Note in documentation for weakrefs

[sbt]

BPO	15412
Nosy	@pitrou

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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assignee = None
closed_at = <Date 2012-08-17.14:08:30.143>
created_at = <Date 2012-07-21.09:41:57.691>
labels = ['docs']
title = 'Note in documentation for weakrefs'
updated_at = <Date 2012-08-17.14:08:30.142>
user = 'https://bugs.python.org/sbt'

bugs.python.org fields:

activity = <Date 2012-08-17.14:08:30.142>
actor = 'sbt'
assignee = 'docs@python'
closed = True
closed_date = <Date 2012-08-17.14:08:30.143>
closer = 'sbt'
components = ['Documentation']
creation = <Date 2012-07-21.09:41:57.691>
creator = 'sbt'
dependencies = []
files = []
hgrepos = []
issue_num = 15412
keywords = []
message_count = 5.0
messages = ['166002', '166006', '166009', '166010', '168446']
nosy_count = 4.0
nosy_names = ['pitrou', 'docs@python', 'python-dev', 'sbt']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue15412'
versions = []

[sbt]

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)

[pitrou]

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).

[sbt]

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.

[pitrou]

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 :-)).

[python-dev]

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

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