[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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[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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[issue36878] ast.parse with type_comments=True should allow extra text after # type: ignore

[Michael J. Sullivan]

Change by Michael J. Sullivan :


--
pull_requests: +13419

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[issue36878] ast.parse with type_comments=True should allow extra text after # type: ignore

[Michael J. Sullivan]

Michael J. Sullivan  added the comment:

I think this is done!

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[issue37017] Use LOAD_METHOD optimization in CallMethod C API functions

[Michael J. Sullivan]

New submission from Michael J. Sullivan :

The different varieties of PyObject_CallMethod* routines all operate by doing a 
PyObject_GetAttr to fetch an object to call. It seems likely to be worthwhile 
to take advantage of the LOAD_METHOD optimization that avoids creating a bound 
method object when calling a method.

--
components: Extension Modules
messages: 343259
nosy: msullivan
priority: normal
severity: normal
status: open
title: Use LOAD_METHOD optimization in CallMethod C API functions
versions: Python 3.8

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[issue37017] Use LOAD_METHOD optimization in CallMethod C API functions

[Michael J. Sullivan]

Change by Michael J. Sullivan :


--
keywords: +patch
pull_requests: +13433
stage:  -> patch review

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[issue37017] Use LOAD_METHOD optimization in CallMethod C API functions

[Michael J. Sullivan]

Change by Michael J. Sullivan :


--
nosy: +brett.cannon, serhiy.storchaka, vstinner, yselivanov

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[issue37017] Use LOAD_METHOD optimization in CallMethod C API functions

[Michael J. Sullivan]

Michael J. Sullivan  added the comment:

I believe that this is orthogonal to PEP 590.

PyObject_CallMethodObjArgs and friends take varargs which need to be copied 
into an array one way or another. It is easy (and efficient) to prepend the 
base while copying the function arguments into the array (see the attached PR). 
I don't think that vector call will change anything about this.

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