Annotations

Description

How to use annotation design pattern to store arbitrary values on Python objects (Plone site, HTTP request) for storage and caching purposes.

Introduction

Annotations is conflict-free way to stick attributes on arbitrary Python objects.

Plone uses annotations for:

  • Storing field data in Archetypes (Annotation storage).
  • Caching values on HTTP request object (plone.memoize cache decorators).
  • Storing settings information in portal or content object (various add-on products).

See zope.annotation package.

HTTP request example

Store cached values on HTTP request during the life cycle of one request processing. This allows you to cache computed values if the computation function is called from the different, unrelated, code paths. 

from zope.annotation.interfaces import IAnnotations

# Non-conflicting key
KEY = "mypackage.something"

annotations = IAnnotations(request)

value = annotations.get(KEY, None)
if value is None:
    # Compute value and store it on request object for further look-ups
    value = annotations[KEY] = something()

Content annotations

Overview and basic usage

If you want to extend any Plone content to contain "custom" settings annotations is the recommended way to do it.

  • Your add-on can store its settings in Plone site root object using local utilities or annotations.
  • You can store custom settings on content objects using annotations.

By default, in content annotations are stored:

  • Assigned portlets and their settings.
  • Archetypes content type fields using AnnotationStorage (like text field on Document).
  • Behavior data from plone.behavior package.

Example:

# Assume context variable refers to some content item

# Non-conflicting key
KEY = "yourcompany.packagename.magicalcontentnavigationsetting"

annotations = IAnnotations(context)

# Store some setting on the content item
annotations[KEY] = True

Advanced content annotation

The above example is enough for storing simple values as annotations. You may provide more complex annotation objects depending on your application logic on various content types. This example shows how to add a simple "Like / Dislike" counter on a content object. 

class LikeDislike(object):
    def __init__(self):
        self.reset()

    def reset(self):
        self._likes = set()
        self._dislikes = set()

    def likedBy(self, user_id):
        self._dislikes.discard(user_id)
        self._likes.add(user_id)

    def dislikedBy(self, user_id):
        self._likes.discard(user_id)
        self._dislikes.add(user_id)

    def status(self):
        return len(self._likes), len(self._dislikes)

At this step it is essential to check that your custom annotation class can be pickled. In the Zope world, this means that you cannot hold in your annotation object any reference to a content too.

Tip

Use the UID of a content object if you need to keep the reference of that content object in an annotation.

The most pythonic recipe to get (and set if not existing) your annotation for a given key is: 

from zope.annotation import IAttributeAnnotatable, IAnnotations

KEY = 'content.like.dislike'  # It's best place is config.py in a real app

def getLikesDislikeFor(item):
    """Factory for LikeDislike as annotation of a contentish
    @param item: any annotatable object, thus any Plone content
    """
    # Ensure the item is annotatable
    assert IAttributeAnnotatable.providedBy(item)  # Won't work otherwise
    annotations = IAnnotations(item)
    return annotations.setdefault(KEY, LikeDislike())

This way, you're sure that :

  • You won't create annotations on an object that can't support them.
  • You will create a new fresh annotation mastered with your LikeDislike for your context object if it does not already exist.
  • You can play with your LikeDislike annotation object as with any Python object, all attributes changes will be stored automatically in the annotations of the associated content object.

Wrapping your annotation with an adapter

zope.annotation comes with the factory() function that transforms the annotation class into an adapter (possibly named as the annotation key).

In addition the annotation created this way have location awareness, having __parent__ and __name__ attributes.

Let's go back to the above sample and use the zope.annotation.factory() function. 

import zope.interface
import zope.component
import zope.annotation

from zope.interface import implements
from zope.annotation import factory

from some.contenttype.interfaces import ISomeContent

KEY = 'content.like.dislike'  # It's best place is config.py in a real app

class ILikeDislike(zope.interface.Interface):
    """Model for like/dislike annotation
    """
    def reset():
        """Reinitialize everything
        """

    def likedBy(user_id):
        """User liked the associated content
        """

    def dislikedBy(user_id):
        """User disliked the associated content
        """


class LikeDislike(object):
    implements(ILikeDislike)
    zope.component.adapts(ISomeContent)

    def __init__(self):
        # Does not expect argument as usual adapters
        # You can access annotated object through ``self.__parent__``
        self.reset()

    def reset(self):
        self._likes = set()
        self._dislikes = set()

    def likedBy(self, user_id):
        self._dislikes.discard(user_id)
        self._likes.add(user_id)

    def dislikedBy(self, user_id):
        self._likes.discard(user_id)
        self._dislikes.add(user_id)

    def status(self):
        return len(self._likes), len(self._dislikes)


# Register as adapter (you may do this in ZCML too)
zope.component.provideAdapter(factory(LikeDislike, key=KEY))

# Lets play with some content
item = getSomeContentImplementingISomeContent()  # Guess what :)

# Let's have its annotation
like_dislike = ILikeDislike(item)

# Play with this annotation
like_dislike.likedBy('joe')
like_dislike.dislikedBy('jane')

assert like_dislike.status() == (1, 1)
assert like_dislike.__parent__ is item
assert like_dislike.__name__ == KEY

Tip

Read a full doc / test / demo of the zope.annotation.factory() in the README.txt file in the root of zope.annotation package for more advanced usages.

Cleaning up content annotations

Warning

If you store full Python objects in annotations you need to clean them up during your add-on uninstallation. Otherwise if Python code is not present you cannot no longer import or export Plone site (annotations are pickled objects in the database and pickles do no longer work if the code is not present).

How to clean up annotations on content objects:

def clean_up_content_annotations(portal, names):
    """
    Remove objects from content annotations in Plone site,

    This is mostly to remove objects which might make the site un-exportable
    when eggs / Python code has been removed.

    @param portal: Plone site object

    @param names: Names of the annotation entries to remove
    """

    output = StringIO()

    def recurse(context):
        """ Recurse through all content on Plone site """

        annotations = IAnnotations(context)

        #print  >> output, "Recusring to item:" + str(context)
        print annotations

        for name in names:
            if name in annotations:
                print >> output, "Cleaning up annotation %s on item %s" % (name, context.absolute_url())
                del annotations[name]

        # Make sure that we recurse to real folders only,
        # otherwise contentItems() might be acquired from higher level
        if IFolderish.providedBy(context):
            for id, item in context.contentItems():
                recurse(item)

    recurse(portal)

    return output

Make your code persistence free

There is one issue with the above methods: you are creating new persistent classes so your data need your source code. That makes your code hard to uninstall (have to keep the code BBB + cleaning up the DB by walking throw all objects)

So here is an other pattern to store data in annotations: Use already existing persistent base code instead of creating your own.

Please use one of theses:

  • BTrees
  • PersistentList
  • PersistentDict

This pattern is used by cioppino.twothumbs and collective.favoriting addons.

How to achieve this: https://gist.github.com/toutpt/7680498