Expose weakref callback for introspection purposes.

[mdickinson]

BPO	17643
Nosy	@terryjreedy, @mdickinson, @pitrou, @benjaminp
Files	weakref___callback__.patch weakref___callback__2.patch

^{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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GitHub fields:

assignee = 'https://github.com/mdickinson'
closed_at = <Date 2013-04-13.14:47:28.637>
created_at = <Date 2013-04-06.12:54:57.703>
labels = ['interpreter-core', 'type-feature']
title = 'Expose weakref callback for introspection purposes.'
updated_at = <Date 2013-04-13.14:47:28.636>
user = 'https://github.com/mdickinson'

bugs.python.org fields:

activity = <Date 2013-04-13.14:47:28.636>
actor = 'mark.dickinson'
assignee = 'mark.dickinson'
closed = True
closed_date = <Date 2013-04-13.14:47:28.637>
closer = 'mark.dickinson'
components = ['Interpreter Core']
creation = <Date 2013-04-06.12:54:57.703>
creator = 'mark.dickinson'
dependencies = []
files = ['29685', '29687']
hgrepos = []
issue_num = 17643
keywords = ['patch']
message_count = 12.0
messages = ['186122', '186124', '186125', '186132', '186151', '186154', '186155', '186655', '186665', '186668', '186722', '186723']
nosy_count = 6.0
nosy_names = ['terry.reedy', 'mark.dickinson', 'pitrou', 'benjamin.peterson', 'python-dev', 'sbt']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue17643'
versions = ['Python 3.4']

[mdickinson]

It would be nice to be able to access the callback of a weakref as an attribute on the weakref itself. For example:

Python 3.4.0a0 (default:2bf154ca43c6+, Apr  6 2013, 13:31:29) 
[GCC 4.2.1 (Apple Inc. build 5664)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import weakref
>>> x = {1, 2, 3}
>>> ref = weakref.ref(x, lambda ref: print("done"))
>>> ref.__callback__
<function <lambda> at 0x1004f56d0>
>>> del x
done
>>> ref.__callback__  # Returns None

I encountered this while writing a tool to show graphs of Python objects and their references to each other: I wanted to be able to annotate each edge of the graph. For something like a function, it's easy to use introspection to compare the reference target with f.__code__, f.__annotations__, etc. For a weakref, I couldn't find an easy way to retrieve the callback (or even determine whether there *was* a callback associated to the weakref). One can do a "gc.get_referents" call and hope that if there's exactly one object returned it's the callback, but that won't work so well with weakref.ref subclasses.

Patch attached: it has tests but no doc updates as yet.

[pitrou]

Sounds fine to me.

[benjaminp]

Instead of a getset, I think you just use a read-only T_OBJECT member.

[mdickinson]

Ah yes; that's easier. New patch including doc updates.

[pitrou]

"del x" will not be enough on non-refcounted implementations.

[mdickinson]

True: I'm not sure what to do about that---there are other tests in that test module that also rely on del resulting in immediate cleanup. I'm not sure what other implementations are currently doing with this test module.

I could mark the new test as cpython only, or add a gc.collect (but I'm not sure if the latter is enough, either).

[pitrou]

Ah, then we can probably keep using the same style.

[terryjreedy]

there are other tests in that test module that also rely on del resulting in immediate cleanup

I would mark them all as 'cpython-only', since they are. Other implementation must be omitting this test module, ignoring its failures, or patching out the specific tests.

[sbt]

I could mark the new test as cpython only, or add a gc.collect (but I'm
not sure if the latter is enough, either).

test.support.gc_collect() should work on non-refcounted implementations.

[mdickinson]

Ah, test.support.gc_collect looks useful. Thanks!

[python-dev]

New changeset 9e7d31b04d78 by Mark Dickinson in branch 'default':
Issue bpo-17643: Add __callback__ attribute to weakref.ref.
http://hg.python.org/cpython/rev/9e7d31b04d78

[mdickinson]

Committed, using support.gc_collect for the test that requires garbage collection. Thanks for the reviews!