How to use annotation design pattern to store arbitrary values on Python objects (Plone site, HTTP request) for storage and caching purposes.
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).
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()
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
textfield on Document).
- Behavior data from
# 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.
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
LikeDislikefor your context object if it does not already exist.
- You can play with your
LikeDislikeannotation 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
Let’s go back to the above sample and use the
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
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¶
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 can 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)
Another pattern to store data in annotations: Use already existing persistent base code instead of creating your own.
Please use one of theses:
This pattern is used by cioppino.twothumbs and collective.favoriting addons.
How to achieve this: https://gist.github.com/toutpt/7680498