[issue46764] Wrapping a bound method with a @classmethod no longer works

[Raymond Hettinger]

Raymond Hettinger  added the comment:

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

--
assignee:  -> rhettinger

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[issue46764] Wrapping a bound method with a @classmethod no longer works

[Michael J. Sullivan]

Change by Michael J. Sullivan :


--
keywords: +patch
pull_requests: +29517
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/31367

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[issue46764] Wrapping a bound method with a @classmethod no longer works

[Alex Waygood]

Change by Alex Waygood :


--
nosy: +rhettinger
type:  -> behavior

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[issue46764] Wrapping a bound method with a @classmethod no longer works

[Michael J. Sullivan]

New submission from Michael J. Sullivan :

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)

--
components: Interpreter Core
messages: 413310
nosy: msullivan
priority: normal
severity: normal
status: open
title: Wrapping a bound method with a @classmethod no longer works
versions: Python 3.10, Python 3.11, Python 3.9

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