documentation tool.

Now we will begin to use the Zope interfaces. For the class given above you can specify the interface like this:

As you can see, the interface inherits from zope.interface.Interface. This use (abuse?) of Python's class statement is how Zope defines an interface. The I prefix for the interface name is a useful convention.

Declaring interfaces
You have already seen how to declare an interface using zope.interface in previous section. This section will explain the concepts in detail.

Consider this example interface:

The interface, IHost has two attributes, name and goodmorning. Recall that, at least in Python, methods are also attributes of classes. The name attribute is defined using zope.interface.Attribute class. When you add the attribute name to the IHost interface, you don't set an initial value. The purpose of defining the attribute name here is merely to indicate that any implementation of this interface will feature an attribute named name. In this case, you don't even say what type of attribute it has to be!. You can pass a documentation string as a first argument to Attribute.

The other attribute, goodmorning is a method defined using a function definition. Note that self is not required in interfaces, because self is an implementation detail of class. For example, a module can implement this interface. If a module implements this interface, there will be a name attribute and goodmorning function defined. And the goodmorning function will accept one argument.

Now you will see how to connect interface-class-object. So object is the real living thing, objects are instances of classes. And interface is the actual definition of the object, so classes are just the implementation details. This is why you should program to an interface and not to an implementation.

Now you should familiarize yourself with two more terms to understand other concepts. The first one is "provide" and the other one is "implement". Object provides interfaces and classes implement interfaces. In other words, objects provide interfaces that their classes implement. In the above example, host (object) provides IHost (interface), and Host (class) implements IHost (interface). One object can provide more than one interface; also one class can implement more than one interface. Objects can also provide interfaces directly, in addition to what their classes implement.


 * Note: Classes are the implementation details of objects. In Python, classes are callable objects, so why can't other callable objects implement an interface? Yes, it is possible. For any callable object you can declare that it produces objects that provide some interfaces by saying that the callable object implements the interfaces. The callable objects are generally called "factories". Since functions are callable objects, a function can be an implementer of an interface.

Implementing interfaces
To declare a class implements a particular interface, use the function zope.interface.implements in the class statement.

Consider this example, here Host implements IHost:


 * Note: If you wonder how implements function works, refer the blog post by James Henstridge (http://blogs.gnome.org/jamesh/2005/09/08/python-class-advisors/) . In the adapter section, you will see an adapts function, it is also working similarly.

Since Host implements IHost, instances of Host provide IHost. There are some utility methods to introspect the declarations. The declaration can write outside the class also. If you don't write interface.implements(IHost) in the above example, then after defining the class statement, you can write like this:

Marker interfaces
An interface can be used to declare that a particular object belongs to a special type. An interface without any attribute or method is called marker interface.

Here is a marker interface:

This interface can be used to declare an object is a special guest.