Describing functionality with interfaces

There is nothing Grok-specific about interfaces, but they are important because they used in various directives for describing or registering components.

Zope interfaces are implemented in the zope.interface package. In addition, zope.schema contains various classes that can be used to describe the type of attributes on an interface (the Dexterity developer manual contains a reference).

Interfaces are typically found in an module, although you will sometimes see schema interfaces kept in the same module as other code (e.g. content classes, event handlers) related to the content type described by that schema.

The simplest interface is a marker interface. This is used as a flag which can either be applied or not to a particular object. A marker interface may look like this:

from zope.interface import Interface

class IImportant(Interface):
    ""Marker interface used for important objects

Notice how we have a docstring on the interface. Interfaces are useful as documentation, and you should endeavour to describe their purpose and behaviour in docstrings on the interface and on any attributes or methods (see below).

Also note that although an interface is created using the class*keyword, they are in fact instances of type *InterfaceClass. For the most part, you don’t need to worry about this.

Interfaces are said to be implemented by classes, in which case instances of that class is said to provide the interface.

from five import grok

class ImportantStuff(object):


Note: The grok.implements() directive is just a convenience alias for the implements() directive from zope.interface.

Adherence to a given interface can be tested like this:

>>> IImportant.implementedBy(ImportantStuff)
>>> stuff = ImportantStuff()
>>> IIimportant.providedBy(stuff)

Again, note that we perform an “implements” check against the class and a “provides” check against an instance.

It is also possible to apply an interface directly to an instance. This is mostly relevant to marker interfaces, since other interfaces promise attributes and methods that you usually cannot guarantee that the object will provide.

>>> from zope.interface import alsoProvides
>>> alsoProvides(someObject, IImportant)

Let’s now take a look at a non-marker interface. This one promises several attributes and methods. Typing and constraints for attributes are described by zope.schema fields.

from zope.interface import Interface
from zope import schema

class IMessage(Interface):
    """An email-like message

    subject = schema.TextLine(title=u"Subject")
    recipients = schema.Tuple(title=u"Recipients",
                              description=u"A list of email addresses",
    body = schema.Text(title=u"Body", required=False)

    def format():
        """Return a formatted string representing the message

Again notice the use of docstrings for methods and titles and descriptions for fields. Also notice how the method does not take the self parameter. If the implementation is a class (as it is likely to be), its methods will of course take the self parameter, but as far as the user of the interface is concerned, this is an implementation detail, and so does not belong in the interface.

Here is a class implementing this interface:

class Message(object):

    subject = u""
    recipients = ()
    body = u""

    def format(self):
        return "Subject: %s\nTo: %s\n%s" % (self.subject, ', '.join(self.recipients), self.body,)

Like classes, interfaces may inherit one another. The derived interface inherits all the attributes and methods of the parent interface. Objects providing the derived interface must provide all attributes and methods of both interfaces.

class ITestContent(Interface):
    """Base interface for content types

    title = schema.TextLine(title=u"Title")

class IDocumentContent(ITestContent):
    """Document content

    text = schema.Text(title=u"Body")

class IFileContent(ITestContent):
    """File content

    data = schema.Bytes(title=u"Octet stream")

A class may implement more than one interface. In addition, a class automatically implements all interfaces from its base classes (unless you use the implementsOnly() directive from zope.interface).

class ImportantMessageDocument(Message):
    grok.implements(IDocumentContent, IImportant)

    title = u"Title"

    def _getText(self):
        return self.body
    def _setText(self, value):
        self.body = value
    text = property(_getText, _setText)

Here, we have implemented text as a property delegating to the body field from the IMessage interface. We inherited the implementation of body from the Message*base class, from which we have also indicated the *implements() specification for the IMessage interface:

>>> doc = ImportantMessageDocument()
>>> IImportant.providedBy(doc)
>>> IMessage.providedBy(doc)
>>> ITestContent.providedBy(doc)
>>> IDocumentContent.providedBy(doc)
>>> IFileContent.providedBy(doc)

There are a few other things you can do with interfaces, such as specifying interfaces provided by modules (used to specify an API for that module) or classes (e.g. in the case of class objects acting as factories), looping through the interfaces provided by an instance, or adding or removing marker interfaces. None of these is terribly common. See the documentation for zope.interface (including its interfaces) for details.