Changed behavior of <instancemethod>.__get__ in Python 3.11

[mschmitzer]

Bug report

Bug description:

There is a change in behavior in Python 3.11 that does not seem to be described in the "what's new" document.

In the following code:

class A:
    def meth(self):
      print(self)

class B:
    pass

a = A()
b = B()
b.meth = a.meth.__get__(b, B)
b.meth()

The b.meth() call receives a as the self argument up to Python 3.10, but b in 3.11 and 3.12.

This pattern is used by the Pyramid framework. The change breaks Pyramid applications that use add_request_method(someobject.somemethod, "name").

I have to admit that I do not understand the descriptor protocol (or the documentation of __get__) well enough to say whether this is a legitimate use case, but it does exist in the wild.

CPython versions tested on:

3.9, 3.10, 3.11, 3.12

Operating systems tested on:

Linux

Linked PRs

• gh-113157 gh-89519: Fix method descriptors #113233
• gh-113157: Document and test __get__ for MethodType #115492

[Eclips4]

Bisected to a918589
cc @msullivan @rhettinger

[rhettinger]

@MarcSchmitzer is correct and the "return a" behavior should be restored.

For Python 3.13, this is straightforward. Since classmethod descriptor chaining has now been removed, we can just add back method_descr_get to PyMethod_Type.

For Python 3.11 and Python 3.12, the situation is more complex and we can't fix any one aspect of the problem without breaking something else. Here are the options that come to mind:

1. Leave Python 3.11 and 3.12 as-is. But that would be a problem for the Pyramid framework.
2. Revert a918589 . But that would break @msullivan 's use case.
3. Apply the 3.13 solution to 3.11 and 3.12. This is the cleanest way in the sense that we will have fully restored the original long-standing behaviors before the classmethod descriptor chaining trainwreck began. However, this path breaks having classmethod wrap property which is something that we know people use — that is why we went through a deprecation period to finally remove it in 3.13.

[rhettinger]

@msullivan Would you be okay with reverting a918589 ? We wouldn't have gone down that path had we known that it would break another part of the API.

[rhettinger]

For 3.11 and 3.12, I suggest that we revert a918589

Any objections?

[erlend-aasland]

For 3.11 and 3.12, I suggest that we revert a918589

Any objections?

cc. @pablogsal as 3.11 RM, and @Yhg1s as 3.12 RM.