Regression in Django with singledispatchmethod on models

Bug report

Bug description:

Also reported upstream to Django here.

The following code works on Python 3.12 and I believe #85160 is the cause.

When traversing a relationship and then calling a singledispatchmethod method registered on a model, the reference to self seems to get cached and locked to the first created model.

class Test(models.Model):
    ...

class Test2(models.Model):
    relationship = models.ForeignKey("Test", on_delete=models.CASCADE)

    @singledispatchmethod
    def bug(self, x):
        print(">", id(self))

    @bug.register
    def _(self, x: int):
        print(">", id(self))

and then it's pretty easy to trigger:

for t in test.test2_set.all():
    print("id:", id(t))  # always changes
    t.bug(4)             # only ever prints a single id

CPython versions tested on:

3.13

Operating systems tested on:

macOS

Linked PRs

I see what the issue is now: https://github.com/AlexWaygood/cpython/blob/3d8d4ba5f5367f2d87488b00afd55f75401c2b7d/Lib/functools.py#L950

Since dictionaries use __hash__/__eq__ for determining if a key is present, we can have collisions. Django hashs models based on the primary key. Here's a way to reproduce the same bug with just the stdlib:

from functools import singledispatchmethod
from dataclasses import dataclass

@dataclass(frozen=True)  # or (unsafe_hash=True)
class Test:
    pk: int

    @singledispatchmethod
    def broken(self, x):
        raise NotImplemenetedError

    @broken.register
    def _(self, x: int):
        return id(self)


t1 = Test(1)
print(id(t1))

t2 = Test(1)
print(id(t2))

assert id(t1) == t1.broken(2)  # both t1 and t2 will share the same dispatcher
assert id(t2) == t2.broken(2)  # fails!

I have a fix, I think. Working on a PR.

pythongh-127750: Improve singledispatchmethod caching

I'm pretty sure the code in question is also leaking memory. See comment here.

@vodik

@vodik There is indeed a behavior change between 3.12 and 3.13, so perhaps that needs to be fixed. Another example showing the issue is:

from functools import lru_cache
from dataclasses import dataclass

@dataclass(frozen=True)  # or (unsafe_hash=True)
class Test:
    pk: int

t1 = Test(1)
t2 = Test(1)  

@lru_cache
def return_id(obj):
    return id(obj)

print(return_id(t1))
print(return_id(t2))

gh-127750: Restore inspect and pydoc support of singledispatchmethod (G…

[3.13] pythongh-127750: Restore inspect and pydoc support of singledi…

[3.13] pythongh-127750: Backport some tests for singledispatchmethod (p…

[3.13] gh-127750: Backport some tests for singledispatchmethod (GH-13…

[3.13] pythongh-127750: Backport some tests for singledispatchmethod (p…

[3.12] gh-127750: Backport some tests for singledispatchmethod (GH-13…

gh-127750: Improve repr of functools.singledispatchmethod (GH-130220)

gh-127750: Fix annotations in singledispatchmethod signature tests (G…

pythongh-127750: Fix annotations in singledispatchmethod signature te…

[3.13] gh-127750: Fix annotations in singledispatchmethod signature t…

[3.14] gh-127750: Fix annotations in singledispatchmethod signature t…

pythongh-127750: Fix annotations in singledispatchmethod signature te…

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Regression in Django with singledispatchmethod on models #127750

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Operating systems tested on:

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Regression in Django with singledispatchmethod on models #127750

Description

Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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vodik commented on Dec 9, 2024

vodik commented on Dec 9, 2024

vodik commented on Dec 9, 2024

eendebakpt commented on Dec 9, 2024

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