functools.partial missing __weakref__ descriptor?

[mr-nfamous]

BPO	43341
Nosy	@rhettinger, @mr-nfamous

^{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 = None
created_at = <Date 2021-02-27.18:37:25.298>
labels = ['type-bug', '3.8']
title = 'functools.partial missing __weakref__ descriptor?'
updated_at = <Date 2021-02-27.21:15:12.591>
user = 'https://proxy-ga.blitzz.co/proxy/123456/github.com/mr-nfamous'

bugs.python.org fields:

activity = <Date 2021-02-27.21:15:12.591>
actor = 'rhettinger'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2021-02-27.18:37:25.298>
creator = 'bup'
dependencies = []
files = []
hgrepos = []
issue_num = 43341
keywords = []
message_count = 1.0
messages = ['387781']
nosy_count = 2.0
nosy_names = ['rhettinger', 'bup']
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priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://proxy-ga.blitzz.co/proxy/123456/bugs.python.org/issue43341'
versions = ['Python 3.8']

[mr-nfamous]

The only way to lookup weak references to functools.partial objects is with weakref.getweakrefs. I don't know if it's normal for extension types that define tp_weaklistoffset to not add a __weakref__ descriptor, but I figured at the very least a subtype specifying "__weakref__" in its __slots__ could get one. Instead, it fails with the partially true TypeError: __weakref__ slot disallowed: either we already got one, or __itemsize__ != 0.