homepage

In the documentation on weakrefs there is the following quote
 Note: Weak references to an object are cleared before the object’s
 __del__() is called, to ensure that the weak reference callback 
 (if any) finds the object still alive.
But I think the weakref is always dead by the time the callback is invoked. The first paragraph from the documentation for weakref.ref(object[, callback]) contains the following:
 If callback is provided and not None, and the returned weakref 
 object is still alive, the callback will be called when the object 
 is about to be finalized; the weak reference object will be passed 
 as the only parameter to the callback; **the referent will no longer 
 be available**.
Which does prompt a question: what use is there for the weakref argument to the callback if it already dead?
(Compare http://bugs.python.org/issue14933)

> In the documentation on weakrefs there is the following quote
> 
> Note: Weak references to an object are cleared before the object’s
> __del__() is called, to ensure that the weak reference callback 
> (if any) finds the object still alive.
> 
> But I think the weakref is always dead by the time the callback is
> invoked.
It's true, the doc needs fixing.
> Which does prompt a question: what use is there for the weakref
> argument to the callback if it already dead?
The weakref is "dead" but it's still a weakref, and it can be used to
e.g. index a container of existing weakrefs (cf. WeakSet,
WeakKeyDictionary, WeakValueDictionary).

> The weakref is "dead" but it's still a weakref, and it can be used to
> e.g. index a container of existing weakrefs (cf. WeakSet,
> WeakKeyDictionary, WeakValueDictionary).
Ah.
I had assumed that since dead weakrefs were unhashable you couldn't safely use them as keys in a dict. (Presumably removal of a no longer hashable key from a dict is O(n)?)
I also hadn't realize that weakrefs were subclassable.

> I had assumed that since dead weakrefs were unhashable you couldn't
> safely use them as keys in a dict. (Presumably removal of a no longer
> hashable key from a dict is O(n)?)
They are unhashable if you didn't hash them alive. Otherwise they retain
their old hash value (which is quite useful :-)).

New changeset 78b0f294674c by Richard Oudkerk in branch '2.7':
Issue #15412: Remove erroneous note about weakrefs
http://hg.python.org/cpython/rev/78b0f294674c
New changeset 24b13be81d61 by Richard Oudkerk in branch '3.2':
Issue #15412: Remove erroneous note about weakrefs
http://hg.python.org/cpython/rev/24b13be81d61