zope.annotation 3.4.0

Zope Annotation

Detailed Documentation

Annotations

There is more to document about annotations, but we’ll just sketch out a scenario on how to use the annotation factory for now. This is one of the easiest ways to use annotations – basically you can see them as persistent, writable adapters.

First, let’s make a persistent object we can create annotations for:

>>> from zope import interface
>>> class IFoo(interface.Interface):
...     pass
>>> from zope.annotation.interfaces import IAttributeAnnotatable
>>> from persistent import Persistent
>>> class Foo(Persistent):
...     interface.implements(IFoo, IAttributeAnnotatable)

We directly say that Foo implements IAttributeAnnotatable here. In practice this is often done in ZCML, using the implements subdirective of the content or class directive.

Now let’s create an annotation for this:

>>> class IBar(interface.Interface):
...     a = interface.Attribute('A')
...     b = interface.Attribute('B')
>>> from zope import component
>>> class Bar(Persistent):
...     interface.implements(IBar)
...     component.adapts(IFoo)
...     def __init__(self):
...         self.a = 1
...         self.b = 2

Note that the annotation implementation does not expect any arguments to its __init__. Otherwise it’s basically an adapter.

Now, we’ll register the annotation as an adapter. To do this we use the factory function provided by zope.annotation:

>>> from zope.annotation import factory
>>> component.provideAdapter(factory(Bar))

Note that we do not need to specify what the adapter provides or what it adapts - we already do this on the annotation class itself.

Now let’s make an instance of Foo, and make an annotation for it.

>>> foo = Foo()
>>> bar = IBar(foo)
>>> bar.a
1
>>> bar.b
2

We’ll change a and get the annotation again. Our change is still there:

>>> bar.a = 3
>>> IBar(foo).a
3

Of course it’s still different for another instance of Foo:

>>> foo2 = Foo()
>>> IBar(foo2).a
1

What if our annotation does not provide what it adapts with component.adapts? It will complain:

>>> class IQux(interface.Interface):
...     pass
>>> class Qux(Persistent):
...     interface.implements(IQux)
>>> component.provideAdapter(factory(Qux)) # doctest: +ELLIPSIS
Traceback (most recent call last):
...
TypeError: Missing 'zope.component.adapts' on annotation

It’s possible to provide an annotation with an explicit key. (If the key is not supplied, the key is deduced from the annotation’s dotted name, provided it is a class.)

>>> class IHoi(interface.Interface):
...     pass
>>> class Hoi(Persistent):
...     interface.implements(IHoi)
...     component.adapts(IFoo)
>>> component.provideAdapter(factory(Hoi, 'my.unique.key'))
>>> isinstance(IHoi(foo), Hoi)
True