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

• gh-127750: Improve caching in singledispatchmethod #127751
• gh-127750: Fix singledispatchmethod caching #127839
• gh-127750: Fix singledispatchmethod caching (v2) #128648
• gh-127750: Fix and optimize functools.singledispatchmethod() #130008
• [3.13] gh-127750: Fix functools.singledispatchmethod()  #130029
• gh-127750: Improve repr of functools.singledispatchmethod #130220
• gh-127750: Restore inspect and pydoc support of singledispatchmethod #130309
• [3.13] gh-127750: Backport some tests for singledispatchmethod (GH-130309) #130340
• [3.12] gh-127750: Backport some tests for singledispatchmethod (GH-130309) (GH-130340) #130341
• gh-127750: Fix annotations in singledispatchmethod signature tests #143571
• [3.14] gh-127750: Fix annotations in singledispatchmethod signature tests (GH-143571) #143707
• [3.13] gh-127750: Fix annotations in singledispatchmethod signature tests (GH-143571) #143708