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.

Sounds fine to me.

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

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

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

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).

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

> 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.

> 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.

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

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

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