Restore weakref support for lru_cache wrappers

[rhettinger]

BPO	40504
Nosy	@rhettinger, @serhiy-storchaka, @sweeneyde
PRs	bpo-40504: Allow weakrefs to lru_cache objects #19938

^{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 = None
closed_at = <Date 2020-05-05.21:15:17.962>
created_at = <Date 2020-05-04.20:36:26.737>
labels = ['extension-modules', 'easy', 'type-bug', '3.9']
title = 'Restore weakref support for lru_cache wrappers'
updated_at = <Date 2020-05-05.21:15:17.961>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2020-05-05.21:15:17.961>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = <Date 2020-05-05.21:15:17.962>
closer = 'rhettinger'
components = ['Extension Modules']
creation = <Date 2020-05-04.20:36:26.737>
creator = 'rhettinger'
dependencies = []
files = []
hgrepos = []
issue_num = 40504
keywords = ['patch', 'easy (C)']
message_count = 5.0
messages = ['368081', '368093', '368097', '368207', '368208']
nosy_count = 3.0
nosy_names = ['rhettinger', 'serhiy.storchaka', 'Dennis Sweeney']
pr_nums = ['19938']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40504'
versions = ['Python 3.9']

[rhettinger]

The pure python version of lru_cache() wrappers supported weak references. It's not essential, but I have used it a couple of times. In the spirit of PEP-399, the C version should add back the weakref support (and a test).

[sweeneyde]

I can submit a PR. Just making sure I understand, is this essentially the desired behavior change?

import weakref
import functools

if 0:
    from test.support import import_fresh_module
    functools = import_fresh_module('functools', blocked=['_functools'])

@functools.lru_cache
def f(x):
    return x

ref_to_f = weakref.ref(f)
print(ref_to_f)

# Current:
# TypeError: cannot create weak reference to 'functools._lru_cache_wrapper' object
# Desired:
# <weakref at 0x000001ABFE0F5090; to 'function' at 0x000001ABFE2D5AF0 (f)>

[rhettinger]

Yes. That is the desired behavior. Use Objects/setobject.c and Include/setobject.h as a model.

[rhettinger]

New changeset 1253c3e by Dennis Sweeney in branch 'master':
bpo-40504: Allow weakrefs to lru_cache objects (GH-19938)
1253c3e

[rhettinger]

Thanks for the PE :-)