Wrapping a bound method with a @classmethod no longer works

[msullivan]

BPO	46764
Nosy	@rhettinger, @msullivan
PRs	bpo-46764: Fix wrapping bound method with @classmethod #31367

^{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/rhettinger'
closed_at = None
created_at = <Date 2022-02-16.01:56:29.217>
labels = ['interpreter-core', 'type-bug', '3.9', '3.10', '3.11']
title = 'Wrapping a bound method with a @classmethod no longer works'
updated_at = <Date 2022-02-17.20:02:42.052>
user = 'https://github.com/msullivan'

bugs.python.org fields:

activity = <Date 2022-02-17.20:02:42.052>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2022-02-16.01:56:29.217>
creator = 'msullivan'
dependencies = []
files = []
hgrepos = []
issue_num = 46764
keywords = ['patch']
message_count = 2.0
messages = ['413310', '413450']
nosy_count = 2.0
nosy_names = ['rhettinger', 'msullivan']
pr_nums = ['31367']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46764'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[msullivan]

class A:
    def foo(self, cls): return 1

class B: pass

class B:
    bar = classmethod(A().foo)

B.bar()

In Python 3.8 and prior, this worked. Since Python 3.9, it produces "TypeError: A.foo() missing 1 required positional argument: 'cls'"

I tracked it down, and the issue was introduced by https://github.com/python/cpython/pull/8405/files, which makes classmethod's tp_descr_get invoke its argument tp_descr_get when present instead of calling PyMethod_New. That this was a semantics change that could break existing code may have been missed (though it is a fairly obscure such change).

The reason it breaks this case in particular of bound methods, though, is that bound methods have a tp_descr_get that does nothing (increfs the method and then returns it). Dropping that tp_descr_get fixes this issue and doesn't introduce any test failures. Not sure if there is some potential downstream breakage of that?

(This issue was originally reported to me by Jared Hance)

[rhettinger]

This seems like a reasonable fix. I'll wait a bit so that others can comment as well.

FWIW, we looking a potentially reverting the whole line of development starting with that commit. It has caused a number of problems and no longer looks tenable. See https://bugs.python.org/issue45356

[iritkatriel]

The PR was merged, and I think it resolves this issue.

[iritkatriel]

Unless it can still be backported.