Behaviour change in partialmethod in Python3.11

Bug report

The behavior of partialmethod changed in python3.11. This caused a library that I use testsuite to fail.

Here is a minimal use case that demonstrates this:

from functools import partialmethod


class Cell:
    def __init__(self):
        self.alive = False

    def set_state(self, state):
        self.alive = bool(state)


class Wrapper:
    def set_state_wrap(self, cell: Cell, state):
        cell.set_state(state)


wrapper = Wrapper()

Cell.set_alive = partialmethod(wrapper.set_state_wrap, True)

cell = Cell()
cell.set_alive()
print(cell.alive)

In python3.10, this prints True. In python3.11 it errors with:

Traceback (most recent call last):
  File "/home/gary/partialmethod.py", line 23, in <module>
    c.set_alive()
TypeError: Wrapper.set_state_wrap() missing 1 required positional argument: 'state'

The arguments passed to Wrapper.set_state_wrap are:
python3.10: wrapper, cell, True
python3.11: cell, True

So partialmethod is forgetting that set_state_wrap is a method of wrapper, and it needs to pass that in as the first arg.

I know that the way that they are using partialmethod here is not how it is intended to be used, and feels wrong. However this change is behavior if intended should be documented.

Your environment

CPython versions tested on: Python 3.11.0 (broken) Python 3.10.7 (working)
Operating system and architecture: Fedora Linux 36

I bisected to here:

a918589 is the first bad commit
commit a918589
Author: Michael J. Sullivan sully@msully.net
Date: Wed May 4 21:00:21 2022 -0700
bpo-46764: Fix wrapping bound method with @classmethod (#31367)

@rhettinger

cc @rhettinger

Having stumbled across this in a Django app where I was using Field.contribute_to_class to add some extra functionality to models that I couldn't change directly, I experimented with a few different workarounds with very little success.
Eventually I decided the easiest solution was an explicit decorator, and this is what I'm using now (in the very specific cases where I need to ensure the old behavior is maintained:

class partialmethod_with_self(partialmethod):
    def __get__(self, obj, cls=None):
        return self._make_unbound_method().__get__(obj, cls)

This works in my case, and it should be backwards-compatible as well (I tested against 3.11 and 3.9.2).

Is this behavior change documented anywhere? Will it ever be fixed? Is it considered as a bug at all by the python devs?

I am also being hit by this problem in the context of django (and the contribute_to_class pattern). This basically boils down to this snippet (works fine until 3.10, errors out in 3.11 and 3.12):

from functools import partialmethod

class A:
    @classmethod
    def f1(cls, self, value):
        self.value = value

class B:
    def __init__(self):
        self.value = None

setattr(B, 'set_value_true', partialmethod(A.f1, True))

b = B()
b.set_value_true()
print(b.value)

This was fixed in d058eae for #113157 (which is effectively a dup of this, I think) and delivered in v.3.13.0.

I believe this issue can be closed.

fix python 3.13+ compat

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Behaviour change in partialmethod in Python3.11 #99152

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Behaviour change in partialmethod in Python3.11 #99152

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sweeneyde commented on Nov 6, 2022

sweeneyde commented on Nov 6, 2022

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