3. Components and Interfaces

Zope uses a component architecture internally in many places. Zope components are nothing but Python objects with interfaces that describe them. As a Zope developer you can use interfaces right now to build your Zope components.

3.1. Zope Components

Components are objects that are associated with interfaces. An interface is a Python object that describes how you work with other Python objects. In this chapter, you’ll see some simple examples of creating components, and a description of interfaces and how they work.

Here is a very simple component that says hello. Like all components, this one consists of two pieces, an interface and an implementation:

from zope.interface import Interface
from zope.interface import implementer

class IHello(Interface):
    """The Hello interface provides greetings."""

    def hello(name):
        """Say hello to the name"""

@implementer(IHello)
class HelloComponent:
    def hello(self, name):
        return "hello %s!" % name

Let’s take a look at this step by step. Here, you see two Python class statements. The first class statement creates the interface, and the second class statement creates the implementation.

The first class statement creates the IHello interface. This interface describes one method, called hello. Notice that there is no implementation for this method. Interfaces do not define behavior, they just describe a specification.

The second class statement creates the HelloComponent class. This class is the actual component that does what IHello describes. This is usually referred to as the implementation of IHello. In order for you to know what interfaces HelloComponent implements, it must somehow associate itself with an interface. The implementer decorator above the class does just that. It says, “I implement these interfaces”. In this case, HelloComponent asserts that it implements one interface, IHello.

The interface describes how you would work with the object, but it doesn’t dictate how that description is implemented. For example, here’s a more complex implementation of the Hello interface:

import xmlrpclib

@implementer(IHello)
class XMLRPCHello:
    def hello(self, name):
        """Delegates the hello call to a remote object
        using XML-RPC.
        """
        s = xmlrpclib.Server('your/rpc/server')
        return s.hello(name)

This component contacts a remote server and gets its hello greeting from a remote component.

And that’s all there is to components, really. The rest of this chapter describes interfaces and how you can work with them from the perspective of components. In Chapter 5, we’ll put all this together into a Zope product.

3.2. Python Interfaces

An interface describes the behavior of an object by containing useful information about the object. This information includes:

Prose documentation about the object. In Python terms, this is called the “doc string” of the interface. In this element, you describe how the object works in prose language and any other useful information about the object.
Descriptions of attributes. Attribute descriptions include the name of the attribute and prose documentation describing the attributes usage.
Descriptions of methods. Method descriptions can include:
- Prose “doc string” documentation about the method and its usage.
- A sequence of parameter objects that describes the parameters expected by the method.
Optional tagged data. Interface objects (and their attributes, methods, and method parameters) can have optional, application specific tagged data associated with them. Examples uses for this are security assertions, pre/post conditions, unit tests, and other possible information you may want to associate with an Interface or its attributes.

Not all of this information is mandatory. For example, you may only want the methods of your interface to have prose documentation and not describe the arguments of the method in exact detail. Interface objects are flexible and let you give or take any of these components.

3.3. Why Use Interfaces?

Interfaces solve a number of problems that arise while developing large systems with lots of developers.

Developers waste a lot of time looking at the source code of your system to figure out how objects work. This is even worse if someone else has already wasted their time doing the same thing.
Developers who are new to your system may misunderstand how an object works, causing, and possibly propagating, usage errors.
Because an object’s interface is inferred from the source, developers may end up using methods and attributes that are meant for “internal use only”.
Code inspection can be hard, and very discouraging to novice programmers trying to understand code written by gurus.

Interfaces try to solve these problems by providing a way for you to describe how to use an object, and a mechanism for discovering that description.

3.4. Creating Interfaces

The first step to creating a component, as you’ve been shown, is to create an interface.

Interface objects can be conveniently constructed using the Python class statement. Keep in mind that this syntax can be a little misleading, because interfaces are not classes. It is important to understand that using Python’s class syntax is just a convenience, and that the resulting object is an interface, not a class.

To create an interface object using Python’s class syntax, create a Python class that subclasses from zope.interface.Interface:

from zope.interface import Interface

class IHello(Interface):

    def hello(name):
        """Say hello to the world"""

This interface does not implement behavior for its methods, it just describes an interface that a typical “Hello” object would realize. By subclassing zope.interface.Interface, the resulting object Hello is an interface object. The Python interpreter confirms this:

>>> IHello
<InterfaceClass __main__.IHello>

Now, you can associate the Hello Interface with your new concrete class in which you define your user behavior. For example:

from zope.interface import implementer

@implementer(IHello)
class HelloComponent:
    def hello(self, name):
        return "Hello %s!" % name

This new class, HelloComponent is a concrete class that implements the IHello interface. A class can realize more than one interface. For example, say you had an interface called IItem that described how an object worked as an item in a “Container” object. If you wanted to assert that HelloComponent instances realized the IItem interface as well as IHello, you can provide a sequence of Interface objects to the HelloComponent class:

@implementer(IHello, IItem)
class HelloComponent:
    ...

3.5. The Interface Model

Interfaces can extend other interfaces. For example, let’s extend the IHello interface by adding an additional method:

class ISmartHello(IHello):
    """A Hello object that remembers who it's greeted"""

    def lastGreeted(self):
        """Returns the name of the last person greeted."""

ISmartHello extends the IHello interface. It does this by using the same syntax a class would use to subclass another class.

Now, you can ask the ISmartHello for a list of the interfaces it extends with getBases:

>>> ISmartHello.getBases()
(<InterfaceClass __main__.IHello>,)

An interface can extend any number of other interfaces, and getBases will return that list of interfaces for you. If you want to know if ISmartHello extends any other interface, you could call getBases and search through the list, but a convenience method called extends is provided that returns true or false for this purpose:

>>> ISmartHello.extends(IHello)
True
>>> class ISandwich(Interface):
...     pass
>>> ISmartHello.extends(ISandwich)
False

Here you can see extends can be used to determine if one interface extends another.

You may notice a similarity between interfaces extending from other interfaces and classes sub-classing from other classes. This is a similar concept, but the two should not be considered equal. There is no assumption that classes and interfaces exist in a one to one relationship; one class may implement several interfaces, and a class may not implement its base classes’s interfaces.

The distinction between a class and an interface should always be kept clear. The purpose of a class is to share the implementation of how an object works. The purpose of an interface is to document how to work with an object, not how the object is implemented. It is possible to have several different classes with very different implementations realizing the same interface. Because of this, interfaces and classes should never be confused.

3.6. Querying an Interface

Interfaces can be queried for information. The simplest case is to ask an interface the names of all the various interface items it describes. From the Python interpreter, for example, you can walk right up to an interface and ask it for its names:

>>> IHello.names()
dict_keys(['hello'])

Interfaces can also give you more interesting information about their items. Interface objects can return a list of ‘(name, description)’ tuples about their items by calling the namesAndDescriptions method.

For example:

>>> IHello.namesAndDescriptions()
dict_items([('hello', <zope.interface.interface.Method object at 0x7fc6875110f0>)])

Note

You cannot access the Method object by index, as namesAndDescriptions returns a dict_view.

You can either use list or next and iter on the result.

As you can see, the “description” of the Interface’s item is a Method object. Description objects can be either Attribute or Method objects. Attribute, Method and Interface objects implement the following interface:

`getName()` -- Returns the name of the object.

`getDoc()` -- Returns the documentation for the object.

Method objects provide a way to describe rich meta-data about Python methods. Method objects have the following methods:

`getSignatureInfo()` -- Returns a dictionary describing the method
parameters.

`getSignatureString()` -- Returns a human-readable string
representation of the method's signature.

For example:

>>> m = list(IHello.namesAndDescriptions())[0][1]
>>> m
<zope.interface.interface.Method object at 0x7fc6875110f0>
>>> m.getSignatureString()
'(name)'
>>> m.getSignatureInfo()
{'positional': ('name',), 'required': ('name',), 'optional': {},
 'varargs': None, 'kwargs': None}

You can use getSignatureInfo to find out the names and types of the method parameters.

3.7. Checking Implementation

You can ask an interface if a certain class that you hand it implements that interface. For example, say you want to know if the HelloComponent class implements IHello:

IHello.implementedBy(HelloComponent)

This is a true expression. If you had an instance of HelloComponent, you can also ask the interface if that instance implements the interface:

IHello.providedBy(my_hello_instance)

This would also return true if my_hello_instance was an instance of HelloComponent, or any other object of a class that implemented the IHello interface.

3.8. Conclusion

Interfaces provide a simple way to describe your Python objects. By using interfaces you document capabilities of objects.