Object Annotations

Annotation factories

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.interface import Interface
>>> from zope.interface import implementer
>>> class IFoo(Interface):
...     pass
>>> from zope.annotation.interfaces import IAttributeAnnotatable
>>> @implementer(IFoo, IAttributeAnnotatable)
... class Foo(object):
...     pass

We directly say that Foo implements interfacesIAttributeAnnotatable 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:

>>> from zope.component import adapts
>>> from zope.interface import Attribute
>>> class IBar(Interface):
...     a = Attribute('A')
...     b = Attribute('B')
>>> from zope import component
>>> @implementer(IBar)
... class Bar(object):
...     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.component import provideAdapter
>>> from zope.annotation import factory
>>> provideAdapter(factory(Bar))
>>> from zope.component import provideAdapter
>>> from zope.annotation.attribute import AttributeAnnotations
>>> provideAdapter(AttributeAnnotations)

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 adapts? It will complain:

>>> class IQux(Interface):
...     pass
>>> @implementer(IQux)
... class Qux(object):
...     pass
>>> provideAdapter(factory(Qux)) 
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):
...     pass
>>> @implementer(IHoi)
... class Hoi(object):
...     adapts(IFoo)
>>> provideAdapter(factory(Hoi, 'my.unique.key'))
>>> isinstance(IHoi(foo), Hoi)
True

Location

Annotation factories are put into the location hierarchy with their parent pointing to the annotated object and the name to the dotted name of the annotation’s class (or the name the adapter was registered under):

>>> foo3 = Foo()
>>> new_hoi = IHoi(foo3)
>>> new_hoi.__parent__
<Foo object at 0x...>
>>> new_hoi.__name__
'my.unique.key'
>>> import zope.location.interfaces
>>> zope.location.interfaces.ILocation.providedBy(new_hoi)
True

Please notice, that our Hoi object does not implement ILocation, so a location proxy will be used. This has to be re-established every time we retrieve the object

(Guard against former bug: proxy wasn’t established when the annotation existed already.)

>>> old_hoi = IHoi(foo3)
>>> old_hoi.__parent__
<Foo object at 0x...>
>>> old_hoi.__name__
'my.unique.key'
>>> zope.location.interfaces.ILocation.providedBy(old_hoi)
True

LocationProxies

Suppose your annotation proxy provides ILocation.

>>> class IPolloi(Interface):
...     pass
>>> @implementer(IPolloi, zope.location.interfaces.ILocation)
... class Polloi(object):
...     adapts(IFoo)
...     __name__ = __parent__ = 0
>>> provideAdapter(factory(Polloi, 'my.other.key'))

Sometimes you’re adapting an object wrapped in a LocationProxy.

>>> foo4 = Foo()
>>> import zope.location.location
>>> wrapped_foo4 = zope.location.location.LocationProxy(foo4, None, 'foo4')
>>> located_polloi = IPolloi(wrapped_foo4)

At first glance it looks as if located_polloi is located under wrapped_foo4.

>>> located_polloi.__parent__ is wrapped_foo4
True
>>> located_polloi.__name__
'my.other.key'

but that’s because we received a LocationProxy

>>> type(located_polloi).__name__
'LocationProxy'

If we unwrap located_polloi and look at it directly, we’ll see it stores a reference to the real Foo object

>>> from zope.proxy import removeAllProxies
>>> removeAllProxies(located_polloi).__parent__ == foo4
True
>>> removeAllProxies(located_polloi).__name__
'my.other.key'