Issue 2116: weakref copy module interaction

The copy module will not properly copy/deepcopy weakrefs, it will bomb
out with __new__ not having enough args. This is a problem b/c it makes
deepcopying of objects that make use of Weak(Key|Value)Dictionaries
difficult.

To replicate:

import copy, weakref
class Test(object): pass
t = Test()
wr = weakref.ref(t)
wr_new = copy.copy(wr)

Rick: In the future, please provide a context or unified diff ("diff
-c") so that we get the file name that the diff is for and some
additional context.

Sorry about forgetting the -c arg! The patch is intended for
python/trunk/Lib/copy.py.

It looks like Raymond Hettinger has made a similar-ish change recently
to make Ellipsis copyable.

Should a deepcopy of a weakref return the same weakref, or a weakref to
a new copied object?
Also, what about the optional callback?

Hmm, forget that question. If we deepcopy the weakref target, it will be
destroyed just afterwards, making the deepcopied weakref useless.

Here is a patch providing proper copy and deepcopy support, including
for weak dicts (the Proxy type is unsupported, though).

Updated patch so that copied weakdicts are decoupled (adding an item to
the copy doesn't mutate the original).

Raymond, Guido, any opinion on this one?

Committed in r72662, r72670. Thanks!

messages: + msg86917

assignee: tim.peters ->
stage: patch review

messages: + msg83612

messages: + msg83609