Persistent weak references
Persistent weak references are used much like Python weak references. The major difference is that you can't specify an object to be called when the object is removed from the database.
Here's an example. We'll start by creating a persistent object and a refernce to it:
>>> import persistent.list >>> import ZODB.tests.util >>> ob = persistent.list.PersistentList() >>> ref = WeakRef(ob) >>> ref() is ob True
The hash of the ref if the same as the hash of the referenced object:
>>> hash(ref) == hash(ob) True
Two refs to the same object are equal:
>>> WeakRef(ob) == ref True
>>> ob2 = persistent.list.PersistentList() >>> WeakRef(ob2) == ref False
Lets save the reference and the referenced object in a database:
>>> db = ZODB.tests.util.DB()
>>> conn1 = db.open() >>> conn1.root()['ob'] = ob >>> conn1.root()['ref'] = ref >>> ZODB.tests.util.commit()
If we open a new connection, we can use the reference:
>>> conn2 = db.open() >>> conn2.root()['ref']() is conn2.root()['ob'] True >>> hash(conn2.root()['ref']) == hash(conn2.root()['ob']) True
But if we delete the referenced object and pack:
>>> del conn2.root()['ob'] >>> ZODB.tests.util.commit() >>> ZODB.tests.util.pack(db)
And then look in a new connection:
>>> conn3 = db.open()
Traceback (most recent call last):
Trying to dereference the reference returns None:
Trying to get a hash, raises a type error:
>>> hash(conn3.root()['ref']) Traceback (most recent call last): ... TypeError: Weakly-referenced object has gone away
Always explicitly close databases: :)
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